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<!-- doc/src/sgml/charset.sgml -->

<chapter id="charset">
 <title>Localization</title>

 <para>
  This chapter describes the available localization features from the
  point of view of the administrator.
  <productname>PostgreSQL</productname> supports two localization
  facilities:

   <itemizedlist>
    <listitem>
     <para>
      Using the locale features of the operating system to provide
      locale-specific collation order, number formatting, translated
      messages, and other aspects.
      This is covered in <xref linkend="locale"/> and
      <xref linkend="collation"/>.
     </para>
    </listitem>

    <listitem>
     <para>
      Providing a number of different character sets to support storing text
      in all kinds of languages, and providing character set translation
      between client and server.
      This is covered in <xref linkend="multibyte"/>.
     </para>
    </listitem>
   </itemizedlist>
  </para>


 <sect1 id="locale">
  <title>Locale Support</title>

  <indexterm zone="locale"><primary>locale</primary></indexterm>

  <para>
   <firstterm>Locale</firstterm> support refers to an application respecting
   cultural preferences regarding alphabets, sorting, number
   formatting, etc.  <productname>PostgreSQL</productname> uses the standard ISO
   C and <acronym>POSIX</acronym> locale facilities provided by the server operating
   system.  For additional information refer to the documentation of your
   system.
  </para>

  <sect2 id="locale-overview">
   <title>Overview</title>

   <para>
    Locale support is automatically initialized when a database
    cluster is created using <command>initdb</command>.
    <command>initdb</command> will initialize the database cluster
    with the locale setting of its execution environment by default,
    so if your system is already set to use the locale that you want
    in your database cluster then there is nothing else you need to
    do.  If you want to use a different locale (or you are not sure
    which locale your system is set to), you can instruct
    <command>initdb</command> exactly which locale to use by
    specifying the <option>--locale</option> option. For example:
<screen>
initdb --locale=sv_SE
</screen>
   </para>

   <para>
    This example for Unix systems sets the locale to Swedish
    (<literal>sv</literal>) as spoken
    in Sweden (<literal>SE</literal>).  Other possibilities might include
    <literal>en_US</literal> (U.S. English) and <literal>fr_CA</literal> (French
    Canadian).  If more than one character set can be used for a
    locale then the specifications can take the form
    <replaceable>language_territory.codeset</replaceable>.  For example,
    <literal>fr_BE.UTF-8</literal> represents the French language (fr) as
    spoken in Belgium (BE), with a <acronym>UTF-8</acronym> character set
    encoding.
   </para>

   <para>
    What locales are available on your
    system under what names depends on what was provided by the operating
    system vendor and what was installed.  On most Unix systems, the command
    <literal>locale -a</literal> will provide a list of available locales.
    Windows uses more verbose locale names, such as <literal>German_Germany</literal>
    or <literal>Swedish_Sweden.1252</literal>, but the principles are the same.
   </para>

   <para>
    Occasionally it is useful to mix rules from several locales, e.g.,
    use English collation rules but Spanish messages.  To support that, a
    set of locale subcategories exist that control only certain
    aspects of the localization rules:

    <informaltable>
     <tgroup cols="2">
      <colspec colname="col1" colwidth="1*"/>
      <colspec colname="col2" colwidth="3*"/>
      <tbody>
       <row>
        <entry><envar>LC_COLLATE</envar></entry>
        <entry>String sort order</entry>
       </row>
       <row>
        <entry><envar>LC_CTYPE</envar></entry>
        <entry>Character classification (What is a letter? Its upper-case equivalent?)</entry>
       </row>
       <row>
        <entry><envar>LC_MESSAGES</envar></entry>
        <entry>Language of messages</entry>
       </row>
       <row>
        <entry><envar>LC_MONETARY</envar></entry>
        <entry>Formatting of currency amounts</entry>
       </row>
       <row>
        <entry><envar>LC_NUMERIC</envar></entry>
        <entry>Formatting of numbers</entry>
       </row>
       <row>
        <entry><envar>LC_TIME</envar></entry>
        <entry>Formatting of dates and times</entry>
       </row>
      </tbody>
     </tgroup>
    </informaltable>

    The category names translate into names of
    <command>initdb</command> options to override the locale choice
    for a specific category.  For instance, to set the locale to
    French Canadian, but use U.S. rules for formatting currency, use
    <literal>initdb --locale=fr_CA --lc-monetary=en_US</literal>.
   </para>

   <para>
    If you want the system to behave as if it had no locale support,
    use the special locale name <literal>C</literal>, or equivalently
    <literal>POSIX</literal>.
   </para>

   <para>
    Some locale categories must have their values
    fixed when the database is created.  You can use different settings
    for different databases, but once a database is created, you cannot
    change them for that database anymore. <literal>LC_COLLATE</literal>
    and <literal>LC_CTYPE</literal> are these categories.  They affect
    the sort order of indexes, so they must be kept fixed, or indexes on
    text columns would become corrupt.
    (But you can alleviate this restriction using collations, as discussed
    in <xref linkend="collation"/>.)
    The default values for these
    categories are determined when <command>initdb</command> is run, and
    those values are used when new databases are created, unless
    specified otherwise in the <command>CREATE DATABASE</command> command.
   </para>

   <para>
    The other locale categories can be changed whenever desired
    by setting the server configuration parameters
    that have the same name as the locale categories (see <xref
    linkend="runtime-config-client-format"/> for details).  The values
    that are chosen by <command>initdb</command> are actually only written
    into the configuration file <filename>postgresql.conf</filename> to
    serve as defaults when the server is started.  If you remove these
    assignments from <filename>postgresql.conf</filename> then the
    server will inherit the settings from its execution environment.
   </para>

   <para>
    Note that the locale behavior of the server is determined by the
    environment variables seen by the server, not by the environment
    of any client.  Therefore, be careful to configure the correct locale settings
    before starting the server.  A consequence of this is that if
    client and server are set up in different locales, messages might
    appear in different languages depending on where they originated.
   </para>

   <note>
    <para>
     When we speak of inheriting the locale from the execution
     environment, this means the following on most operating systems:
     For a given locale category, say the collation, the following
     environment variables are consulted in this order until one is
     found to be set: <envar>LC_ALL</envar>, <envar>LC_COLLATE</envar>
     (or the variable corresponding to the respective category),
     <envar>LANG</envar>.  If none of these environment variables are
     set then the locale defaults to <literal>C</literal>.
    </para>

    <para>
     Some message localization libraries also look at the environment
     variable <envar>LANGUAGE</envar> which overrides all other locale
     settings for the purpose of setting the language of messages.  If
     in doubt, please refer to the documentation of your operating
     system, in particular the documentation about
     <application>gettext</application>.
    </para>
   </note>

   <para>
    To enable messages to be translated to the user's preferred language,
    <acronym>NLS</acronym> must have been selected at build time
    (<literal>configure --enable-nls</literal>).  All other locale support is
    built in automatically.
   </para>
  </sect2>

  <sect2 id="locale-behavior">
   <title>Behavior</title>

   <para>
    The locale settings influence the following SQL features:

    <itemizedlist>
     <listitem>
      <para>
       Sort order in queries using <literal>ORDER BY</literal> or the standard
       comparison operators on textual data
       <indexterm><primary>ORDER BY</primary><secondary>and locales</secondary></indexterm>
      </para>
     </listitem>

     <listitem>
      <para>
       The <function>upper</function>, <function>lower</function>, and <function>initcap</function>
       functions
       <indexterm><primary>upper</primary><secondary>and locales</secondary></indexterm>
       <indexterm><primary>lower</primary><secondary>and locales</secondary></indexterm>
      </para>
     </listitem>

     <listitem>
      <para>
       Pattern matching operators (<literal>LIKE</literal>, <literal>SIMILAR TO</literal>,
       and POSIX-style regular expressions); locales affect both case
       insensitive matching and the classification of characters by
       character-class regular expressions
       <indexterm><primary>LIKE</primary><secondary>and locales</secondary></indexterm>
       <indexterm><primary>regular expressions</primary><secondary>and locales</secondary></indexterm>
      </para>
     </listitem>

     <listitem>
      <para>
       The <function>to_char</function> family of functions
       <indexterm><primary>to_char</primary><secondary>and locales</secondary></indexterm>
      </para>
     </listitem>

     <listitem>
      <para>
       The ability to use indexes with <literal>LIKE</literal> clauses
      </para>
     </listitem>
    </itemizedlist>
   </para>

   <para>
    The drawback of using locales other than <literal>C</literal> or
    <literal>POSIX</literal> in <productname>PostgreSQL</productname> is its performance
    impact. It slows character handling and prevents ordinary indexes
    from being used by <literal>LIKE</literal>. For this reason use locales
    only if you actually need them.
   </para>

   <para>
    As a workaround to allow <productname>PostgreSQL</productname> to use indexes
    with <literal>LIKE</literal> clauses under a non-C locale, several custom
    operator classes exist. These allow the creation of an index that
    performs a strict character-by-character comparison, ignoring
    locale comparison rules. Refer to <xref linkend="indexes-opclass"/>
    for more information.  Another approach is to create indexes using
    the <literal>C</literal> collation, as discussed in
    <xref linkend="collation"/>.
   </para>
  </sect2>

  <sect2 id="locale-selecting-locales">
   <title>Selecting Locales</title>

   <para>
    Locales can be selected in different scopes depending on requirements.
    The above overview showed how locales are specified using
    <command>initdb</command> to set the defaults for the entire cluster.  The
    following list shows where locales can be selected.  Each item provides
    the defaults for the subsequent items, and each lower item allows
    overriding the defaults on a finer granularity.
   </para>

   <orderedlist>
    <listitem>
     <para>
      As explained above, the environment of the operating system provides the
      defaults for the locales of a newly initialized database cluster.  In
      many cases, this is enough: if the operating system is configured for
      the desired language/territory, by default
      <productname>PostgreSQL</productname> will also behave according
      to that locale.
     </para>
    </listitem>

    <listitem>
     <para>
      As shown above, command-line options for <command>initdb</command>
      specify the locale settings for a newly initialized database cluster.
      Use this if the operating system does not have the locale configuration
      you want for your database system.
     </para>
    </listitem>

    <listitem>
     <para>
      A locale can be selected separately for each database.  The SQL command
      <command>CREATE DATABASE</command> and its command-line equivalent
      <command>createdb</command> have options for that.  Use this for example
      if a database cluster houses databases for multiple tenants with
      different requirements.
     </para>
    </listitem>

    <listitem>
     <para>
      Locale settings can be made for individual table columns.  This uses an
      SQL object called <firstterm>collation</firstterm> and is explained in
      <xref linkend="collation"/>.  Use this for example to sort data in
      different languages or customize the sort order of a particular table.
     </para>
    </listitem>

    <listitem>
     <para>
      Finally, locales can be selected for an individual query.  Again, this
      uses SQL collation objects.  This could be used to change the sort order
      based on run-time choices or for ad-hoc experimentation.
     </para>
    </listitem>
   </orderedlist>
  </sect2>

  <sect2 id="locale-providers">
   <title>Locale Providers</title>

   <para>
    A locale provider specifies which library defines the locale behavior for
    collations and character classifications.
   </para>

   <para>
    The commands and tools that select the locale settings, as described
    above, each have an option to select the locale provider. Here is an
    example to initialize a database cluster using the ICU provider:
<programlisting>
initdb --locale-provider=icu --icu-locale=en
</programlisting>
    See the description of the respective commands and programs for
    details.  Note that you can mix locale providers at different
    granularities, for example use <literal>libc</literal> by default for the
    cluster but have one database that uses the <literal>icu</literal>
    provider, and then have collation objects using either provider within
    those databases.
   </para>

   <para>
    Regardless of the locale provider, the operating system is still used to
    provide some locale-aware behavior, such as messages (see <xref
    linkend="guc-lc-messages"/>).
   </para>

   <para>
    The available locale providers are listed below:
   </para>

   <variablelist>
    <varlistentry>
     <term><literal>builtin</literal></term>
     <listitem>
      <para>
       The <literal>builtin</literal> provider uses built-in operations. Only
       the <literal>C</literal> and <literal>C.UTF-8</literal> locales are
       supported for this provider.
      </para>
      <para>
       The <literal>C</literal> locale behavior is identical to the
       <literal>C</literal> locale in the libc provider. When using this
       locale, the behavior may depend on the database encoding.
      </para>
      <para>
       The <literal>C.UTF-8</literal> locale is available only for when the
       database encoding is <literal>UTF-8</literal>, and the behavior is
       based on Unicode. The collation uses the code point values only. The
       regular expression character classes are based on the "POSIX
       Compatible" semantics, and the case mapping is the "simple" variant.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>icu</literal></term>
     <listitem>
      <para>
       The <literal>icu</literal> provider uses the external
       ICU<indexterm><primary>ICU</primary></indexterm>
       library. <productname>PostgreSQL</productname> must have been
       configured with support.
      </para>
      <para>
       ICU provides collation and character classification behavior that is
       independent of the operating system and database encoding, which is
       preferable if you expect to transition to other platforms without any
       change in results. <literal>LC_COLLATE</literal> and
       <literal>LC_CTYPE</literal> can be set independently of the ICU
       locale.
      </para>
      <note>
       <para>
        For the ICU provider, results may depend on the version of the ICU
        library used, as it is updated to reflect changes in natural language
        over time.
       </para>
      </note>
     </listitem>
    </varlistentry>

    <varlistentry>
     <term><literal>libc</literal></term>
     <listitem>
      <para>
       The <literal>libc</literal> provider uses the operating system's C
       library. The collation and character classification behavior is
       controlled by the settings <literal>LC_COLLATE</literal> and
       <literal>LC_CTYPE</literal>, so they cannot be set independently.
      </para>
      <note>
       <para>
        The same locale name may have different behavior on different
        platforms when using the libc provider.
       </para>
      </note>
     </listitem>
    </varlistentry>
   </variablelist>
  </sect2>

  <sect2 id="icu-locales">
   <title>ICU Locales</title>

   <sect3 id="icu-locale-names">
    <title>ICU Locale Names</title>

    <para>
     The ICU format for the locale name is a <link
     linkend="icu-language-tag">Language Tag</link>.

<programlisting>
CREATE COLLATION mycollation1 (provider = icu, locale = 'ja-JP');
CREATE COLLATION mycollation2 (provider = icu, locale = 'fr');
</programlisting>
    </para>
   </sect3>

   <sect3 id="icu-canonicalization">
    <title>Locale Canonicalization and Validation</title>
    <para>
     When defining a new ICU collation object or database with ICU as the
     provider, the given locale name is transformed ("canonicalized") into a
     language tag if not already in that form. For instance,

<screen>
CREATE COLLATION mycollation3 (provider = icu, locale = 'en-US-u-kn-true');
NOTICE:  using standard form "en-US-u-kn" for locale "en-US-u-kn-true"
CREATE COLLATION mycollation4 (provider = icu, locale = 'de_DE.utf8');
NOTICE:  using standard form "de-DE" for locale "de_DE.utf8"
</screen>

     If you see this notice, ensure that the <symbol>provider</symbol> and
     <symbol>locale</symbol> are the expected result. For consistent results
     when using the ICU provider, specify the canonical <link
     linkend="icu-language-tag">language tag</link> instead of relying on the
     transformation.
    </para>

    <para>
     A locale with no language name, or the special language name
     <literal>root</literal>, is transformed to have the language
     <literal>und</literal> ("undefined").
    </para>

    <para>
     ICU can transform most libc locale names, as well as some other formats,
     into language tags for easier transition to ICU. If a libc locale name is
     used in ICU, it may not have precisely the same behavior as in libc.
    </para>

    <para>
     If there is a problem interpreting the locale name, or if the locale name
     represents a language or region that ICU does not recognize, you will see
     the following warning:

<screen>
CREATE COLLATION nonsense (provider = icu, locale = 'nonsense');
WARNING:  ICU locale "nonsense" has unknown language "nonsense"
HINT:  To disable ICU locale validation, set parameter icu_validation_level to DISABLED.
CREATE COLLATION
</screen>

     <xref linkend="guc-icu-validation-level"/> controls how the message is
     reported. Unless set to <literal>ERROR</literal>, the collation will
     still be created, but the behavior may not be what the user intended.
    </para>
   </sect3>

   <sect3 id="icu-language-tag">
    <title>Language Tag</title>

    <para>
     A language tag, defined in BCP 47, is a standardized identifier used to
     identify languages, regions, and other information about a locale.
    </para>

    <para>
     Basic language tags are simply
     <replaceable>language</replaceable><literal>-</literal><replaceable>region</replaceable>;
     or even just <replaceable>language</replaceable>. The
     <replaceable>language</replaceable> is a language code
     (e.g. <literal>fr</literal> for French), and
     <replaceable>region</replaceable> is a region code
     (e.g. <literal>CA</literal> for Canada). Examples:
     <literal>ja-JP</literal>, <literal>de</literal>, or
     <literal>fr-CA</literal>.
    </para>

    <para>
     Collation settings may be included in the language tag to customize
     collation behavior. ICU allows extensive customization, such as
     sensitivity (or insensitivity) to accents, case, and punctuation;
     treatment of digits within text; and many other options to satisfy a
     variety of uses.
    </para>

    <para>
     To include this additional collation information in a language tag,
     append <literal>-u</literal>, which indicates there are additional
     collation settings, followed by one or more
     <literal>-</literal><replaceable>key</replaceable><literal>-</literal><replaceable>value</replaceable>
     pairs. The <replaceable>key</replaceable> is the key for a <link
     linkend="icu-collation-settings">collation setting</link> and
     <replaceable>value</replaceable> is a valid value for that setting. For
     boolean settings, the <literal>-</literal><replaceable>key</replaceable>
     may be specified without a corresponding
     <literal>-</literal><replaceable>value</replaceable>, which implies a
     value of <literal>true</literal>.
    </para>

    <para>
     For example, the language tag <literal>en-US-u-kn-ks-level2</literal>
     means the locale with the English language in the US region, with
     collation settings <literal>kn</literal> set to <literal>true</literal>
     and <literal>ks</literal> set to <literal>level2</literal>. Those
     settings mean the collation will be case-insensitive and treat a sequence
     of digits as a single number:

<screen>
CREATE COLLATION mycollation5 (provider = icu, deterministic = false, locale = 'en-US-u-kn-ks-level2');
SELECT 'aB' = 'Ab' COLLATE mycollation5 as result;
 result
--------
 t
(1 row)

SELECT 'N-45' &lt; 'N-123' COLLATE mycollation5 as result;
 result
--------
 t
(1 row)
</screen>
    </para>

    <para>
     See <xref linkend="icu-custom-collations"/> for details and additional
     examples of using language tags with custom collation information for the
     locale.
    </para>
   </sect3>
  </sect2>

  <sect2 id="locale-problems">
   <title>Problems</title>

   <para>
    If locale support doesn't work according to the explanation above,
    check that the locale support in your operating system is
    correctly configured.  To check what locales are installed on your
    system, you can use the command <literal>locale -a</literal> if
    your operating system provides it.
   </para>

   <para>
    Check that <productname>PostgreSQL</productname> is actually using the locale
    that you think it is.  The <envar>LC_COLLATE</envar> and <envar>LC_CTYPE</envar>
    settings are determined when a database is created, and cannot be
    changed except by creating a new database.  Other locale
    settings including <envar>LC_MESSAGES</envar> and <envar>LC_MONETARY</envar>
    are initially determined by the environment the server is started
    in, but can be changed on-the-fly.  You can check the active locale
    settings using the <command>SHOW</command> command.
   </para>

   <para>
    The directory <filename>src/test/locale</filename> in the source
    distribution contains a test suite for
    <productname>PostgreSQL</productname>'s locale support.
   </para>

   <para>
    Client applications that handle server-side errors by parsing the
    text of the error message will obviously have problems when the
    server's messages are in a different language.  Authors of such
    applications are advised to make use of the error code scheme
    instead.
   </para>

   <para>
    Maintaining catalogs of message translations requires the on-going
    efforts of many volunteers that want to see
    <productname>PostgreSQL</productname> speak their preferred language well.
    If messages in your language are currently not available or not fully
    translated, your assistance would be appreciated.  If you want to
    help, refer to <xref linkend="nls"/> or write to the developers'
    mailing list.
   </para>
  </sect2>
 </sect1>


 <sect1 id="collation">
  <title>Collation Support</title>

  <indexterm zone="collation"><primary>collation</primary></indexterm>

  <para>
   The collation feature allows specifying the sort order and character
   classification behavior of data per-column, or even per-operation.
   This alleviates the restriction that the
   <symbol>LC_COLLATE</symbol> and <symbol>LC_CTYPE</symbol> settings
   of a database cannot be changed after its creation.
  </para>

  <sect2 id="collation-concepts">
   <title>Concepts</title>

   <para>
    Conceptually, every expression of a collatable data type has a
    collation.  (The built-in collatable data types are
    <type>text</type>, <type>varchar</type>, and <type>char</type>.
    User-defined base types can also be marked collatable, and of course
    a <glossterm linkend="glossary-domain">domain</glossterm> over a
    collatable data type is collatable.)  If the
    expression is a column reference, the collation of the expression is the
    defined collation of the column.  If the expression is a constant, the
    collation is the default collation of the data type of the
    constant.  The collation of a more complex expression is derived
    from the collations of its inputs, as described below.
   </para>

   <para>
    The collation of an expression can be the <quote>default</quote>
    collation, which means the locale settings defined for the
    database.  It is also possible for an expression's collation to be
    indeterminate.  In such cases, ordering operations and other
    operations that need to know the collation will fail.
   </para>

   <para>
    When the database system has to perform an ordering or a character
    classification, it uses the collation of the input expression.  This
    happens, for example, with <literal>ORDER BY</literal> clauses
    and function or operator calls such as <literal>&lt;</literal>.
    The collation to apply for an <literal>ORDER BY</literal> clause
    is simply the collation of the sort key.  The collation to apply for a
    function or operator call is derived from the arguments, as described
    below.  In addition to comparison operators, collations are taken into
    account by functions that convert between lower and upper case
    letters, such as <function>lower</function>, <function>upper</function>, and
    <function>initcap</function>; by pattern matching operators; and by
    <function>to_char</function> and related functions.
   </para>

   <para>
    For a function or operator call, the collation that is derived by
    examining the argument collations is used at run time for performing
    the specified operation.  If the result of the function or operator
    call is of a collatable data type, the collation is also used at parse
    time as the defined collation of the function or operator expression,
    in case there is a surrounding expression that requires knowledge of
    its collation.
   </para>

   <para>
    The <firstterm>collation derivation</firstterm> of an expression can be
    implicit or explicit.  This distinction affects how collations are
    combined when multiple different collations appear in an
    expression.  An explicit collation derivation occurs when a
    <literal>COLLATE</literal> clause is used; all other collation
    derivations are implicit.  When multiple collations need to be
    combined, for example in a function call, the following rules are
    used:

    <orderedlist>
     <listitem>
      <para>
       If any input expression has an explicit collation derivation, then
       all explicitly derived collations among the input expressions must be
       the same, otherwise an error is raised.  If any explicitly
       derived collation is present, that is the result of the
       collation combination.
      </para>
     </listitem>

     <listitem>
      <para>
       Otherwise, all input expressions must have the same implicit
       collation derivation or the default collation.  If any non-default
       collation is present, that is the result of the collation combination.
       Otherwise, the result is the default collation.
      </para>
     </listitem>

     <listitem>
      <para>
       If there are conflicting non-default implicit collations among the
       input expressions, then the combination is deemed to have indeterminate
       collation.  This is not an error condition unless the particular
       function being invoked requires knowledge of the collation it should
       apply.  If it does, an error will be raised at run-time.
      </para>
     </listitem>
    </orderedlist>

    For example, consider this table definition:
<programlisting>
CREATE TABLE test1 (
    a text COLLATE "de_DE",
    b text COLLATE "es_ES",
    ...
);
</programlisting>

    Then in
<programlisting>
SELECT a &lt; 'foo' FROM test1;
</programlisting>
    the <literal>&lt;</literal> comparison is performed according to
    <literal>de_DE</literal> rules, because the expression combines an
    implicitly derived collation with the default collation.  But in
<programlisting>
SELECT a &lt; ('foo' COLLATE "fr_FR") FROM test1;
</programlisting>
    the comparison is performed using <literal>fr_FR</literal> rules,
    because the explicit collation derivation overrides the implicit one.
    Furthermore, given
<programlisting>
SELECT a &lt; b FROM test1;
</programlisting>
    the parser cannot determine which collation to apply, since the
    <structfield>a</structfield> and <structfield>b</structfield> columns have conflicting
    implicit collations.  Since the <literal>&lt;</literal> operator
    does need to know which collation to use, this will result in an
    error.  The error can be resolved by attaching an explicit collation
    specifier to either input expression, thus:
<programlisting>
SELECT a &lt; b COLLATE "de_DE" FROM test1;
</programlisting>
    or equivalently
<programlisting>
SELECT a COLLATE "de_DE" &lt; b FROM test1;
</programlisting>
    On the other hand, the structurally similar case
<programlisting>
SELECT a || b FROM test1;
</programlisting>
    does not result in an error, because the <literal>||</literal> operator
    does not care about collations: its result is the same regardless
    of the collation.
   </para>

   <para>
    The collation assigned to a function or operator's combined input
    expressions is also considered to apply to the function or operator's
    result, if the function or operator delivers a result of a collatable
    data type.  So, in
<programlisting>
SELECT * FROM test1 ORDER BY a || 'foo';
</programlisting>
    the ordering will be done according to <literal>de_DE</literal> rules.
    But this query:
<programlisting>
SELECT * FROM test1 ORDER BY a || b;
</programlisting>
    results in an error, because even though the <literal>||</literal> operator
    doesn't need to know a collation, the <literal>ORDER BY</literal> clause does.
    As before, the conflict can be resolved with an explicit collation
    specifier:
<programlisting>
SELECT * FROM test1 ORDER BY a || b COLLATE "fr_FR";
</programlisting>
   </para>
  </sect2>

  <sect2 id="collation-managing">
   <title>Managing Collations</title>

   <para>
    A collation is an SQL schema object that maps an SQL name to locales
    provided by libraries installed in the operating system.  A collation
    definition has a <firstterm>provider</firstterm> that specifies which
    library supplies the locale data.  One standard provider name
    is <literal>libc</literal>, which uses the locales provided by the
    operating system C library.  These are the locales used by most tools
    provided by the operating system.  Another provider
    is <literal>icu</literal>, which uses the external
    ICU<indexterm><primary>ICU</primary></indexterm> library.  ICU locales can only be
    used if support for ICU was configured when PostgreSQL was built.
   </para>

   <para>
    A collation object provided by <literal>libc</literal> maps to a
    combination of <symbol>LC_COLLATE</symbol> and <symbol>LC_CTYPE</symbol>
    settings, as accepted by the <literal>setlocale()</literal> system library call.  (As
    the name would suggest, the main purpose of a collation is to set
    <symbol>LC_COLLATE</symbol>, which controls the sort order.  But
    it is rarely necessary in practice to have an
    <symbol>LC_CTYPE</symbol> setting that is different from
    <symbol>LC_COLLATE</symbol>, so it is more convenient to collect
    these under one concept than to create another infrastructure for
    setting <symbol>LC_CTYPE</symbol> per expression.)  Also,
    a <literal>libc</literal> collation
    is tied to a character set encoding (see <xref linkend="multibyte"/>).
    The same collation name may exist for different encodings.
   </para>

   <para>
    A collation object provided by <literal>icu</literal> maps to a named
    collator provided by the ICU library.  ICU does not support
    separate <quote>collate</quote> and <quote>ctype</quote> settings, so
    they are always the same.  Also, ICU collations are independent of the
    encoding, so there is always only one ICU collation of a given name in
    a database.
   </para>

   <sect3 id="collation-managing-standard">
    <title>Standard Collations</title>

   <para>
    On all platforms, the following collations are supported:

    <variablelist>
     <varlistentry>
      <term><literal>unicode</literal></term>
      <listitem>
       <para>
        This SQL standard collation sorts using the Unicode Collation
        Algorithm with the Default Unicode Collation Element Table.  It is
        available in all encodings.  ICU support is required to use this
        collation, and behavior may change if <productname>PostgreSQL</productname> is built with a
        different version of ICU.  (This collation has the same behavior as
        the ICU root locale; see <xref
        linkend="collation-managing-predefined-icu-und-x-icu"/>.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>ucs_basic</literal></term>
      <listitem>
       <para>
        This SQL standard collation sorts using the Unicode code point values
        rather than natural language order, and only the ASCII letters
        <quote><literal>A</literal></quote> through
        <quote><literal>Z</literal></quote> are treated as letters.  The
        behavior is efficient and stable across all versions.  Only available
        for encoding <literal>UTF8</literal>.  (This collation has the same
        behavior as the libc locale specification <literal>C</literal> in
        <literal>UTF8</literal> encoding.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>pg_c_utf8</literal></term>
      <listitem>
       <para>
        This collation sorts by Unicode code point values rather than natural
        language order.  For the functions <function>lower</function>,
        <function>initcap</function>, and <function>upper</function>, it uses
        Unicode simple case mapping.  For pattern matching (including regular
        expressions), it uses the POSIX Compatible variant of Unicode <ulink
        url="https://www.unicode.org/reports/tr18/#Compatibility_Properties">Compatibility
        Properties</ulink>.  Behavior is efficient and stable within a
        <productname>PostgreSQL</productname> major version.  This collation is
        only available for encoding <literal>UTF8</literal>.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>C</literal> (equivalent to <literal>POSIX</literal>)</term>
      <listitem>
       <para>
        The <literal>C</literal> and <literal>POSIX</literal> collations are
        based on <quote>traditional C</quote> behavior.  They sort by byte
        values rather than natural language order, and only the ASCII letters
        <quote><literal>A</literal></quote> through
        <quote><literal>Z</literal></quote> are treated as letters.  The
        behavior is efficient and stable across all versions for a given
        database encoding, but behavior may vary between different database
        encodings.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>default</literal></term>
      <listitem>
       <para>
        The <literal>default</literal> collation selects the locale specified
        at database creation time.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Additional collations may be available depending on operating system
    support.  The efficiency and stability of these additional collations
    depend on the collation provider, the provider version, and the locale.
   </para>
  </sect3>

  <sect3 id="collation-managing-predefined">
   <title>Predefined Collations</title>

   <para>
    If the operating system provides support for using multiple locales
    within a single program (<function>newlocale</function> and related functions),
    or if support for ICU is configured,
    then when a database cluster is initialized, <command>initdb</command>
    populates the system catalog <literal>pg_collation</literal> with
    collations based on all the locales it finds in the operating
    system at the time.
   </para>

   <para>
    To inspect the currently available locales, use the query <literal>SELECT
    * FROM pg_collation</literal>, or the command <command>\dOS+</command>
    in <application>psql</application>.
   </para>

  <sect4 id="collation-managing-predefined-libc">
   <title>libc Collations</title>

   <para>
    For example, the operating system might
    provide a locale named <literal>de_DE.utf8</literal>.
    <command>initdb</command> would then create a collation named
    <literal>de_DE.utf8</literal> for encoding <literal>UTF8</literal>
    that has both <symbol>LC_COLLATE</symbol> and
    <symbol>LC_CTYPE</symbol> set to <literal>de_DE.utf8</literal>.
    It will also create a collation with the <literal>.utf8</literal>
    tag stripped off the name.  So you could also use the collation
    under the name <literal>de_DE</literal>, which is less cumbersome
    to write and makes the name less encoding-dependent.  Note that,
    nevertheless, the initial set of collation names is
    platform-dependent.
   </para>

   <para>
    The default set of collations provided by <literal>libc</literal> map
    directly to the locales installed in the operating system, which can be
    listed using the command <literal>locale -a</literal>.  In case
    a <literal>libc</literal> collation is needed that has different values
    for <symbol>LC_COLLATE</symbol> and <symbol>LC_CTYPE</symbol>, or if new
    locales are installed in the operating system after the database system
    was initialized, then a new collation may be created using
    the <xref linkend="sql-createcollation"/> command.
    New operating system locales can also be imported en masse using
    the <link linkend="functions-admin-collation"><function>pg_import_system_collations()</function></link> function.
   </para>

   <para>
    Within any particular database, only collations that use that
    database's encoding are of interest.  Other entries in
    <literal>pg_collation</literal> are ignored.  Thus, a stripped collation
    name such as <literal>de_DE</literal> can be considered unique
    within a given database even though it would not be unique globally.
    Use of the stripped collation names is recommended, since it will
    make one fewer thing you need to change if you decide to change to
    another database encoding.  Note however that the <literal>default</literal>,
    <literal>C</literal>, and <literal>POSIX</literal> collations can be used regardless of
    the database encoding.
   </para>

   <para>
    <productname>PostgreSQL</productname> considers distinct collation
    objects to be incompatible even when they have identical properties.
    Thus for example,
<programlisting>
SELECT a COLLATE "C" &lt; b COLLATE "POSIX" FROM test1;
</programlisting>
    will draw an error even though the <literal>C</literal> and <literal>POSIX</literal>
    collations have identical behaviors.  Mixing stripped and non-stripped
    collation names is therefore not recommended.
   </para>
  </sect4>

  <sect4 id="collation-managing-predefined-icu">
   <title>ICU Collations</title>

   <para>
    With ICU, it is not sensible to enumerate all possible locale names.  ICU
    uses a particular naming system for locales, but there are many more ways
    to name a locale than there are actually distinct locales.
    <command>initdb</command> uses the ICU APIs to extract a set of distinct
    locales to populate the initial set of collations.  Collations provided by
    ICU are created in the SQL environment with names in BCP 47 language tag
    format, with a <quote>private use</quote>
    extension <literal>-x-icu</literal> appended, to distinguish them from
    libc locales.
   </para>

   <para>
    Here are some example collations that might be created:

    <variablelist>
     <varlistentry id="collation-managing-predefined-icu-de-x-icu">
      <term><literal>de-x-icu</literal></term>
      <listitem>
       <para>German collation, default variant</para>
      </listitem>
     </varlistentry>

     <varlistentry id="collation-managing-predefined-icu-de-at-x-icu">
      <term><literal>de-AT-x-icu</literal></term>
      <listitem>
       <para>German collation for Austria, default variant</para>
       <para>
        (There are also, say, <literal>de-DE-x-icu</literal>
        or <literal>de-CH-x-icu</literal>, but as of this writing, they are
        equivalent to <literal>de-x-icu</literal>.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="collation-managing-predefined-icu-und-x-icu">
      <term><literal>und-x-icu</literal> (for <quote>undefined</quote>)</term>
      <listitem>
       <para>
        ICU <quote>root</quote> collation.  Use this to get a reasonable
        language-agnostic sort order.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>

   <para>
    Some (less frequently used) encodings are not supported by ICU.  When the
    database encoding is one of these, ICU collation entries
    in <literal>pg_collation</literal> are ignored.  Attempting to use one
    will draw an error along the lines of <quote>collation "de-x-icu" for
    encoding "WIN874" does not exist</quote>.
   </para>
  </sect4>
  </sect3>

  <sect3 id="collation-create">
   <title>Creating New Collation Objects</title>

   <para>
    If the standard and predefined collations are not sufficient, users can
    create their own collation objects using the SQL
    command <xref linkend="sql-createcollation"/>.
   </para>

   <para>
    The standard and predefined collations are in the
    schema <literal>pg_catalog</literal>, like all predefined objects.
    User-defined collations should be created in user schemas.  This also
    ensures that they are saved by <command>pg_dump</command>.
   </para>

   <sect4 id="collation-managing-create-libc">
    <title>libc Collations</title>

    <para>
     New libc collations can be created like this:
<programlisting>
CREATE COLLATION german (provider = libc, locale = 'de_DE');
</programlisting>
     The exact values that are acceptable for the <literal>locale</literal>
     clause in this command depend on the operating system.  On Unix-like
     systems, the command <literal>locale -a</literal> will show a list.
    </para>

    <para>
     Since the predefined libc collations already include all collations
     defined in the operating system when the database instance is
     initialized, it is not often necessary to manually create new ones.
     Reasons might be if a different naming system is desired (in which case
     see also <xref linkend="collation-copy"/>) or if the operating system has
     been upgraded to provide new locale definitions (in which case see
     also <link linkend="functions-admin-collation"><function>pg_import_system_collations()</function></link>).
    </para>
   </sect4>

   <sect4 id="collation-managing-create-icu">
    <title>ICU Collations</title>

    <para>
     ICU collations can be created like:

<programlisting>
CREATE COLLATION german (provider = icu, locale = 'de-DE');
</programlisting>

     ICU locales are specified as a BCP 47 <link
     linkend="icu-language-tag">Language Tag</link>, but can also accept most
     libc-style locale names. If possible, libc-style locale names are
     transformed into language tags.
    </para>
    <para>
     New ICU collations can customize collation behavior extensively by
     including collation attributes in the language tag. See <xref
     linkend="icu-custom-collations"/> for details and examples.
    </para>
   </sect4>
   <sect4 id="collation-copy">
   <title>Copying Collations</title>

   <para>
    The command <xref linkend="sql-createcollation"/> can also be used to
    create a new collation from an existing collation, which can be useful to
    be able to use operating-system-independent collation names in
    applications, create compatibility names, or use an ICU-provided collation
    under a more readable name.  For example:
<programlisting>
CREATE COLLATION german FROM "de_DE";
CREATE COLLATION french FROM "fr-x-icu";
</programlisting>
   </para>
   </sect4>
   </sect3>

   <sect3 id="collation-nondeterministic">
    <title>Nondeterministic Collations</title>

    <para>
     A collation is either <firstterm>deterministic</firstterm> or
     <firstterm>nondeterministic</firstterm>.  A deterministic collation uses
     deterministic comparisons, which means that it considers strings to be
     equal only if they consist of the same byte sequence.  Nondeterministic
     comparison may determine strings to be equal even if they consist of
     different bytes.  Typical situations include case-insensitive comparison,
     accent-insensitive comparison, as well as comparison of strings in
     different Unicode normal forms.  It is up to the collation provider to
     actually implement such insensitive comparisons; the deterministic flag
     only determines whether ties are to be broken using bytewise comparison.
     See also <ulink url="https://www.unicode.org/reports/tr10">Unicode Technical
     Standard 10</ulink> for more information on the terminology.
    </para>

    <para>
     To create a nondeterministic collation, specify the property
     <literal>deterministic = false</literal> to <command>CREATE
     COLLATION</command>, for example:
<programlisting>
CREATE COLLATION ndcoll (provider = icu, locale = 'und', deterministic = false);
</programlisting>
     This example would use the standard Unicode collation in a
     nondeterministic way.  In particular, this would allow strings in
     different normal forms to be compared correctly.  More interesting
     examples make use of the ICU customization facilities explained above.
     For example:
<programlisting>
CREATE COLLATION case_insensitive (provider = icu, locale = 'und-u-ks-level2', deterministic = false);
CREATE COLLATION ignore_accents (provider = icu, locale = 'und-u-ks-level1-kc-true', deterministic = false);
</programlisting>
    </para>

    <para>
     All standard and predefined collations are deterministic, all
     user-defined collations are deterministic by default.  While
     nondeterministic collations give a more <quote>correct</quote> behavior,
     especially when considering the full power of Unicode and its many
     special cases, they also have some drawbacks.  Foremost, their use leads
     to a performance penalty.  Note, in particular, that B-tree cannot use
     deduplication with indexes that use a nondeterministic collation.  Also,
     certain operations are not possible with nondeterministic collations,
     such as some pattern matching operations.  Therefore, they should be used
     only in cases where they are specifically wanted.
    </para>

    <tip>
     <para>
      To deal with text in different Unicode normalization forms, it is also
      an option to use the functions/expressions
      <function>normalize</function> and <literal>is normalized</literal> to
      preprocess or check the strings, instead of using nondeterministic
      collations.  There are different trade-offs for each approach.
     </para>
    </tip>
   </sect3>
  </sect2>

  <sect2 id="icu-custom-collations">
   <title>ICU Custom Collations</title>

   <para>
    ICU allows extensive control over collation behavior by defining new
    collations with collation settings as a part of the language tag. These
    settings can modify the collation order to suit a variety of needs. For
    instance:

<programlisting>
-- ignore differences in accents and case
CREATE COLLATION ignore_accent_case (provider = icu, deterministic = false, locale = 'und-u-ks-level1');
SELECT 'Å' = 'A' COLLATE ignore_accent_case; -- true
SELECT 'z' = 'Z' COLLATE ignore_accent_case; -- true

-- upper case letters sort before lower case.
CREATE COLLATION upper_first (provider = icu, locale = 'und-u-kf-upper');
SELECT 'B' &lt; 'b' COLLATE upper_first; -- true

-- treat digits numerically and ignore punctuation
CREATE COLLATION num_ignore_punct (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-kn');
SELECT 'id-45' &lt; 'id-123' COLLATE num_ignore_punct; -- true
SELECT 'w;x*y-z' = 'wxyz' COLLATE num_ignore_punct; -- true
</programlisting>

    Many of the available options are described in <xref
    linkend="icu-collation-settings"/>, or see <xref
    linkend="icu-external-references"/> for more details.
   </para>

   <sect3 id="icu-collation-comparison-levels">
    <title>ICU Comparison Levels</title>

    <para>
     Comparison of two strings (collation) in ICU is determined by a
     multi-level process, where textual features are grouped into
     "levels". Treatment of each level is controlled by the <link
     linkend="icu-collation-settings-table">collation settings</link>. Higher
     levels correspond to finer textual features.
    </para>

    <para>
     <xref linkend="icu-collation-levels"/> shows which textual feature
     differences are considered significant when determining equality at the
     given level. The Unicode character <literal>U+2063</literal> is an
     invisible separator, and as seen in the table, is ignored for at all
     levels of comparison less than <literal>identic</literal>.
    </para>

     <table id="icu-collation-levels">
      <title>ICU Collation Levels</title>
      <tgroup cols="8">
       <colspec colname="col1" colwidth="1*"/>
       <colspec colname="col2" colwidth="1.25*"/>
       <colspec colname="col3" colwidth="1*"/>
       <colspec colname="col4" colwidth="1*"/>
       <colspec colname="col5" colwidth="1*"/>
       <colspec colname="col6" colwidth="1*"/>
       <colspec colname="col7" colwidth="1*"/>
       <colspec colname="col8" colwidth="1*"/>

       <thead>
        <row>
         <entry>Level</entry>
         <entry>Description</entry>
         <entry><literal>'f' = 'f'</literal></entry>
         <entry><literal>'ab' = U&amp;'a\2063b'</literal></entry>
         <entry><literal>'x-y' = 'x_y'</literal></entry>
         <entry><literal>'g' = 'G'</literal></entry>
         <entry><literal>'n' = 'ñ'</literal></entry>
         <entry><literal>'y' = 'z'</literal></entry>
        </row>
       </thead>

       <tbody>
        <row>
         <entry>level1</entry>
         <entry>Base Character</entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>false</literal></entry>
        </row>
        <row>
         <entry>level2</entry>
         <entry>Accents</entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
        </row>
        <row>
         <entry>level3</entry>
         <entry>Case/Variants</entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
        </row>
        <row>
         <entry>level4</entry>
         <entry>Punctuation<footnote><para>only with
         <literal>ka-shifted</literal>; see <xref
         linkend="icu-collation-settings-table"/></para></footnote></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>true</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
        </row>
        <row>
         <entry>identic</entry>
         <entry>All</entry>
         <entry><literal>true</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry><literal>false</literal></entry>
        </row>
       </tbody>
      </tgroup>
     </table>

    <para>
     At every level, even with full normalization off, basic normalization is
     performed. For example, <literal>'á'</literal> may be composed of the
     code points <literal>U&amp;'\0061\0301'</literal> or the single code
     point <literal>U&amp;'\00E1'</literal>, and those sequences will be
     considered equal even at the <literal>identic</literal> level. To treat
     any difference in code point representation as distinct, use a collation
     created with <symbol>deterministic</symbol> set to
     <literal>true</literal>.
    </para>

    <sect4 id="icu-collation-level-examples">
     <title>Collation Level Examples</title>

<programlisting>
CREATE COLLATION level3 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level3');
CREATE COLLATION level4 (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-level4');
CREATE COLLATION identic (provider = icu, deterministic = false, locale = 'und-u-ka-shifted-ks-identic');

-- invisible separator ignored at all levels except identic
SELECT 'ab' = U&amp;'a\2063b' COLLATE level4; -- true
SELECT 'ab' = U&amp;'a\2063b' COLLATE identic; -- false

-- punctuation ignored at level3 but not at level 4
SELECT 'x-y' = 'x_y' COLLATE level3; -- true
SELECT 'x-y' = 'x_y' COLLATE level4; -- false
</programlisting>

    </sect4>
   </sect3>

   <sect3 id="icu-collation-settings">
    <title>Collation Settings for an ICU Locale</title>

    <para>
     <xref linkend="icu-collation-settings-table"/> shows the available
     collation settings, which can be used as part of a language tag to
     customize a collation.
    </para>

     <table id="icu-collation-settings-table">
      <title>ICU Collation Settings</title>
      <tgroup cols="4">
       <colspec colname="col1" colwidth="1*"/>
       <colspec colname="col2" colwidth="2*"/>
       <colspec colname="col3" colwidth="2*"/>
       <colspec colname="col4" colwidth="5*"/>

       <thead>
        <row>
         <entry>Key</entry>
         <entry>Values</entry>
         <entry>Default</entry>
         <entry>Description</entry>
        </row>
       </thead>

       <tbody>
        <row>
         <entry><literal>co</literal></entry>
         <entry><literal>emoji</literal>, <literal>phonebk</literal>, <literal>standard</literal>, <replaceable>...</replaceable></entry>
         <entry><literal>standard</literal></entry>
         <entry>
          Collation type. See <xref linkend="icu-external-references"/> for additional options and details.
         </entry>
        </row>

        <row>
         <entry><literal>ka</literal></entry>
         <entry><literal>noignore</literal>, <literal>shifted</literal></entry>
         <entry><literal>noignore</literal></entry>
         <entry>
          If set to <literal>shifted</literal>, causes some characters
          (e.g. punctuation or space) to be ignored in comparison. Key
          <literal>ks</literal> must be set to <literal>level3</literal> or
          lower to take effect. Set key <literal>kv</literal> to control which
          character classes are ignored.
         </entry>
        </row>

        <row>
         <entry><literal>kb</literal></entry>
         <entry><literal>true</literal>, <literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry>
          Backwards comparison for the level 2 differences. For example,
          locale <literal>und-u-kb</literal> sorts <literal>'àe'</literal>
          before <literal>'aé'</literal>.
         </entry>
        </row>

        <row>
         <entry><literal>kc</literal></entry>
         <entry><literal>true</literal>, <literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry>
          <para>
           Separates case into a "level 2.5" that falls between accents and
           other level 3 features.
          </para>
          <para>
           If set to <literal>true</literal> and <literal>ks</literal> is set
           to <literal>level1</literal>, will ignore accents but take case
           into account.
          </para>
         </entry>
        </row>

        <row>
         <entry><literal>kf</literal></entry>
         <entry>
          <literal>upper</literal>, <literal>lower</literal>,
          <literal>false</literal>
         </entry>
         <entry><literal>false</literal></entry>
         <entry>
          If set to <literal>upper</literal>, upper case sorts before lower
          case. If set to <literal>lower</literal>, lower case sorts before
          upper case. If set to <literal>false</literal>, the sort depends on
          the rules of the locale.
         </entry>
        </row>

        <row>
         <entry><literal>kn</literal></entry>
         <entry><literal>true</literal>, <literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry>
          If set to <literal>true</literal>, numbers within a string are
          treated as a single numeric value rather than a sequence of
          digits. For example, <literal>'id-45'</literal> sorts before
          <literal>'id-123'</literal>.
         </entry>
        </row>

        <row>
         <entry><literal>kk</literal></entry>
         <entry><literal>true</literal>, <literal>false</literal></entry>
         <entry><literal>false</literal></entry>
         <entry>
          <para>
           Enable full normalization; may affect performance. Basic
           normalization is performed even when set to
           <literal>false</literal>. Locales for languages that require full
           normalization typically enable it by default.
          </para>
          <para>
           Full normalization is important in some cases, such as when
           multiple accents are applied to a single character. For example,
           the code point sequences <literal>U&amp;'\0065\0323\0302'</literal>
           and <literal>U&amp;'\0065\0302\0323'</literal> represent
           an <literal>e</literal> with circumflex and dot-below accents
           applied in different orders. With full normalization
           on, these code point sequences are treated as equal; otherwise they
           are unequal.
          </para>
         </entry>
        </row>

        <row>
         <entry><literal>kr</literal></entry>
         <entry>
          <literal>space</literal>, <literal>punct</literal>,
          <literal>symbol</literal>, <literal>currency</literal>,
          <literal>digit</literal>, <replaceable>script-id</replaceable>
         </entry>
         <entry></entry>
         <entry>
          <para>
           Set to one or more of the valid values, or any BCP 47
           <replaceable>script-id</replaceable>, e.g. <literal>latn</literal>
           ("Latin") or <literal>grek</literal> ("Greek"). Multiple values are
           separated by "<literal>-</literal>".
          </para>
          <para>
           Redefines the ordering of classes of characters; those characters
           belonging to a class earlier in the list sort before characters
           belonging to a class later in the list. For instance, the value
           <literal>digit-currency-space</literal> (as part of a language tag
           like <literal>und-u-kr-digit-currency-space</literal>) sorts
           punctuation before digits and spaces.
          </para>
         </entry>
        </row>

        <row>
         <entry><literal>ks</literal></entry>
         <entry><literal>level1</literal>, <literal>level2</literal>, <literal>level3</literal>, <literal>level4</literal>, <literal>identic</literal></entry>
         <entry><literal>level3</literal></entry>
         <entry>
          Sensitivity (or "strength") when determining equality, with
          <literal>level1</literal> the least sensitive to differences and
          <literal>identic</literal> the most sensitive to differences. See
          <xref linkend="icu-collation-levels"/> for details.
         </entry>
        </row>

        <row>
         <entry><literal>kv</literal></entry>
         <entry>
          <literal>space</literal>, <literal>punct</literal>,
          <literal>symbol</literal>, <literal>currency</literal>
         </entry>
         <entry><literal>punct</literal></entry>
         <entry>
          Classes of characters ignored during comparison at level 3. Setting
          to a later value includes earlier values;
          e.g. <literal>symbol</literal> also includes
          <literal>punct</literal> and <literal>space</literal> in the
          characters to be ignored. Key <literal>ka</literal> must be set to
          <literal>shifted</literal> and key <literal>ks</literal> must be set
          to <literal>level3</literal> or lower to take effect.
         </entry>
        </row>
       </tbody>
      </tgroup>
     </table>

    <para>
     Defaults may depend on locale. The above table is not meant to be
     complete. See <xref linkend="icu-external-references"/> for additional
     options and details.
    </para>

    <note>
     <para>
      For many collation settings, you must create the collation with
      <option>deterministic</option> set to <literal>false</literal> for the
      setting to have the desired effect (see <xref
      linkend="collation-nondeterministic"/>). Additionally, some settings
      only take effect when the key <literal>ka</literal> is set to
      <literal>shifted</literal> (see <xref
      linkend="icu-collation-settings-table"/>).
     </para>
    </note>
   </sect3>

   <sect3 id="icu-locale-examples">
    <title>Collation Settings Examples</title>

     <variablelist>
      <varlistentry id="collation-managing-create-icu-de-u-co-phonebk-x-icu">
       <term><literal>CREATE COLLATION "de-u-co-phonebk-x-icu" (provider = icu, locale = 'de-u-co-phonebk');</literal></term>
       <listitem>
        <para>German collation with phone book collation type</para>
       </listitem>
      </varlistentry>

      <varlistentry id="collation-managing-create-icu-und-u-co-emoji-x-icu">
       <term><literal>CREATE COLLATION "und-u-co-emoji-x-icu" (provider = icu, locale = 'und-u-co-emoji');</literal></term>
       <listitem>
        <para>
         Root collation with Emoji collation type, per Unicode Technical Standard #51
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="collation-managing-create-icu-en-u-kr-grek-latn">
       <term><literal>CREATE COLLATION latinlast (provider = icu, locale = 'en-u-kr-grek-latn');</literal></term>
       <listitem>
        <para>
         Sort Greek letters before Latin ones.  (The default is Latin before Greek.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="collation-managing-create-icu-en-u-kf-upper">
       <term><literal>CREATE COLLATION upperfirst (provider = icu, locale = 'en-u-kf-upper');</literal></term>
       <listitem>
        <para>
         Sort upper-case letters before lower-case letters.  (The default is
         lower-case letters first.)
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="collation-managing-create-icu-en-u-kf-upper-kr-grek-latn">
       <term><literal>CREATE COLLATION special (provider = icu, locale = 'en-u-kf-upper-kr-grek-latn');</literal></term>
       <listitem>
        <para>
         Combines both of the above options.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
   </sect3>

   <sect3 id="icu-tailoring-rules">
    <title>ICU Tailoring Rules</title>

    <para>
     If the options provided by the collation settings shown above are not
     sufficient, the order of collation elements can be changed with tailoring
     rules, whose syntax is detailed at <ulink
     url="https://unicode-org.github.io/icu/userguide/collation/customization/"></ulink>.
    </para>

    <para>
     This small example creates a collation based on the root locale with a
     tailoring rule:
<programlisting>
<![CDATA[CREATE COLLATION custom (provider = icu, locale = 'und', rules = '&V << w <<< W');]]>
</programlisting>
     With this rule, the letter <quote>W</quote> is sorted after
     <quote>V</quote>, but is treated as a secondary difference similar to an
     accent.  Rules like this are contained in the locale definitions of some
     languages.  (Of course, if a locale definition already contains the
     desired rules, then they don't need to be specified again explicitly.)
    </para>

    <para>
     Here is a more complex example.  The following statement sets up a
     collation named <literal>ebcdic</literal> with rules to sort US-ASCII
     characters in the order of the EBCDIC encoding.

<programlisting>
<![CDATA[CREATE COLLATION ebcdic (provider = icu, locale = 'und',
rules = $$
& ' ' < '.' < '<' < '(' < '+' < \|
< '&' < '!' < '$' < '*' < ')' < ';'
< '-' < '/' < ',' < '%' < '_' < '>' < '?'
< '`' < ':' < '#' < '@' < \' < '=' < '"'
<*a-r < '~' <*s-z < '^' < '[' < ']'
< '{' <*A-I < '}' <*J-R < '\' <*S-Z <*0-9
$$);]]>

SELECT c
FROM (VALUES ('a'), ('b'), ('A'), ('B'), ('1'), ('2'), ('!'), ('^')) AS x(c)
ORDER BY c COLLATE ebcdic;
 c
---
 !
 a
 b
 ^
 A
 B
 1
 2
</programlisting>
    </para>
   </sect3>

   <sect3 id="icu-external-references">
    <title>External References for ICU</title>

    <para>
     This section (<xref linkend="icu-custom-collations"/>) is only a brief
     overview of ICU behavior and language tags. Refer to the following
     documents for technical details, additional options, and new behavior:
    </para>

    <itemizedlist>
     <listitem>
      <para>
       <ulink url="https://www.unicode.org/reports/tr35/tr35-collation.html">Unicode Technical Standard #35</ulink>
      </para>
     </listitem>
     <listitem>
      <para>
       <ulink url="https://www.rfc-editor.org/info/bcp47">BCP 47</ulink>
      </para>
     </listitem>
     <listitem>
      <para>
       <ulink url="https://github.com/unicode-org/cldr/blob/master/common/bcp47/collation.xml">CLDR repository</ulink>
      </para>
     </listitem>
     <listitem>
      <para>
       <ulink url="https://unicode-org.github.io/icu/userguide/locale/"></ulink>
      </para>
     </listitem>
     <listitem>
      <para>
       <ulink url="https://unicode-org.github.io/icu/userguide/collation/"></ulink>
      </para>
     </listitem>
    </itemizedlist>
   </sect3>
  </sect2>
 </sect1>

 <sect1 id="multibyte">
  <title>Character Set Support</title>

  <indexterm zone="multibyte"><primary>character set</primary></indexterm>

  <para>
   The character set support in <productname>PostgreSQL</productname>
   allows you to store text in a variety of character sets (also called
   encodings), including
   single-byte character sets such as the ISO 8859 series and
   multiple-byte character sets such as <acronym>EUC</acronym> (Extended Unix
   Code), UTF-8, and Mule internal code.  All supported character sets
   can be used transparently by clients, but a few are not supported
   for use within the server (that is, as a server-side encoding).
   The default character set is selected while
   initializing your <productname>PostgreSQL</productname> database
   cluster using <command>initdb</command>.  It can be overridden when you
   create a database, so you can have multiple
   databases each with a different character set.
  </para>

  <para>
   An important restriction, however, is that each database's character set
   must be compatible with the database's <envar>LC_CTYPE</envar> (character
   classification) and <envar>LC_COLLATE</envar> (string sort order) locale
   settings. For <literal>C</literal> or
   <literal>POSIX</literal> locale, any character set is allowed, but for other
   libc-provided locales there is only one character set that will work
   correctly.
   (On Windows, however, UTF-8 encoding can be used with any locale.)
   If you have ICU support configured, ICU-provided locales can be used
   with most but not all server-side encodings.
  </para>

   <sect2 id="multibyte-charset-supported">
    <title>Supported Character Sets</title>

    <para>
     <xref linkend="charset-table"/> shows the character sets available
     for use in <productname>PostgreSQL</productname>.
    </para>

     <table id="charset-table">
      <title><productname>PostgreSQL</productname> Character Sets</title>
      <tgroup cols="7">
       <colspec colname="col1" colwidth="3*"/>
       <colspec colname="col2" colwidth="2*"/>
       <colspec colname="col3" colwidth="2*"/>
       <colspec colname="col4" colwidth="1.25*"/>
       <colspec colname="col5" colwidth="1*"/>
       <colspec colname="col6" colwidth="1*"/>
       <colspec colname="col7" colwidth="2*"/>
       <thead>
        <row>
         <entry>Name</entry>
         <entry>Description</entry>
         <entry>Language</entry>
         <entry>Server?</entry>
         <entry>ICU?</entry>
         <!--
          The Bytes/Char field is populated by looking at the values returned
          by pg_wchar_table.mblen function for each encoding.
         -->
         <entry>Bytes/&zwsp;Char</entry>
         <entry>Aliases</entry>
        </row>
       </thead>
       <tbody>
        <row>
         <entry><literal>BIG5</literal></entry>
         <entry>Big Five</entry>
         <entry>Traditional Chinese</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;2</entry>
         <entry><literal>WIN950</literal>, <literal>Windows950</literal></entry>
        </row>
        <row>
         <entry><literal>EUC_CN</literal></entry>
         <entry>Extended UNIX Code-CN</entry>
         <entry>Simplified Chinese</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1&ndash;3</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>EUC_JP</literal></entry>
         <entry>Extended UNIX Code-JP</entry>
         <entry>Japanese</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1&ndash;3</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>EUC_JIS_2004</literal></entry>
         <entry>Extended UNIX Code-JP, JIS X 0213</entry>
         <entry>Japanese</entry>
         <entry>Yes</entry>
         <entry>No</entry>
         <entry>1&ndash;3</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>EUC_KR</literal></entry>
         <entry>Extended UNIX Code-KR</entry>
         <entry>Korean</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1&ndash;3</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>EUC_TW</literal></entry>
         <entry>Extended UNIX Code-TW</entry>
         <entry>Traditional Chinese, Taiwanese</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1&ndash;3</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>GB18030</literal></entry>
         <entry>National Standard</entry>
         <entry>Chinese</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;4</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>GBK</literal></entry>
         <entry>Extended National Standard</entry>
         <entry>Simplified Chinese</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;2</entry>
         <entry><literal>WIN936</literal>, <literal>Windows936</literal></entry>
        </row>
        <row>
         <entry><literal>ISO_8859_5</literal></entry>
         <entry>ISO 8859-5, <acronym>ECMA</acronym> 113</entry>
         <entry>Latin/Cyrillic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>ISO_8859_6</literal></entry>
         <entry>ISO 8859-6, <acronym>ECMA</acronym> 114</entry>
         <entry>Latin/Arabic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>ISO_8859_7</literal></entry>
         <entry>ISO 8859-7, <acronym>ECMA</acronym> 118</entry>
         <entry>Latin/Greek</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>ISO_8859_8</literal></entry>
         <entry>ISO 8859-8, <acronym>ECMA</acronym> 121</entry>
         <entry>Latin/Hebrew</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>JOHAB</literal></entry>
         <entry><acronym>JOHAB</acronym></entry>
         <entry>Korean (Hangul)</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;3</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>KOI8R</literal></entry>
         <entry><acronym>KOI</acronym>8-R</entry>
         <entry>Cyrillic (Russian)</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>KOI8</literal></entry>
        </row>
        <row>
         <entry><literal>KOI8U</literal></entry>
         <entry><acronym>KOI</acronym>8-U</entry>
         <entry>Cyrillic (Ukrainian)</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>LATIN1</literal></entry>
         <entry>ISO 8859-1, <acronym>ECMA</acronym> 94</entry>
         <entry>Western European</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO88591</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN2</literal></entry>
         <entry>ISO 8859-2, <acronym>ECMA</acronym> 94</entry>
         <entry>Central European</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO88592</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN3</literal></entry>
         <entry>ISO 8859-3, <acronym>ECMA</acronym> 94</entry>
         <entry>South European</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO88593</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN4</literal></entry>
         <entry>ISO 8859-4, <acronym>ECMA</acronym> 94</entry>
         <entry>North European</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO88594</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN5</literal></entry>
         <entry>ISO 8859-9, <acronym>ECMA</acronym> 128</entry>
         <entry>Turkish</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO88599</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN6</literal></entry>
         <entry>ISO 8859-10, <acronym>ECMA</acronym> 144</entry>
         <entry>Nordic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO885910</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN7</literal></entry>
         <entry>ISO 8859-13</entry>
         <entry>Baltic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO885913</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN8</literal></entry>
         <entry>ISO 8859-14</entry>
         <entry>Celtic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO885914</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN9</literal></entry>
         <entry>ISO 8859-15</entry>
         <entry>LATIN1 with Euro and accents</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ISO885915</literal></entry>
        </row>
        <row>
         <entry><literal>LATIN10</literal></entry>
         <entry>ISO 8859-16, <acronym>ASRO</acronym> SR 14111</entry>
         <entry>Romanian</entry>
         <entry>Yes</entry>
         <entry>No</entry>
         <entry>1</entry>
         <entry><literal>ISO885916</literal></entry>
        </row>
        <row>
         <entry><literal>MULE_INTERNAL</literal></entry>
         <entry>Mule internal code</entry>
         <entry>Multilingual Emacs</entry>
         <entry>Yes</entry>
         <entry>No</entry>
         <entry>1&ndash;4</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>SJIS</literal></entry>
         <entry>Shift JIS</entry>
         <entry>Japanese</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;2</entry>
         <entry><literal>Mskanji</literal>, <literal>ShiftJIS</literal>, <literal>WIN932</literal>, <literal>Windows932</literal></entry>
        </row>
        <row>
         <entry><literal>SHIFT_JIS_2004</literal></entry>
         <entry>Shift JIS, JIS X 0213</entry>
         <entry>Japanese</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;2</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>SQL_ASCII</literal></entry>
         <entry>unspecified (see text)</entry>
         <entry><emphasis>any</emphasis></entry>
         <entry>Yes</entry>
         <entry>No</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>UHC</literal></entry>
         <entry>Unified Hangul Code</entry>
         <entry>Korean</entry>
         <entry>No</entry>
         <entry>No</entry>
         <entry>1&ndash;2</entry>
         <entry><literal>WIN949</literal>, <literal>Windows949</literal></entry>
        </row>
        <row>
         <entry><literal>UTF8</literal></entry>
         <entry>Unicode, 8-bit</entry>
         <entry><emphasis>all</emphasis></entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1&ndash;4</entry>
         <entry><literal>Unicode</literal></entry>
        </row>
        <row>
         <entry><literal>WIN866</literal></entry>
         <entry>Windows CP866</entry>
         <entry>Cyrillic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ALT</literal></entry>
        </row>
        <row>
         <entry><literal>WIN874</literal></entry>
         <entry>Windows CP874</entry>
         <entry>Thai</entry>
         <entry>Yes</entry>
         <entry>No</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1250</literal></entry>
         <entry>Windows CP1250</entry>
         <entry>Central European</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1251</literal></entry>
         <entry>Windows CP1251</entry>
         <entry>Cyrillic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>WIN</literal></entry>
        </row>
        <row>
         <entry><literal>WIN1252</literal></entry>
         <entry>Windows CP1252</entry>
         <entry>Western European</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1253</literal></entry>
         <entry>Windows CP1253</entry>
         <entry>Greek</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1254</literal></entry>
         <entry>Windows CP1254</entry>
         <entry>Turkish</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1255</literal></entry>
         <entry>Windows CP1255</entry>
         <entry>Hebrew</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1256</literal></entry>
         <entry>Windows CP1256</entry>
         <entry>Arabic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1257</literal></entry>
         <entry>Windows CP1257</entry>
         <entry>Baltic</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry></entry>
        </row>
        <row>
         <entry><literal>WIN1258</literal></entry>
         <entry>Windows CP1258</entry>
         <entry>Vietnamese</entry>
         <entry>Yes</entry>
         <entry>Yes</entry>
         <entry>1</entry>
         <entry><literal>ABC</literal>, <literal>TCVN</literal>, <literal>TCVN5712</literal>, <literal>VSCII</literal></entry>
        </row>
       </tbody>
      </tgroup>
     </table>

     <para>
      Not all client <acronym>API</acronym>s support all the listed character sets. For example, the
      <productname>PostgreSQL</productname>
      JDBC driver does not support <literal>MULE_INTERNAL</literal>, <literal>LATIN6</literal>,
      <literal>LATIN8</literal>, and <literal>LATIN10</literal>.
     </para>

     <para>
      The <literal>SQL_ASCII</literal> setting behaves considerably differently
      from the other settings.  When the server character set is
      <literal>SQL_ASCII</literal>, the server interprets byte values 0&ndash;127
      according to the ASCII standard, while byte values 128&ndash;255 are taken
      as uninterpreted characters.  No encoding conversion will be done when
      the setting is <literal>SQL_ASCII</literal>.  Thus, this setting is not so
      much a declaration that a specific encoding is in use, as a declaration
      of ignorance about the encoding.  In most cases, if you are
      working with any non-ASCII data, it is unwise to use the
      <literal>SQL_ASCII</literal> setting because
      <productname>PostgreSQL</productname> will be unable to help you by
      converting or validating non-ASCII characters.
     </para>
    </sect2>

   <sect2 id="multibyte-setting">
    <title>Setting the Character Set</title>

    <para>
     <command>initdb</command> defines the default character set (encoding)
     for a <productname>PostgreSQL</productname> cluster. For example,

<screen>
initdb -E EUC_JP
</screen>

     sets the default character set to
     <literal>EUC_JP</literal> (Extended Unix Code for Japanese).  You
     can use <option>--encoding</option> instead of
     <option>-E</option> if you prefer longer option strings.
     If no <option>-E</option> or <option>--encoding</option> option is
     given, <command>initdb</command> attempts to determine the appropriate
     encoding to use based on the specified or default locale.
    </para>

    <para>
     You can specify a non-default encoding at database creation time,
     provided that the encoding is compatible with the selected locale:

<screen>
createdb -E EUC_KR -T template0 --lc-collate=ko_KR.euckr --lc-ctype=ko_KR.euckr korean
</screen>

     This will create a database named <literal>korean</literal> that
     uses the character set <literal>EUC_KR</literal>, and locale <literal>ko_KR</literal>.
     Another way to accomplish this is to use this SQL command:

<programlisting>
CREATE DATABASE korean WITH ENCODING 'EUC_KR' LC_COLLATE='ko_KR.euckr' LC_CTYPE='ko_KR.euckr' TEMPLATE=template0;
</programlisting>

     Notice that the above commands specify copying the <literal>template0</literal>
     database.  When copying any other database, the encoding and locale
     settings cannot be changed from those of the source database, because
     that might result in corrupt data.  For more information see
     <xref linkend="manage-ag-templatedbs"/>.
    </para>

    <para>
     The encoding for a database is stored in the system catalog
     <literal>pg_database</literal>.  You can see it by using the
     <command>psql</command> <option>-l</option> option or the
     <command>\l</command> command.

<screen>
$ <userinput>psql -l</userinput>
                                         List of databases
   Name    |  Owner   | Encoding  |  Collation  |    Ctype    |          Access Privileges
-----------+----------+-----------+-------------+-------------+-------------------------------------
 clocaledb | hlinnaka | SQL_ASCII | C           | C           |
 englishdb | hlinnaka | UTF8      | en_GB.UTF8  | en_GB.UTF8  |
 japanese  | hlinnaka | UTF8      | ja_JP.UTF8  | ja_JP.UTF8  |
 korean    | hlinnaka | EUC_KR    | ko_KR.euckr | ko_KR.euckr |
 postgres  | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  |
 template0 | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
 template1 | hlinnaka | UTF8      | fi_FI.UTF8  | fi_FI.UTF8  | {=c/hlinnaka,hlinnaka=CTc/hlinnaka}
(7 rows)
</screen>
    </para>

    <important>
     <para>
      On most modern operating systems, <productname>PostgreSQL</productname>
      can determine which character set is implied by the <envar>LC_CTYPE</envar>
      setting, and it will enforce that only the matching database encoding is
      used.  On older systems it is your responsibility to ensure that you use
      the encoding expected by the locale you have selected.  A mistake in
      this area is likely to lead to strange behavior of locale-dependent
      operations such as sorting.
     </para>

     <para>
      <productname>PostgreSQL</productname> will allow superusers to create
      databases with <literal>SQL_ASCII</literal> encoding even when
      <envar>LC_CTYPE</envar> is not <literal>C</literal> or <literal>POSIX</literal>.  As noted
      above, <literal>SQL_ASCII</literal> does not enforce that the data stored in
      the database has any particular encoding, and so this choice poses risks
      of locale-dependent misbehavior.  Using this combination of settings is
      deprecated and may someday be forbidden altogether.
     </para>
    </important>
   </sect2>

   <sect2 id="multibyte-automatic-conversion">
    <title>Automatic Character Set Conversion Between Server and Client</title>

    <para>
     <productname>PostgreSQL</productname> supports automatic character
     set conversion between server and client for many combinations of
     character sets (<xref linkend="multibyte-conversions-supported"/>
     shows which ones).
    </para>

    <para>
     To enable automatic character set conversion, you have to
     tell <productname>PostgreSQL</productname> the character set
     (encoding) you would like to use in the client. There are several
     ways to accomplish this:

     <itemizedlist>
      <listitem>
       <para>
        Using the <command>\encoding</command> command in
        <application>psql</application>.
        <command>\encoding</command> allows you to change client
        encoding on the fly. For
        example, to change the encoding to <literal>SJIS</literal>, type:

<programlisting>
\encoding SJIS
</programlisting>
       </para>
      </listitem>

      <listitem>
       <para>
        <application>libpq</application> (<xref linkend="libpq-control"/>) has functions to control the client encoding.
       </para>
      </listitem>

      <listitem>
       <para>
        Using <command>SET client_encoding TO</command>.

        Setting the client encoding can be done with this SQL command:

<programlisting>
SET CLIENT_ENCODING TO '<replaceable>value</replaceable>';
</programlisting>

        Also you can use the standard SQL syntax <literal>SET NAMES</literal>
        for this purpose:

<programlisting>
SET NAMES '<replaceable>value</replaceable>';
</programlisting>

        To query the current client encoding:

<programlisting>
SHOW client_encoding;
</programlisting>

        To return to the default encoding:

<programlisting>
RESET client_encoding;
</programlisting>
       </para>
      </listitem>

      <listitem>
       <para>
        Using <envar>PGCLIENTENCODING</envar>. If the environment variable
        <envar>PGCLIENTENCODING</envar> is defined in the client's
        environment, that client encoding is automatically selected
        when a connection to the server is made.  (This can
        subsequently be overridden using any of the other methods
        mentioned above.)
       </para>
      </listitem>

      <listitem>
      <para>
       Using the configuration variable <xref
       linkend="guc-client-encoding"/>. If the
       <varname>client_encoding</varname> variable is set, that client
       encoding is automatically selected when a connection to the
       server is made.  (This can subsequently be overridden using any
       of the other methods mentioned above.)
       </para>
      </listitem>

     </itemizedlist>
    </para>

    <para>
     If the conversion of a particular character is not possible
     &mdash; suppose you chose <literal>EUC_JP</literal> for the
     server and <literal>LATIN1</literal> for the client, and some
     Japanese characters are returned that do not have a representation in
     <literal>LATIN1</literal> &mdash; an error is reported.
    </para>

    <para>
     If the client character set is defined as <literal>SQL_ASCII</literal>,
     encoding conversion is disabled, regardless of the server's character
     set.  (However, if the server's character set is
     not <literal>SQL_ASCII</literal>, the server will still check that
     incoming data is valid for that encoding; so the net effect is as
     though the client character set were the same as the server's.)
     Just as for the server, use of <literal>SQL_ASCII</literal> is unwise
     unless you are working with all-ASCII data.
    </para>
   </sect2>

   <sect2 id="multibyte-conversions-supported">
    <title>Available Character Set Conversions</title>

    <para>
     <productname>PostgreSQL</productname> allows conversion between any
     two character sets for which a conversion function is listed in the
     <link linkend="catalog-pg-conversion"><structname>pg_conversion</structname></link>
     system catalog.  <productname>PostgreSQL</productname> comes with
     some predefined conversions, as summarized in
     <xref linkend="multibyte-translation-table"/> and shown in more
     detail in <xref linkend="builtin-conversions-table"/>.  You can
     create a new conversion using the SQL command
     <xref linkend="sql-createconversion"/>.  (To be used for automatic
     client/server conversions, a conversion must be marked
     as <quote>default</quote> for its character set pair.)
    </para>

    <table id="multibyte-translation-table">
     <title>Built-in Client/Server Character Set Conversions</title>
     <tgroup cols="2">
      <colspec colname="col1" colwidth="1*"/>
      <colspec colname="col2" colwidth="3*"/>
      <thead>
       <row>
        <entry>Server Character Set</entry>
        <entry>Available Client Character Sets</entry>
       </row>
      </thead>
      <tbody>
       <row>
        <entry><literal>BIG5</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>EUC_CN</literal></entry>
        <entry><emphasis>EUC_CN</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>EUC_JP</literal></entry>
        <entry><emphasis>EUC_JP</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>SJIS</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>EUC_JIS_2004</literal></entry>
        <entry><emphasis>EUC_JIS_2004</emphasis>,
        <literal>SHIFT_JIS_2004</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>EUC_KR</literal></entry>
        <entry><emphasis>EUC_KR</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>EUC_TW</literal></entry>
        <entry><emphasis>EUC_TW</emphasis>,
        <literal>BIG5</literal>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>GB18030</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>GBK</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>ISO_8859_5</literal></entry>
        <entry><emphasis>ISO_8859_5</emphasis>,
        <literal>KOI8R</literal>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>,
        <literal>WIN866</literal>,
        <literal>WIN1251</literal>
        </entry>
       </row>
       <row>
        <entry><literal>ISO_8859_6</literal></entry>
        <entry><emphasis>ISO_8859_6</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>ISO_8859_7</literal></entry>
        <entry><emphasis>ISO_8859_7</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>ISO_8859_8</literal></entry>
        <entry><emphasis>ISO_8859_8</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>JOHAB</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>KOI8R</literal></entry>
        <entry><emphasis>KOI8R</emphasis>,
        <literal>ISO_8859_5</literal>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>,
        <literal>WIN866</literal>,
        <literal>WIN1251</literal>
        </entry>
       </row>
       <row>
        <entry><literal>KOI8U</literal></entry>
        <entry><emphasis>KOI8U</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN1</literal></entry>
        <entry><emphasis>LATIN1</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN2</literal></entry>
        <entry><emphasis>LATIN2</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>,
        <literal>WIN1250</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN3</literal></entry>
        <entry><emphasis>LATIN3</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN4</literal></entry>
        <entry><emphasis>LATIN4</emphasis>,
        <literal>MULE_INTERNAL</literal>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN5</literal></entry>
        <entry><emphasis>LATIN5</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN6</literal></entry>
        <entry><emphasis>LATIN6</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN7</literal></entry>
        <entry><emphasis>LATIN7</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN8</literal></entry>
        <entry><emphasis>LATIN8</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN9</literal></entry>
        <entry><emphasis>LATIN9</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>LATIN10</literal></entry>
        <entry><emphasis>LATIN10</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><emphasis>MULE_INTERNAL</emphasis>,
         <literal>BIG5</literal>,
         <literal>EUC_CN</literal>,
         <literal>EUC_JP</literal>,
         <literal>EUC_KR</literal>,
         <literal>EUC_TW</literal>,
         <literal>ISO_8859_5</literal>,
         <literal>KOI8R</literal>,
         <literal>LATIN1</literal> to <literal>LATIN4</literal>,
         <literal>SJIS</literal>,
         <literal>WIN866</literal>,
         <literal>WIN1250</literal>,
         <literal>WIN1251</literal>
        </entry>
       </row>
       <row>
        <entry><literal>SJIS</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>SHIFT_JIS_2004</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>SQL_ASCII</literal></entry>
        <entry><emphasis>any (no conversion will be performed)</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>UHC</literal></entry>
        <entry><emphasis>not supported as a server encoding</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>UTF8</literal></entry>
        <entry><emphasis>all supported encodings</emphasis>
        </entry>
       </row>
       <row>
        <entry><literal>WIN866</literal></entry>
        <entry><emphasis>WIN866</emphasis>,
         <literal>ISO_8859_5</literal>,
         <literal>KOI8R</literal>,
         <literal>MULE_INTERNAL</literal>,
         <literal>UTF8</literal>,
         <literal>WIN1251</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN874</literal></entry>
        <entry><emphasis>WIN874</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1250</literal></entry>
        <entry><emphasis>WIN1250</emphasis>,
         <literal>LATIN2</literal>,
         <literal>MULE_INTERNAL</literal>,
         <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1251</literal></entry>
        <entry><emphasis>WIN1251</emphasis>,
         <literal>ISO_8859_5</literal>,
         <literal>KOI8R</literal>,
         <literal>MULE_INTERNAL</literal>,
         <literal>UTF8</literal>,
         <literal>WIN866</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1252</literal></entry>
        <entry><emphasis>WIN1252</emphasis>,
         <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1253</literal></entry>
        <entry><emphasis>WIN1253</emphasis>,
         <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1254</literal></entry>
        <entry><emphasis>WIN1254</emphasis>,
         <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1255</literal></entry>
        <entry><emphasis>WIN1255</emphasis>,
         <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1256</literal></entry>
        <entry><emphasis>WIN1256</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1257</literal></entry>
        <entry><emphasis>WIN1257</emphasis>,
         <literal>UTF8</literal>
        </entry>
       </row>
       <row>
        <entry><literal>WIN1258</literal></entry>
        <entry><emphasis>WIN1258</emphasis>,
        <literal>UTF8</literal>
        </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <table id="builtin-conversions-table">
     <title>All Built-in Character Set Conversions</title>
     <tgroup cols="3">
      <colspec colname="col1" colwidth="2*"/>
      <colspec colname="col2" colwidth="1*"/>
      <colspec colname="col3" colwidth="1*"/>
      <thead>
       <row>
        <entry>Conversion Name
         <footnote>
          <para>
           The conversion names follow a standard naming scheme: The
           official name of the source encoding with all
           non-alphanumeric characters replaced by underscores, followed
           by <literal>_to_</literal>, followed by the similarly processed
           destination encoding name.  Therefore, these names sometimes
           deviate from the customary encoding names shown in
           <xref linkend="charset-table"/>.
          </para>
         </footnote>
        </entry>
        <entry>Source Encoding</entry>
        <entry>Destination Encoding</entry>
       </row>
      </thead>

      <tbody>
       <row>
        <entry><literal>big5_to_euc_tw</literal></entry>
        <entry><literal>BIG5</literal></entry>
        <entry><literal>EUC_TW</literal></entry>
       </row>
       <row>
        <entry><literal>big5_to_mic</literal></entry>
        <entry><literal>BIG5</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>big5_to_utf8</literal></entry>
        <entry><literal>BIG5</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>euc_cn_to_mic</literal></entry>
        <entry><literal>EUC_CN</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>euc_cn_to_utf8</literal></entry>
        <entry><literal>EUC_CN</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>euc_jp_to_mic</literal></entry>
        <entry><literal>EUC_JP</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>euc_jp_to_sjis</literal></entry>
        <entry><literal>EUC_JP</literal></entry>
        <entry><literal>SJIS</literal></entry>
       </row>
       <row>
        <entry><literal>euc_jp_to_utf8</literal></entry>
        <entry><literal>EUC_JP</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>euc_kr_to_mic</literal></entry>
        <entry><literal>EUC_KR</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>euc_kr_to_utf8</literal></entry>
        <entry><literal>EUC_KR</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>euc_tw_to_big5</literal></entry>
        <entry><literal>EUC_TW</literal></entry>
        <entry><literal>BIG5</literal></entry>
       </row>
       <row>
        <entry><literal>euc_tw_to_mic</literal></entry>
        <entry><literal>EUC_TW</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>euc_tw_to_utf8</literal></entry>
        <entry><literal>EUC_TW</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>gb18030_to_utf8</literal></entry>
        <entry><literal>GB18030</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>gbk_to_utf8</literal></entry>
        <entry><literal>GBK</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_10_to_utf8</literal></entry>
        <entry><literal>LATIN6</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_13_to_utf8</literal></entry>
        <entry><literal>LATIN7</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_14_to_utf8</literal></entry>
        <entry><literal>LATIN8</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_15_to_utf8</literal></entry>
        <entry><literal>LATIN9</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_16_to_utf8</literal></entry>
        <entry><literal>LATIN10</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_1_to_mic</literal></entry>
        <entry><literal>LATIN1</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_1_to_utf8</literal></entry>
        <entry><literal>LATIN1</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_2_to_mic</literal></entry>
        <entry><literal>LATIN2</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_2_to_utf8</literal></entry>
        <entry><literal>LATIN2</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_2_to_windows_1250</literal></entry>
        <entry><literal>LATIN2</literal></entry>
        <entry><literal>WIN1250</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_3_to_mic</literal></entry>
        <entry><literal>LATIN3</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_3_to_utf8</literal></entry>
        <entry><literal>LATIN3</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_4_to_mic</literal></entry>
        <entry><literal>LATIN4</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_4_to_utf8</literal></entry>
        <entry><literal>LATIN4</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_5_to_koi8_r</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
        <entry><literal>KOI8R</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_5_to_mic</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_5_to_utf8</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_5_to_windows_1251</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
        <entry><literal>WIN1251</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_5_to_windows_866</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
        <entry><literal>WIN866</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_6_to_utf8</literal></entry>
        <entry><literal>ISO_8859_6</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_7_to_utf8</literal></entry>
        <entry><literal>ISO_8859_7</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_8_to_utf8</literal></entry>
        <entry><literal>ISO_8859_8</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>iso_8859_9_to_utf8</literal></entry>
        <entry><literal>LATIN5</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>johab_to_utf8</literal></entry>
        <entry><literal>JOHAB</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>koi8_r_to_iso_8859_5</literal></entry>
        <entry><literal>KOI8R</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
       </row>
       <row>
        <entry><literal>koi8_r_to_mic</literal></entry>
        <entry><literal>KOI8R</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>koi8_r_to_utf8</literal></entry>
        <entry><literal>KOI8R</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>koi8_r_to_windows_1251</literal></entry>
        <entry><literal>KOI8R</literal></entry>
        <entry><literal>WIN1251</literal></entry>
       </row>
       <row>
        <entry><literal>koi8_r_to_windows_866</literal></entry>
        <entry><literal>KOI8R</literal></entry>
        <entry><literal>WIN866</literal></entry>
       </row>
       <row>
        <entry><literal>koi8_u_to_utf8</literal></entry>
        <entry><literal>KOI8U</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_big5</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>BIG5</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_euc_cn</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>EUC_CN</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_euc_jp</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>EUC_JP</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_euc_kr</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>EUC_KR</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_euc_tw</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>EUC_TW</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_iso_8859_1</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>LATIN1</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_iso_8859_2</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>LATIN2</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_iso_8859_3</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>LATIN3</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_iso_8859_4</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>LATIN4</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_iso_8859_5</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_koi8_r</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>KOI8R</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_sjis</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>SJIS</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_windows_1250</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>WIN1250</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_windows_1251</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>WIN1251</literal></entry>
       </row>
       <row>
        <entry><literal>mic_to_windows_866</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
        <entry><literal>WIN866</literal></entry>
       </row>
       <row>
        <entry><literal>sjis_to_euc_jp</literal></entry>
        <entry><literal>SJIS</literal></entry>
        <entry><literal>EUC_JP</literal></entry>
       </row>
       <row>
        <entry><literal>sjis_to_mic</literal></entry>
        <entry><literal>SJIS</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>sjis_to_utf8</literal></entry>
        <entry><literal>SJIS</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1258_to_utf8</literal></entry>
        <entry><literal>WIN1258</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>uhc_to_utf8</literal></entry>
        <entry><literal>UHC</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_big5</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>BIG5</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_euc_cn</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>EUC_CN</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_euc_jp</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>EUC_JP</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_euc_kr</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>EUC_KR</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_euc_tw</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>EUC_TW</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_gb18030</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>GB18030</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_gbk</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>GBK</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_1</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN1</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_10</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN6</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_13</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN7</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_14</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN8</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_15</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN9</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_16</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN10</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_2</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN2</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_3</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN3</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_4</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN4</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_5</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_6</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>ISO_8859_6</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_7</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>ISO_8859_7</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_8</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>ISO_8859_8</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_iso_8859_9</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>LATIN5</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_johab</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>JOHAB</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_koi8_r</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>KOI8R</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_koi8_u</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>KOI8U</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_sjis</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>SJIS</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1258</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1258</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_uhc</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>UHC</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1250</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1250</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1251</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1251</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1252</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1252</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1253</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1253</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1254</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1254</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1255</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1255</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1256</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1256</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_1257</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN1257</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_866</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN866</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_windows_874</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>WIN874</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1250_to_iso_8859_2</literal></entry>
        <entry><literal>WIN1250</literal></entry>
        <entry><literal>LATIN2</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1250_to_mic</literal></entry>
        <entry><literal>WIN1250</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1250_to_utf8</literal></entry>
        <entry><literal>WIN1250</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1251_to_iso_8859_5</literal></entry>
        <entry><literal>WIN1251</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1251_to_koi8_r</literal></entry>
        <entry><literal>WIN1251</literal></entry>
        <entry><literal>KOI8R</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1251_to_mic</literal></entry>
        <entry><literal>WIN1251</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1251_to_utf8</literal></entry>
        <entry><literal>WIN1251</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1251_to_windows_866</literal></entry>
        <entry><literal>WIN1251</literal></entry>
        <entry><literal>WIN866</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1252_to_utf8</literal></entry>
        <entry><literal>WIN1252</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>windows_1256_to_utf8</literal></entry>
        <entry><literal>WIN1256</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>windows_866_to_iso_8859_5</literal></entry>
        <entry><literal>WIN866</literal></entry>
        <entry><literal>ISO_8859_5</literal></entry>
       </row>
       <row>
        <entry><literal>windows_866_to_koi8_r</literal></entry>
        <entry><literal>WIN866</literal></entry>
        <entry><literal>KOI8R</literal></entry>
       </row>
       <row>
        <entry><literal>windows_866_to_mic</literal></entry>
        <entry><literal>WIN866</literal></entry>
        <entry><literal>MULE_INTERNAL</literal></entry>
       </row>
       <row>
        <entry><literal>windows_866_to_utf8</literal></entry>
        <entry><literal>WIN866</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>windows_866_to_windows_1251</literal></entry>
        <entry><literal>WIN866</literal></entry>
        <entry><literal>WIN</literal></entry>
       </row>
       <row>
        <entry><literal>windows_874_to_utf8</literal></entry>
        <entry><literal>WIN874</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>euc_jis_2004_to_utf8</literal></entry>
        <entry><literal>EUC_JIS_2004</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_euc_jis_2004</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>EUC_JIS_2004</literal></entry>
       </row>
       <row>
        <entry><literal>shift_jis_2004_to_utf8</literal></entry>
        <entry><literal>SHIFT_JIS_2004</literal></entry>
        <entry><literal>UTF8</literal></entry>
       </row>
       <row>
        <entry><literal>utf8_to_shift_jis_2004</literal></entry>
        <entry><literal>UTF8</literal></entry>
        <entry><literal>SHIFT_JIS_2004</literal></entry>
       </row>
       <row>
        <entry><literal>euc_jis_2004_to_shift_jis_2004</literal></entry>
        <entry><literal>EUC_JIS_2004</literal></entry>
        <entry><literal>SHIFT_JIS_2004</literal></entry>
       </row>
       <row>
        <entry><literal>shift_jis_2004_to_euc_jis_2004</literal></entry>
        <entry><literal>SHIFT_JIS_2004</literal></entry>
        <entry><literal>EUC_JIS_2004</literal></entry>
       </row>
      </tbody>
     </tgroup>
    </table>
   </sect2>

   <sect2 id="multibyte-further-reading">
    <title>Further Reading</title>

    <para>
     These are good sources to start learning about various kinds of encoding
     systems.

     <variablelist>
      <varlistentry>
       <term><citetitle>CJKV Information Processing: Chinese, Japanese, Korean &amp; Vietnamese Computing</citetitle></term>

       <listitem>
        <para>
         Contains detailed explanations of <literal>EUC_JP</literal>,
         <literal>EUC_CN</literal>, <literal>EUC_KR</literal>,
         <literal>EUC_TW</literal>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><ulink url="https://www.unicode.org/"></ulink></term>

       <listitem>
        <para>
         The web site of the Unicode Consortium.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><ulink url="https://datatracker.ietf.org/doc/html/rfc3629">RFC 3629</ulink></term>

       <listitem>
        <para>
         <acronym>UTF</acronym>-8 (8-bit UCS/Unicode Transformation
         Format) is defined here.
        </para>
       </listitem>
      </varlistentry>
     </variablelist>
    </para>
   </sect2>

  </sect1>

</chapter>