doc/src/sgml/ref/alter_table.sgml

   1 <!--
   2 $PostgreSQL$
   3 PostgreSQL documentation
   4 -->
   5
   6 <refentry id="SQL-ALTERTABLE">
   7  <refmeta>
   8   <refentrytitle id="sql-altertable-title">ALTER TABLE</refentrytitle>
   9   <manvolnum>7</manvolnum>
  10   <refmiscinfo>SQL - Language Statements</refmiscinfo>
  11  </refmeta>
  12
  13  <refnamediv>
  14   <refname>ALTER TABLE</refname>
  15   <refpurpose>change the definition of a table</refpurpose>
  16  </refnamediv>
  17
  18  <indexterm zone="sql-altertable">
  19   <primary>ALTER TABLE</primary>
  20  </indexterm>
  21
  22  <refsynopsisdiv>
  23 <synopsis>
  24 ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
  25     <replaceable class="PARAMETER">action</replaceable> [, ... ]
  26 ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
  27     RENAME [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> TO <replaceable class="PARAMETER">new_column</replaceable>
  28 ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
  29     RENAME TO <replaceable class="PARAMETER">new_name</replaceable>
  30 ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
  31     SET SCHEMA <replaceable class="PARAMETER">new_schema</replaceable>
  32
  33 where <replaceable class="PARAMETER">action</replaceable> is one of:
  34
  35     ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">type</replaceable> [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
  36     DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
  37     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">type</replaceable> [ USING <replaceable class="PARAMETER">expression</replaceable> ]
  38     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
  39     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> DROP DEFAULT
  40     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL
  41     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>
  42     ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
  43     ADD <replaceable class="PARAMETER">table_constraint</replaceable>
  44     DROP CONSTRAINT <replaceable class="PARAMETER">constraint_name</replaceable> [ RESTRICT | CASCADE ]
  45     DISABLE TRIGGER [ <replaceable class="PARAMETER">trigger_name</replaceable> | ALL | USER ]
  46     ENABLE TRIGGER [ <replaceable class="PARAMETER">trigger_name</replaceable> | ALL | USER ]
  47     ENABLE REPLICA TRIGGER <replaceable class="PARAMETER">trigger_name</replaceable>
  48     ENABLE ALWAYS TRIGGER <replaceable class="PARAMETER">trigger_name</replaceable>
  49     DISABLE RULE <replaceable class="PARAMETER">rewrite_rule_name</replaceable>
  50     ENABLE RULE <replaceable class="PARAMETER">rewrite_rule_name</replaceable>
  51     ENABLE REPLICA RULE <replaceable class="PARAMETER">rewrite_rule_name</replaceable>
  52     ENABLE ALWAYS RULE <replaceable class="PARAMETER">rewrite_rule_name</replaceable>
  53     CLUSTER ON <replaceable class="PARAMETER">index_name</replaceable>
  54     SET WITHOUT CLUSTER
  55     SET WITH OIDS
  56     SET WITHOUT OIDS
  57     SET ( <replaceable class="PARAMETER">storage_parameter</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] )
  58     RESET ( <replaceable class="PARAMETER">storage_parameter</replaceable> [, ... ] )
  59     INHERIT <replaceable class="PARAMETER">parent_table</replaceable>
  60     NO INHERIT <replaceable class="PARAMETER">parent_table</replaceable>
  61     OWNER TO <replaceable class="PARAMETER">new_owner</replaceable>
  62     SET TABLESPACE <replaceable class="PARAMETER">new_tablespace</replaceable>
  63 </synopsis>
  64  </refsynopsisdiv>
  65
  66  <refsect1>
  67   <title>Description</title>
  68
  69   <para>
  70    <command>ALTER TABLE</command> changes the definition of an existing table.
  71    There are several subforms:
  72
  73   <variablelist>
  74    <varlistentry>
  75     <term><literal>ADD COLUMN</literal></term>
  76     <listitem>
  77      <para>
  78       This form adds a new column to the table, using the same syntax as
  79       <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-TITLE">.
  80      </para>
  81     </listitem>
  82    </varlistentry>
  83
  84    <varlistentry>
  85     <term><literal>DROP COLUMN</literal></term>
  86     <listitem>
  87      <para>
  88       This form drops a column from a table.  Indexes and
  89       table constraints involving the column will be automatically
  90       dropped as well.  You will need to say <literal>CASCADE</> if
  91       anything outside the table depends on the column, for example,
  92       foreign key references or views.
  93      </para>
  94     </listitem>
  95    </varlistentry>
  96
  97    <varlistentry>
  98     <term><literal>SET DATA TYPE</literal></term>
  99     <listitem>
 100      <para>
 101       This form changes the type of a column of a table. Indexes and
 102       simple table constraints involving the column will be automatically
 103       converted to use the new column type by reparsing the originally
 104       supplied expression.  The optional <literal>USING</literal>
 105       clause specifies how to compute the new column value from the old;
 106       if omitted, the default conversion is the same as an assignment
 107       cast from old data type to new.  A  <literal>USING</literal>
 108       clause must be provided if there is no implicit or assignment
 109       cast from old to new type.
 110      </para>
 111     </listitem>
 112    </varlistentry>
 113
 114    <varlistentry>
 115     <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
 116     <listitem>
 117      <para>
 118       These forms set or remove the default value for a column.
 119       The default values only apply to subsequent <command>INSERT</command>
 120       commands; they do not cause rows already in the table to change.
 121       Defaults can also be created for views, in which case they are
 122       inserted into <command>INSERT</> statements on the view before
 123       the view's <literal>ON INSERT</literal> rule is applied.
 124      </para>
 125     </listitem>
 126    </varlistentry>
 127
 128    <varlistentry>
 129     <term><literal>SET</literal>/<literal>DROP NOT NULL</literal></term>
 130     <listitem>
 131      <para>
 132       These forms change whether a column is marked to allow null
 133       values or to reject null values.  You can only use <literal>SET
 134       NOT NULL</> when the column contains no null values.
 135      </para>
 136     </listitem>
 137    </varlistentry>
 138
 139    <varlistentry>
 140     <term><literal>SET STATISTICS</literal></term>
 141     <listitem>
 142      <para>
 143       This form
 144       sets the per-column statistics-gathering target for subsequent
 145       <xref linkend="sql-analyze" endterm="sql-analyze-title"> operations.
 146       The target can be set in the range 0 to 10000; alternatively, set it
 147       to -1 to revert to using the system default statistics
 148       target (<xref linkend="guc-default-statistics-target">).
 149       For more information on the use of statistics by the
 150       <productname>PostgreSQL</productname> query planner, refer to
 151       <xref linkend="planner-stats">.
 152      </para>
 153     </listitem>
 154    </varlistentry>
 155
 156    <varlistentry>
 157     <indexterm>
 158      <primary>TOAST</primary>
 159      <secondary>per-column storage settings</secondary>
 160     </indexterm>
 161
 162     <term><literal>SET STORAGE</literal></term>
 163     <listitem>
 164      <para>
 165       This form sets the storage mode for a column. This controls whether this
 166       column is held inline or in a supplementary table, and whether the data
 167       should be compressed or not. <literal>PLAIN</literal> must be used
 168       for fixed-length values such as <type>integer</type> and is
 169       inline, uncompressed. <literal>MAIN</literal> is for inline,
 170       compressible data. <literal>EXTERNAL</literal> is for external,
 171       uncompressed data, and <literal>EXTENDED</literal> is for external,
 172       compressed data.  <literal>EXTENDED</literal> is the default for most
 173       data types that support non-<literal>PLAIN</literal> storage.
 174       Use of <literal>EXTERNAL</literal> will
 175       make substring operations on <type>text</type> and <type>bytea</type>
 176       columns faster, at the penalty of increased storage space.  Note that
 177       <literal>SET STORAGE</> doesn't itself change anything in the table,
 178       it just sets the strategy to be pursued during future table updates.
 179       See <xref linkend="storage-toast"> for more information.
 180      </para>
 181     </listitem>
 182    </varlistentry>
 183
 184    <varlistentry>
 185     <term><literal>ADD <replaceable class="PARAMETER">table_constraint</replaceable></literal></term>
 186     <listitem>
 187      <para>
 188       This form adds a new constraint to a table using the same syntax as
 189       <xref linkend="SQL-CREATETABLE" endterm="SQL-CREATETABLE-TITLE">.
 190      </para>
 191     </listitem>
 192    </varlistentry>
 193
 194    <varlistentry>
 195     <term><literal>DROP CONSTRAINT</literal></term>
 196     <listitem>
 197      <para>
 198       This form drops the specified constraint on a table.
 199      </para>
 200     </listitem>
 201    </varlistentry>
 202
 203    <varlistentry>
 204     <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] TRIGGER</literal></term>
 205     <listitem>
 206      <para>
 207       These forms configure the firing of trigger(s) belonging to the table.
 208       A disabled trigger is still known to the system, but is not executed
 209       when its triggering event occurs.  For a deferred trigger, the enable
 210       status is checked when the event occurs, not when the trigger function
 211       is actually executed.  One can disable or enable a single
 212       trigger specified by name, or all triggers on the table, or only
 213       user triggers (this option excludes triggers that are used to implement
 214       foreign key constraints).  Disabling or enabling constraint triggers
 215       requires superuser privileges; it should be done with caution since
 216       of course the integrity of the constraint cannot be guaranteed if the
 217       triggers are not executed.
 218       The trigger firing mechanism is also affected by the configuration
 219       variable <xref linkend="guc-session-replication-role">. Simply enabled
 220       triggers will fire when the replication role is <quote>origin</>
 221       (the default) or <quote>local</>. Triggers configured as <literal>ENABLE
 222       REPLICA</literal> will only fire if the session is in <quote>replica</>
 223       mode, and triggers configured as <literal>ENABLE ALWAYS</literal> will
 224       fire regardless of the current replication mode.
 225      </para>
 226     </listitem>
 227    </varlistentry>
 228
 229    <varlistentry>
 230     <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] RULE</literal></term>
 231     <listitem>
 232      <para>
 233       These forms configure the firing of rewrite rules belonging to the table.
 234       A disabled rule is still known to the system, but is not applied
 235       during query rewriting. The semantics are as for disabled/enabled
 236       triggers. This configuration is ignored for <literal>ON SELECT</literal> rules, which
 237       are always applied in order to keep views working even if the current
 238       session is in a non-default replication role.
 239      </para>
 240     </listitem>
 241    </varlistentry>
 242
 243    <varlistentry>
 244     <term><literal>CLUSTER</literal></term>
 245     <listitem>
 246      <para>
 247       This form selects the default index for future
 248       <xref linkend="SQL-CLUSTER" endterm="sql-cluster-title">
 249       operations.  It does not actually re-cluster the table.
 250      </para>
 251     </listitem>
 252    </varlistentry>
 253
 254    <varlistentry>
 255     <term><literal>SET WITHOUT CLUSTER</literal></term>
 256     <listitem>
 257      <para>
 258       This form removes the most recently used
 259       <xref linkend="SQL-CLUSTER" endterm="sql-cluster-title">
 260       index specification from the table.  This affects
 261       future cluster operations that don't specify an index.
 262      </para>
 263     </listitem>
 264    </varlistentry>
 265
 266    <varlistentry>
 267     <term><literal>SET WITH OIDS</literal></term>
 268     <listitem>
 269      <para>
 270       This form adds an <literal>oid</literal> system column to the
 271       table (see <xref linkend="ddl-system-columns">).
 272       It does nothing if the table already has OIDs.
 273      </para>
 274
 275      <para>
 276       Note that this is not equivalent to <literal>ADD COLUMN oid oid</>;
 277       that would add a normal column that happened to be named
 278       <literal>oid</>, not a system column.
 279      </para>
 280     </listitem>
 281    </varlistentry>
 282
 283    <varlistentry>
 284     <term><literal>SET WITHOUT OIDS</literal></term>
 285     <listitem>
 286      <para>
 287       This form removes the <literal>oid</literal> system column from the
 288       table.  This is exactly equivalent to
 289       <literal>DROP COLUMN oid RESTRICT</literal>,
 290       except that it will not complain if there is already no
 291       <literal>oid</literal> column.
 292      </para>
 293     </listitem>
 294    </varlistentry>
 295
 296    <varlistentry>
 297     <term><literal>SET ( <replaceable class="PARAMETER">storage_parameter</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] )</literal></term>
 298     <listitem>
 299      <para>
 300       This form changes one or more storage parameters for the table.  See
 301       <xref linkend="SQL-CREATETABLE-storage-parameters"
 302       endterm="sql-createtable-storage-parameters-title">
 303       for details on the available parameters.  Note that the table contents
 304       will not be modified immediately by this command; depending on the
 305       parameter you might need to rewrite the table to get the desired effects.
 306       That can be done with <xref linkend="SQL-CLUSTER"
 307       endterm="sql-cluster-title"> or one of the forms of <command>ALTER
 308       TABLE</> that forces a table rewrite.
 309      </para>
 310
 311      <note>
 312       <para>
 313        While <command>CREATE TABLE</> allows <literal>OIDS</> to be specified
 314        in the <literal>WITH (<replaceable
 315        class="PARAMETER">storage_parameter</>)</literal> syntax,
 316        <command>ALTER TABLE</> does not treat <literal>OIDS</> as a
 317        storage parameter.  Instead use the <literal>SET WITH OIDS</>
 318        and <literal>SET WITHOUT OIDS</> forms to change OID status.
 319       </para>
 320      </note>
 321     </listitem>
 322    </varlistentry>
 323
 324    <varlistentry>
 325     <term><literal>RESET ( <replaceable class="PARAMETER">storage_parameter</replaceable> [, ... ] )</literal></term>
 326     <listitem>
 327      <para>
 328       This form resets one or more storage parameters to their
 329       defaults.  As with <literal>SET</>, a table rewrite might be
 330       needed to update the table entirely.
 331      </para>
 332     </listitem>
 333    </varlistentry>
 334
 335    <varlistentry>
 336     <term><literal>INHERIT <replaceable class="PARAMETER">parent_table</replaceable></literal></term>
 337     <listitem>
 338      <para>
 339       This form adds the target table as a new child of the specified parent
 340       table.  Subsequently, queries against the parent will include records
 341       of the target table.  To be added as a child, the target table must
 342       already contain all the same columns as the parent (it could have
 343       additional columns, too).  The columns must have matching data types,
 344       and if they have <literal>NOT NULL</literal> constraints in the parent
 345       then they must also have <literal>NOT NULL</literal> constraints in the
 346       child.
 347      </para>
 348
 349      <para>
 350       There must also be matching child-table constraints for all
 351       <literal>CHECK</literal> constraints of the parent. Currently
 352       <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and
 353       <literal>FOREIGN KEY</literal> constraints are not considered, but
 354       this might change in the future.
 355      </para>
 356     </listitem>
 357    </varlistentry>
 358
 359    <varlistentry>
 360     <term><literal>NO INHERIT <replaceable class="PARAMETER">parent_table</replaceable></literal></term>
 361     <listitem>
 362      <para>
 363       This form removes the target table from the list of children of the
 364       specified parent table.
 365       Queries against the parent table will no longer include records drawn
 366       from the target table.
 367      </para>
 368     </listitem>
 369    </varlistentry>
 370
 371    <varlistentry>
 372     <term><literal>OWNER</literal></term>
 373     <listitem>
 374      <para>
 375       This form changes the owner of the table, sequence, or view to the
 376       specified user.
 377      </para>
 378     </listitem>
 379    </varlistentry>
 380
 381    <varlistentry>
 382     <term><literal>SET TABLESPACE</literal></term>
 383     <listitem>
 384      <para>
 385       This form changes the table's tablespace to the specified tablespace and
 386       moves the data file(s) associated with the table to the new tablespace.
 387       Indexes on the table, if any, are not moved; but they can be moved
 388       separately with additional <literal>SET TABLESPACE</literal> commands.
 389       See also
 390       <xref linkend="SQL-CREATETABLESPACE" endterm="sql-createtablespace-title">.
 391      </para>
 392     </listitem>
 393    </varlistentry>
 394
 395    <varlistentry>
 396     <term><literal>RENAME</literal></term>
 397     <listitem>
 398      <para>
 399       The <literal>RENAME</literal> forms change the name of a table
 400       (or an index, sequence, or view) or the name of an individual column in
 401       a table. There is no effect on the stored data.
 402      </para>
 403     </listitem>
 404    </varlistentry>
 405
 406    <varlistentry>
 407     <term><literal>SET SCHEMA</literal></term>
 408     <listitem>
 409      <para>
 410       This form moves the table into another schema.  Associated indexes,
 411       constraints, and sequences owned by table columns are moved as well.
 412      </para>
 413     </listitem>
 414    </varlistentry>
 415
 416   </variablelist>
 417   </para>
 418
 419   <para>
 420    All the actions except <literal>RENAME</literal> and <literal>SET SCHEMA</>
 421    can be combined into
 422    a list of multiple alterations to apply in parallel.  For example, it
 423    is possible to add several columns and/or alter the type of several
 424    columns in a single command.  This is particularly useful with large
 425    tables, since only one pass over the table need be made.
 426   </para>
 427
 428   <para>
 429    You must own the table to use <command>ALTER TABLE</>.
 430    To change the schema of a table, you must also have
 431    <literal>CREATE</literal> privilege on the new schema.
 432    To add the table as a new child of a parent table, you must own the
 433    parent table as well.
 434    To alter the owner, you must also be a direct or indirect member of the new
 435    owning role, and that role must have <literal>CREATE</literal> privilege on
 436    the table's schema.  (These restrictions enforce that altering the owner
 437    doesn't do anything you couldn't do by dropping and recreating the table.
 438    However, a superuser can alter ownership of any table anyway.)
 439   </para>
 440  </refsect1>
 441
 442  <refsect1>
 443   <title>Parameters</title>
 444
 445     <variablelist>
 446
 447      <varlistentry>
 448       <term><replaceable class="PARAMETER">name</replaceable></term>
 449       <listitem>
 450        <para>
 451         The name (possibly schema-qualified) of an existing table to
 452         alter. If <literal>ONLY</> is specified, only that table is
 453         altered. If <literal>ONLY</> is not specified, the table and any
 454         descendant tables are altered.
 455        </para>
 456       </listitem>
 457      </varlistentry>
 458
 459      <varlistentry>
 460       <term><replaceable class="PARAMETER">column</replaceable></term>
 461       <listitem>
 462        <para>
 463         Name of a new or existing column.
 464        </para>
 465       </listitem>
 466      </varlistentry>
 467
 468      <varlistentry>
 469       <term><replaceable class="PARAMETER">new_column</replaceable></term>
 470       <listitem>
 471        <para>
 472         New name for an existing column.
 473        </para>
 474       </listitem>
 475      </varlistentry>
 476
 477      <varlistentry>
 478       <term><replaceable class="PARAMETER">new_name</replaceable></term>
 479       <listitem>
 480        <para>
 481         New name for the table.
 482        </para>
 483       </listitem>
 484      </varlistentry>
 485
 486      <varlistentry>
 487       <term><replaceable class="PARAMETER">type</replaceable></term>
 488       <listitem>
 489        <para>
 490         Data type of the new column, or new data type for an existing
 491         column.
 492        </para>
 493       </listitem>
 494      </varlistentry>
 495
 496      <varlistentry>
 497       <term><replaceable class="PARAMETER">table_constraint</replaceable></term>
 498       <listitem>
 499        <para>
 500         New table constraint for the table.
 501        </para>
 502       </listitem>
 503      </varlistentry>
 504
 505      <varlistentry>
 506       <term><replaceable class="PARAMETER">constraint_name</replaceable></term>
 507       <listitem>
 508        <para>
 509         Name of an existing constraint to drop.
 510        </para>
 511       </listitem>
 512      </varlistentry>
 513
 514      <varlistentry>
 515       <term><literal>CASCADE</literal></term>
 516       <listitem>
 517        <para>
 518         Automatically drop objects that depend on the dropped column
 519         or constraint (for example, views referencing the column).
 520        </para>
 521       </listitem>
 522      </varlistentry>
 523
 524      <varlistentry>
 525       <term><literal>RESTRICT</literal></term>
 526       <listitem>
 527        <para>
 528         Refuse to drop the column or constraint if there are any dependent
 529         objects. This is the default behavior.
 530        </para>
 531       </listitem>
 532      </varlistentry>
 533
 534      <varlistentry>
 535       <term><replaceable class="PARAMETER">trigger_name</replaceable></term>
 536       <listitem>
 537        <para>
 538         Name of a single trigger to disable or enable.
 539        </para>
 540       </listitem>
 541      </varlistentry>
 542
 543      <varlistentry>
 544       <term><literal>ALL</literal></term>
 545       <listitem>
 546        <para>
 547         Disable or enable all triggers belonging to the table.
 548         (This requires superuser privilege if any of the triggers are for
 549         foreign key constraints.)
 550        </para>
 551       </listitem>
 552      </varlistentry>
 553
 554      <varlistentry>
 555       <term><literal>USER</literal></term>
 556       <listitem>
 557        <para>
 558         Disable or enable all triggers belonging to the table except for
 559         foreign key constraint triggers.
 560        </para>
 561       </listitem>
 562      </varlistentry>
 563
 564      <varlistentry>
 565       <term><replaceable class="PARAMETER">index_name</replaceable></term>
 566       <listitem>
 567        <para>
 568         The index name on which the table should be marked for clustering.
 569        </para>
 570       </listitem>
 571      </varlistentry>
 572
 573      <varlistentry>
 574       <term><replaceable class="PARAMETER">storage_parameter</replaceable></term>
 575       <listitem>
 576        <para>
 577         The name of a table storage parameter.
 578        </para>
 579       </listitem>
 580      </varlistentry>
 581
 582      <varlistentry>
 583       <term><replaceable class="PARAMETER">value</replaceable></term>
 584       <listitem>
 585        <para>
 586         The new value for a table storage parameter.
 587         This might be a number or a word depending on the parameter.
 588        </para>
 589       </listitem>
 590      </varlistentry>
 591
 592      <varlistentry>
 593       <term><replaceable class="PARAMETER">parent_table</replaceable></term>
 594       <listitem>
 595        <para>
 596         A parent table to associate or de-associate with this table.
 597        </para>
 598       </listitem>
 599      </varlistentry>
 600
 601      <varlistentry>
 602       <term><replaceable class="PARAMETER">new_owner</replaceable></term>
 603       <listitem>
 604        <para>
 605         The user name of the new owner of the table.
 606        </para>
 607       </listitem>
 608      </varlistentry>
 609
 610      <varlistentry>
 611       <term><replaceable class="PARAMETER">new_tablespace</replaceable></term>
 612       <listitem>
 613        <para>
 614         The name of the tablespace to which the table will be moved.
 615        </para>
 616       </listitem>
 617      </varlistentry>
 618
 619      <varlistentry>
 620       <term><replaceable class="PARAMETER">new_schema</replaceable></term>
 621       <listitem>
 622        <para>
 623         The name of the schema to which the table will be moved.
 624        </para>
 625       </listitem>
 626      </varlistentry>
 627
 628     </variablelist>
 629  </refsect1>
 630
 631  <refsect1>
 632   <title>Notes</title>
 633
 634    <para>
 635     The key word <literal>COLUMN</literal> is noise and can be omitted.
 636    </para>
 637
 638    <para>
 639     When a column is added with <literal>ADD COLUMN</literal>, all existing
 640     rows in the table are initialized with the column's default value
 641     (NULL if no <literal>DEFAULT</> clause is specified).
 642    </para>
 643
 644    <para>
 645     Adding a column with a non-null default or changing the type of an
 646     existing column will require the entire table to be rewritten.  This
 647     might take a significant amount of time for a large table; and it will
 648     temporarily require double the disk space.  Adding or removing a system
 649     <literal>oid</> column likewise requires rewriting the entire table.
 650    </para>
 651
 652    <para>
 653     Adding a <literal>CHECK</> or <literal>NOT NULL</> constraint requires
 654     scanning the table to verify that existing rows meet the constraint.
 655    </para>
 656
 657    <para>
 658     The main reason for providing the option to specify multiple changes
 659     in a single <command>ALTER TABLE</> is that multiple table scans or
 660     rewrites can thereby be combined into a single pass over the table.
 661    </para>
 662
 663    <para>
 664     The <literal>DROP COLUMN</literal> form does not physically remove
 665     the column, but simply makes it invisible to SQL operations.  Subsequent
 666     insert and update operations in the table will store a null value for the
 667     column. Thus, dropping a column is quick but it will not immediately
 668     reduce the on-disk size of your table, as the space occupied
 669     by the dropped column is not reclaimed.  The space will be
 670     reclaimed over time as existing rows are updated.  (These statements do
 671     not apply when dropping the system <literal>oid</> column; that is done
 672     with an immediate rewrite.)
 673    </para>
 674
 675    <para>
 676     The fact that <literal>ALTER TYPE</> requires rewriting the whole table
 677     is sometimes an advantage, because the rewriting process eliminates
 678     any dead space in the table.  For example, to reclaim the space occupied
 679     by a dropped column immediately, the fastest way is:
 680 <programlisting>
 681 ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
 682 </programlisting>
 683     where <literal>anycol</> is any remaining table column and
 684     <literal>anytype</> is the same type that column already has.
 685     This results in no semantically-visible change in the table,
 686     but the command forces rewriting, which gets rid of no-longer-useful
 687     data.
 688    </para>
 689
 690    <para>
 691     The <literal>USING</literal> option of <literal>ALTER TYPE</> can actually
 692     specify any expression involving the old values of the row; that is, it
 693     can refer to other columns as well as the one being converted.  This allows
 694     very general conversions to be done with the <literal>ALTER TYPE</>
 695     syntax.  Because of this flexibility, the <literal>USING</literal>
 696     expression is not applied to the column's default value (if any); the
 697     result might not be a constant expression as required for a default.
 698     This means that when there is no implicit or assignment cast from old to
 699     new type, <literal>ALTER TYPE</> might fail to convert the default even
 700     though a <literal>USING</literal> clause is supplied.  In such cases,
 701     drop the default with <literal>DROP DEFAULT</>, perform the <literal>ALTER
 702     TYPE</>, and then use <literal>SET DEFAULT</> to add a suitable new
 703     default.  Similar considerations apply to indexes and constraints involving
 704     the column.
 705    </para>
 706
 707    <para>
 708     If a table has any descendant tables, it is not permitted to add,
 709     rename, or change the type of a column in the parent table without doing
 710     the same to the descendants.  That is, <command>ALTER TABLE ONLY</command>
 711     will be rejected.  This ensures that the descendants always have
 712     columns matching the parent.
 713    </para>
 714
 715    <para>
 716     A recursive <literal>DROP COLUMN</literal> operation will remove a
 717     descendant table's column only if the descendant does not inherit
 718     that column from any other parents and never had an independent
 719     definition of the column.  A nonrecursive <literal>DROP
 720     COLUMN</literal> (i.e., <command>ALTER TABLE ONLY ... DROP
 721     COLUMN</command>) never removes any descendant columns, but
 722     instead marks them as independently defined rather than inherited.
 723    </para>
 724
 725    <para>
 726     The <literal>TRIGGER</>, <literal>CLUSTER</>, <literal>OWNER</>,
 727     and <literal>TABLESPACE</> actions never recurse to descendant tables;
 728     that is, they always act as though <literal>ONLY</> were specified.
 729     Adding a constraint can recurse only for <literal>CHECK</> constraints,
 730     and is required to do so for such constraints.
 731    </para>
 732
 733    <para>
 734     Changing any part of a system catalog table is not permitted.
 735    </para>
 736
 737    <para>
 738     Refer to <xref linkend="sql-createtable"
 739     endterm="sql-createtable-title"> for a further description of valid
 740     parameters. <xref linkend="ddl"> has further information on
 741     inheritance.
 742    </para>
 743  </refsect1>
 744
 745  <refsect1>
 746   <title>Examples</title>
 747
 748   <para>
 749    To add a column of type <type>varchar</type> to a table:
 750 <programlisting>
 751 ALTER TABLE distributors ADD COLUMN address varchar(30);
 752 </programlisting>
 753   </para>
 754
 755   <para>
 756    To drop a column from a table:
 757 <programlisting>
 758 ALTER TABLE distributors DROP COLUMN address RESTRICT;
 759 </programlisting>
 760   </para>
 761
 762   <para>
 763    To change the types of two existing columns in one operation:
 764 <programlisting>
 765 ALTER TABLE distributors
 766     ALTER COLUMN address TYPE varchar(80),
 767     ALTER COLUMN name TYPE varchar(100);
 768 </programlisting>
 769   </para>
 770
 771   <para>
 772    To change an integer column containing UNIX timestamps to <type>timestamp
 773    with time zone</type> via a <literal>USING</literal> clause:
 774 <programlisting>
 775 ALTER TABLE foo
 776     ALTER COLUMN foo_timestamp SET DATA TYPE timestamp with time zone
 777     USING
 778         timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';
 779 </programlisting>
 780   </para>
 781
 782   <para>
 783    The same, when the column has a default expression that won't automatically
 784    cast to the new data type:
 785 <programlisting>
 786 ALTER TABLE foo
 787     ALTER COLUMN foo_timestamp DROP DEFAULT,
 788     ALTER COLUMN foo_timestamp TYPE timestamp with time zone
 789     USING
 790         timestamp with time zone 'epoch' + foo_timestamp * interval '1 second',
 791     ALTER COLUMN foo_timestamp SET DEFAULT now();
 792 </programlisting>
 793   </para>
 794
 795   <para>
 796    To rename an existing column:
 797 <programlisting>
 798 ALTER TABLE distributors RENAME COLUMN address TO city;
 799 </programlisting>
 800   </para>
 801
 802   <para>
 803    To rename an existing table:
 804 <programlisting>
 805 ALTER TABLE distributors RENAME TO suppliers;
 806 </programlisting>
 807   </para>
 808
 809   <para>
 810    To add a not-null constraint to a column:
 811 <programlisting>
 812 ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
 813 </programlisting>
 814    To remove a not-null constraint from a column:
 815 <programlisting>
 816 ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
 817 </programlisting>
 818   </para>
 819
 820   <para>
 821    To add a check constraint to a table and all its children:
 822 <programlisting>
 823 ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
 824 </programlisting>
 825   </para>
 826
 827   <para>
 828    To remove a check constraint from a table and all its children:
 829 <programlisting>
 830 ALTER TABLE distributors DROP CONSTRAINT zipchk;
 831 </programlisting>
 832   </para>
 833
 834   <para>
 835    To remove a check constraint from a table only:
 836 <programlisting>
 837 ALTER TABLE ONLY distributors DROP CONSTRAINT zipchk;
 838 </programlisting>
 839    (The check constraint remains in place for any child tables.)
 840   </para>
 841
 842   <para>
 843    To add a foreign key constraint to a table:
 844 <programlisting>
 845 ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) MATCH FULL;
 846 </programlisting>
 847   </para>
 848
 849   <para>
 850    To add a (multicolumn) unique constraint to a table:
 851 <programlisting>
 852 ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
 853 </programlisting>
 854   </para>
 855
 856   <para>
 857    To add an automatically named primary key constraint to a table, noting
 858    that a table can only ever have one primary key:
 859 <programlisting>
 860 ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
 861 </programlisting>
 862   </para>
 863
 864   <para>
 865    To move a table to a different tablespace:
 866 <programlisting>
 867 ALTER TABLE distributors SET TABLESPACE fasttablespace;
 868 </programlisting>
 869   </para>
 870
 871   <para>
 872    To move a table to a different schema:
 873 <programlisting>
 874 ALTER TABLE myschema.distributors SET SCHEMA yourschema;
 875 </programlisting>
 876   </para>
 877
 878  </refsect1>
 879
 880  <refsect1>
 881   <title>Compatibility</title>
 882
 883   <para>
 884    The forms <literal>ADD</literal>, <literal>DROP</>, <literal>SET DEFAULT</>,
 885    and <literal>SET DATA TYPE</literal> (without <literal>USING</literal>)
 886    conform with the SQL standard.  The other forms are
 887    <productname>PostgreSQL</productname> extensions of the SQL standard.
 888    Also, the ability to specify more than one manipulation in a single
 889    <command>ALTER TABLE</> command is an extension.
 890   </para>
 891
 892   <para>
 893    <command>ALTER TABLE DROP COLUMN</> can be used to drop the only
 894    column of a table, leaving a zero-column table.  This is an
 895    extension of SQL, which disallows zero-column tables.
 896   </para>
 897  </refsect1>
 898 </refentry>