doc/src/sgml/ref/lock.sgml

   1 <!--
   2 $PostgreSQL$
   3 PostgreSQL documentation
   4 -->
   5
   6 <refentry id="SQL-LOCK">
   7  <refmeta>
   8   <refentrytitle id="sql-lock-title">LOCK</refentrytitle>
   9   <manvolnum>7</manvolnum>
  10   <refmiscinfo>SQL - Language Statements</refmiscinfo>
  11  </refmeta>
  12
  13  <refnamediv>
  14   <refname>LOCK</refname>
  15   <refpurpose>lock a table</refpurpose>
  16  </refnamediv>
  17
  18  <indexterm zone="sql-lock">
  19   <primary>LOCK</primary>
  20  </indexterm>
  21
  22  <refsynopsisdiv>
  23 <synopsis>
  24 LOCK [ TABLE ] [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [, ...] [ IN <replaceable class="PARAMETER">lockmode</replaceable> MODE ] [ NOWAIT ]
  25
  26 where <replaceable class="PARAMETER">lockmode</replaceable> is one of:
  27
  28     ACCESS SHARE | ROW SHARE | ROW EXCLUSIVE | SHARE UPDATE EXCLUSIVE
  29     | SHARE | SHARE ROW EXCLUSIVE | EXCLUSIVE | ACCESS EXCLUSIVE
  30 </synopsis>
  31  </refsynopsisdiv>
  32
  33  <refsect1>
  34   <title>Description</title>
  35
  36   <para>
  37    <command>LOCK TABLE</command> obtains a table-level lock, waiting
  38    if necessary for any conflicting locks to be released.  If
  39    <literal>NOWAIT</literal> is specified, <command>LOCK
  40    TABLE</command> does not wait to acquire the desired lock: if it
  41    cannot be acquired immediately, the command is aborted and an
  42    error is emitted.  Once obtained, the lock is held for the
  43    remainder of the current transaction.  (There is no <command>UNLOCK
  44    TABLE</command> command; locks are always released at transaction
  45    end.)
  46   </para>
  47
  48   <para>
  49    When acquiring locks automatically for commands that reference
  50    tables, <productname>PostgreSQL</productname> always uses the least
  51    restrictive lock mode possible. <command>LOCK TABLE</command>
  52    provides for cases when you might need more restrictive locking.
  53    For example, suppose an application runs a transaction at the
  54    Read Committed isolation level and needs to ensure that data in a
  55    table remains stable for the duration of the transaction. To
  56    achieve this you could obtain <literal>SHARE</> lock mode over the
  57    table before querying. This will prevent concurrent data changes
  58    and ensure subsequent reads of the table see a stable view of
  59    committed data, because <literal>SHARE</> lock mode conflicts with
  60    the <literal>ROW EXCLUSIVE</> lock acquired by writers, and your
  61    <command>LOCK TABLE <replaceable
  62    class="PARAMETER">name</replaceable> IN SHARE MODE</command>
  63    statement will wait until any concurrent holders of <literal>ROW
  64    EXCLUSIVE</literal> mode locks commit or roll back. Thus, once you
  65    obtain the lock, there are no uncommitted writes outstanding;
  66    furthermore none can begin until you release the lock.
  67   </para>
  68
  69   <para>
  70    To achieve a similar effect when running a transaction at the Serializable
  71    isolation level, you have to execute the <command>LOCK TABLE</> statement
  72    before executing any <command>SELECT</> or data modification statement.
  73    A serializable transaction's view of data will be frozen when its first
  74    <command>SELECT</> or data modification statement begins.  A <command>LOCK
  75    TABLE</> later in the transaction will still prevent concurrent writes
  76    &mdash; but it won't ensure that what the transaction reads corresponds to
  77    the latest committed values.
  78   </para>
  79
  80   <para>
  81    If a transaction of this sort is going to change the data in the
  82    table, then it should use <literal>SHARE ROW EXCLUSIVE</> lock mode
  83    instead of <literal>SHARE</> mode.  This ensures that only one
  84    transaction of this type runs at a time.  Without this, a deadlock
  85    is possible: two transactions might both acquire <literal>SHARE</>
  86    mode, and then be unable to also acquire <literal>ROW EXCLUSIVE</>
  87    mode to actually perform their updates.  (Note that a transaction's
  88    own locks never conflict, so a transaction can acquire <literal>ROW
  89    EXCLUSIVE</> mode when it holds <literal>SHARE</> mode &mdash; but not
  90    if anyone else holds <literal>SHARE</> mode.)  To avoid deadlocks,
  91    make sure all transactions acquire locks on the same objects in the
  92    same order, and if multiple lock modes are involved for a single
  93    object, then transactions should always acquire the most
  94    restrictive mode first.
  95   </para>
  96
  97   <para>
  98    More information about the lock modes and locking strategies can be
  99    found in <xref linkend="explicit-locking">.
 100   </para>
 101  </refsect1>
 102
 103  <refsect1>
 104   <title>Parameters</title>
 105
 106   <variablelist>
 107    <varlistentry>
 108     <term><replaceable class="PARAMETER">name</replaceable></term>
 109     <listitem>
 110      <para>
 111       The name (optionally schema-qualified) of an existing table to
 112       lock.  If <literal>ONLY</> is specified, only that table is
 113       locked.  If <literal>ONLY</> is not specified, the table and all
 114       its descendant tables (if any) are locked.
 115      </para>
 116
 117      <para>
 118       The command <literal>LOCK TABLE a, b;</> is equivalent to
 119       <literal>LOCK TABLE a; LOCK TABLE b;</>. The tables are locked
 120       one-by-one in the order specified in the <command>LOCK
 121       TABLE</command> command.
 122      </para>
 123     </listitem>
 124    </varlistentry>
 125
 126    <varlistentry>
 127     <term><replaceable class="parameter">lockmode</replaceable></term>
 128     <listitem>
 129      <para>
 130       The lock mode specifies which locks this lock conflicts with.
 131       Lock modes are described in <xref linkend="explicit-locking">.
 132      </para>
 133
 134      <para>
 135       If no lock mode is specified, then <literal>ACCESS
 136       EXCLUSIVE</literal>, the most restrictive mode, is used.
 137      </para>
 138     </listitem>
 139    </varlistentry>
 140
 141    <varlistentry>
 142     <term><literal>NOWAIT</literal></term>
 143     <listitem>
 144      <para>
 145       Specifies that <command>LOCK TABLE</command> should not wait for
 146       any conflicting locks to be released: if the specified lock(s)
 147       cannot be acquired immediately without waiting, the transaction
 148       is aborted.
 149      </para>
 150     </listitem>
 151    </varlistentry>
 152   </variablelist>
 153  </refsect1>
 154
 155  <refsect1>
 156   <title>Notes</title>
 157
 158    <para>
 159     <literal>LOCK TABLE ... IN ACCESS SHARE MODE</> requires <literal>SELECT</>
 160     privileges on the target table.  All other forms of <command>LOCK</>
 161     require at least one of <literal>UPDATE</>, <literal>DELETE</>, or
 162     <literal>TRUNCATE</> privileges.
 163    </para>
 164
 165    <para>
 166     <command>LOCK TABLE</> is useless outside a transaction block: the lock
 167     would remain held only to the completion of the statement.  Therefore
 168     <productname>PostgreSQL</productname> reports an error if <command>LOCK</>
 169     is used outside a transaction block.
 170     Use
 171     <xref linkend="sql-begin" endterm="sql-begin-title"> and
 172     <xref linkend="sql-commit" endterm="sql-commit-title">
 173     (or <xref linkend="sql-rollback" endterm="sql-rollback-title">)
 174     to define a transaction block.
 175    </para>
 176
 177   <para>
 178    <command>LOCK TABLE</> only deals with table-level locks, and so
 179    the mode names involving <literal>ROW</> are all misnomers.  These
 180    mode names should generally be read as indicating the intention of
 181    the user to acquire row-level locks within the locked table.  Also,
 182    <literal>ROW EXCLUSIVE</> mode is a sharable table lock.  Keep in
 183    mind that all the lock modes have identical semantics so far as
 184    <command>LOCK TABLE</> is concerned, differing only in the rules
 185    about which modes conflict with which. For information on how to
 186    acquire an actual row-level lock, see <xref linkend="locking-rows">
 187    and the <xref linkend="sql-for-update-share"
 188    endterm="sql-for-update-share-title"> in the <command>SELECT</command>
 189    reference documentation.
 190   </para>
 191  </refsect1>
 192
 193  <refsect1>
 194   <title>Examples</title>
 195
 196   <para>
 197    Obtain a <literal>SHARE</> lock on a primary key table when going to perform
 198    inserts into a foreign key table:
 199
 200 <programlisting>
 201 BEGIN WORK;
 202 LOCK TABLE films IN SHARE MODE;
 203 SELECT id FROM films
 204     WHERE name = 'Star Wars: Episode I - The Phantom Menace';
 205 -- Do ROLLBACK if record was not returned
 206 INSERT INTO films_user_comments VALUES
 207     (_id_, 'GREAT! I was waiting for it for so long!');
 208 COMMIT WORK;
 209 </programlisting>
 210   </para>
 211
 212   <para>
 213    Take a <literal>SHARE ROW EXCLUSIVE</> lock on a primary key table when going to perform
 214    a delete operation:
 215
 216 <programlisting>
 217 BEGIN WORK;
 218 LOCK TABLE films IN SHARE ROW EXCLUSIVE MODE;
 219 DELETE FROM films_user_comments WHERE id IN
 220     (SELECT id FROM films WHERE rating &lt; 5);
 221 DELETE FROM films WHERE rating &lt; 5;
 222 COMMIT WORK;
 223 </programlisting>
 224   </para>
 225  </refsect1>
 226
 227  <refsect1>
 228   <title>Compatibility</title>
 229
 230   <para>
 231    There is no <command>LOCK TABLE</command> in the SQL standard,
 232    which instead uses <command>SET TRANSACTION</command> to specify
 233    concurrency levels on transactions.  <productname>PostgreSQL</productname> supports that too;
 234    see <xref linkend="SQL-SET-TRANSACTION"
 235    endterm="SQL-SET-TRANSACTION-TITLE"> for details.
 236   </para>
 237
 238   <para>
 239    Except for <literal>ACCESS SHARE</>, <literal>ACCESS EXCLUSIVE</>,
 240    and <literal>SHARE UPDATE EXCLUSIVE</> lock modes, the
 241    <productname>PostgreSQL</productname> lock modes and the
 242    <command>LOCK TABLE</command> syntax are compatible with those
 243    present in <productname>Oracle</productname>.
 244   </para>
 245  </refsect1>
 246 </refentry>