doc/src/sgml/ref/alter_table.sgml - master - githubmirror/postgres

doc/src/sgml/ref/alter_table.sgml
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Alexander Korotkov
doc: clarify MERGE PARTITIONS adjacency requirement
30 июн 2026, 22:30
30 июн 2026, 22:3057f1977
<!--
doc/src/sgml/ref/alter_table.sgml
PostgreSQL documentation
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<refentry id="sql-altertable">
 <indexterm zone="sql-altertable">
  <primary>ALTER TABLE</primary>
 </indexterm>

 <refmeta>
  <refentrytitle>ALTER TABLE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>ALTER TABLE</refname>
  <refpurpose>change the definition of a table</refpurpose>
 </refnamediv>

 <refsynopsisdiv>
<synopsis>
ALTER TABLE [ IF EXISTS ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ]
    <replaceable class="parameter">action</replaceable> [, ... ]
ALTER TABLE [ IF EXISTS ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ]
    RENAME [ COLUMN ] <replaceable class="parameter">column_name</replaceable> TO <replaceable class="parameter">new_column_name</replaceable>
ALTER TABLE [ IF EXISTS ] [ ONLY ] <replaceable class="parameter">name</replaceable> [ * ]
    RENAME CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> TO <replaceable class="parameter">new_constraint_name</replaceable>
ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
    RENAME TO <replaceable class="parameter">new_name</replaceable>
ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
    SET SCHEMA <replaceable class="parameter">new_schema</replaceable>
ALTER TABLE ALL IN TABLESPACE <replaceable class="parameter">name</replaceable> [ OWNED BY <replaceable class="parameter">role_name</replaceable> [, ... ] ]
    SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable> [ NOWAIT ]
ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
    ATTACH PARTITION <replaceable class="parameter">partition_name</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }
ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
    DETACH PARTITION <replaceable class="parameter">partition_name</replaceable> [ CONCURRENTLY | FINALIZE ]
ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
    MERGE PARTITIONS (<replaceable class="parameter">partition_name1</replaceable>, <replaceable class="parameter">partition_name2</replaceable> [, ...]) INTO <replaceable class="parameter">partition_name</replaceable>
ALTER TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceable>
    SPLIT PARTITION <replaceable class="parameter">partition_name</replaceable> INTO
        (PARTITION <replaceable class="parameter">partition_name1</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT },
         PARTITION <replaceable class="parameter">partition_name2</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT } [, ...])

<phrase>where <replaceable class="parameter">action</replaceable> is one of:</phrase>

    ADD [ COLUMN ] [ IF NOT EXISTS ] <replaceable class="parameter">column_name</replaceable> <replaceable class="parameter">data_type</replaceable> [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ <replaceable class="parameter">column_constraint</replaceable> [ ... ] ]
    DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="parameter">column_name</replaceable> [ RESTRICT | CASCADE ]
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> [ SET DATA ] TYPE <replaceable class="parameter">data_type</replaceable> [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ USING <replaceable class="parameter">expression</replaceable> ]
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET DEFAULT <replaceable class="parameter">expression</replaceable>
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP DEFAULT
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET | DROP } NOT NULL
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET EXPRESSION AS ( <replaceable class="parameter">expression</replaceable> )
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP EXPRESSION [ IF EXISTS ]
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ]
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> { SET GENERATED { ALWAYS | BY DEFAULT } | SET <replaceable>sequence_option</replaceable> | RESTART [ [ WITH ] <replaceable class="parameter">restart</replaceable> ] } [...]
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> DROP IDENTITY [ IF EXISTS ]
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET STATISTICS { <replaceable class="parameter">integer</replaceable> | DEFAULT }
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] )
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN | DEFAULT }
    ALTER [ COLUMN ] <replaceable class="parameter">column_name</replaceable> SET COMPRESSION <replaceable class="parameter">compression_method</replaceable>
    ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]
    ADD <replaceable class="parameter">table_constraint_using_index</replaceable>
    ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] [ ENFORCED | NOT ENFORCED ]
    ALTER CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> [ INHERIT | NO INHERIT ]
    VALIDATE CONSTRAINT <replaceable class="parameter">constraint_name</replaceable>
    DROP CONSTRAINT [ IF EXISTS ]  <replaceable class="parameter">constraint_name</replaceable> [ RESTRICT | CASCADE ]
    DISABLE TRIGGER [ <replaceable class="parameter">trigger_name</replaceable> | ALL | USER ]
    ENABLE TRIGGER [ <replaceable class="parameter">trigger_name</replaceable> | ALL | USER ]
    ENABLE REPLICA TRIGGER <replaceable class="parameter">trigger_name</replaceable>
    ENABLE ALWAYS TRIGGER <replaceable class="parameter">trigger_name</replaceable>
    DISABLE RULE <replaceable class="parameter">rewrite_rule_name</replaceable>
    ENABLE RULE <replaceable class="parameter">rewrite_rule_name</replaceable>
    ENABLE REPLICA RULE <replaceable class="parameter">rewrite_rule_name</replaceable>
    ENABLE ALWAYS RULE <replaceable class="parameter">rewrite_rule_name</replaceable>
    DISABLE ROW LEVEL SECURITY
    ENABLE ROW LEVEL SECURITY
    FORCE ROW LEVEL SECURITY
    NO FORCE ROW LEVEL SECURITY
    CLUSTER ON <replaceable class="parameter">index_name</replaceable>
    SET WITHOUT CLUSTER
    SET WITHOUT OIDS
    SET ACCESS METHOD { <replaceable class="parameter">new_access_method</replaceable> | DEFAULT }
    SET TABLESPACE <replaceable class="parameter">new_tablespace</replaceable>
    SET { LOGGED | UNLOGGED }
    SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )
    RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )
    INHERIT <replaceable class="parameter">parent_table</replaceable>
    NO INHERIT <replaceable class="parameter">parent_table</replaceable>
    OF <replaceable class="parameter">type_name</replaceable>
    NOT OF
    OWNER TO { <replaceable class="parameter">new_owner</replaceable> | CURRENT_ROLE | CURRENT_USER | SESSION_USER }
    REPLICA IDENTITY { DEFAULT | USING INDEX <replaceable class="parameter">index_name</replaceable> | FULL | NOTHING }

<phrase>and <replaceable class="parameter">partition_bound_spec</replaceable> is:</phrase>

IN ( <replaceable class="parameter">partition_bound_expr</replaceable> [, ...] ) |
FROM ( { <replaceable class="parameter">partition_bound_expr</replaceable> | MINVALUE | MAXVALUE } [, ...] )
  TO ( { <replaceable class="parameter">partition_bound_expr</replaceable> | MINVALUE | MAXVALUE } [, ...] ) |
WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REMAINDER <replaceable class="parameter">numeric_literal</replaceable> )

<phrase>and <replaceable class="parameter">column_constraint</replaceable> is:</phrase>

[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
{ NOT NULL [ NO INHERIT ] |
  NULL |
  CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
  DEFAULT <replaceable>default_expr</replaceable> |
  GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ] |
  GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
  UNIQUE [ NULLS [ NOT ] DISTINCT ] <replaceable class="parameter">index_parameters</replaceable> |
  PRIMARY KEY <replaceable class="parameter">index_parameters</replaceable> |
  REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> ) ] [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ]
    [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] [ ENFORCED | NOT ENFORCED ]

<phrase>and <replaceable class="parameter">table_constraint</replaceable> is:</phrase>

[ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
{ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
  NOT NULL <replaceable class="parameter">column_name</replaceable> [ NO INHERIT ] |
  UNIQUE [ NULLS [ NOT ] DISTINCT ] ( <replaceable class="parameter">column_name</replaceable> [, ... ] [, <replaceable class="parameter">column_name</replaceable> WITHOUT OVERLAPS ] ) <replaceable class="parameter">index_parameters</replaceable> |
  PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] [, <replaceable class="parameter">column_name</replaceable> WITHOUT OVERLAPS ] ) <replaceable class="parameter">index_parameters</replaceable> |
  EXCLUDE [ USING <replaceable class="parameter">index_method</replaceable> ] ( <replaceable class="parameter">exclude_element</replaceable> WITH <replaceable class="parameter">operator</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> [ WHERE ( <replaceable class="parameter">predicate</replaceable> ) ] |
  FOREIGN KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] [, PERIOD <replaceable class="parameter">column_name</replaceable> ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ]  [, PERIOD <replaceable class="parameter">refcolumn</replaceable> ] ) ]
    [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE <replaceable class="parameter">referential_action</replaceable> ] [ ON UPDATE <replaceable class="parameter">referential_action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ] [ ENFORCED | NOT ENFORCED ]

<phrase>and <replaceable class="parameter">table_constraint_using_index</replaceable> is:</phrase>

    [ CONSTRAINT <replaceable class="parameter">constraint_name</replaceable> ]
    { UNIQUE | PRIMARY KEY } USING INDEX <replaceable class="parameter">index_name</replaceable>
    [ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

<phrase><replaceable class="parameter">index_parameters</replaceable> in <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and <literal>EXCLUDE</literal> constraints are:</phrase>

[ INCLUDE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ]
[ WITH ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] ) ]
[ USING INDEX TABLESPACE <replaceable class="parameter">tablespace_name</replaceable> ]

<phrase><replaceable class="parameter">exclude_element</replaceable> in an <literal>EXCLUDE</literal> constraint is:</phrase>

{ <replaceable class="parameter">column_name</replaceable> | ( <replaceable class="parameter">expression</replaceable> ) } [ COLLATE <replaceable class="parameter">collation</replaceable> ] [ <replaceable class="parameter">opclass</replaceable> [ ( <replaceable class="parameter">opclass_parameter</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] ) ] ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]

<phrase><replaceable class="parameter">referential_action</replaceable> in a <literal>FOREIGN KEY</literal>/<literal>REFERENCES</literal> constraint is:</phrase>

{ NO ACTION | RESTRICT | CASCADE | SET NULL [ ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ] | SET DEFAULT [ ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) ] }
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER TABLE</command> changes the definition of an existing table.
   There are several subforms described below. Note that the lock level required
   may differ for each subform. An <literal>ACCESS EXCLUSIVE</literal> lock is
   acquired unless explicitly noted. When multiple subcommands are given, the
   lock acquired will be the strictest one required by any subcommand.

  <variablelist>
   <varlistentry id="sql-altertable-desc-add-column">
    <term><literal>ADD [ COLUMN ] [ IF NOT EXISTS ]</literal></term>
    <listitem>
     <para>
      This form adds a new column to the table, using the same syntax as
      <link linkend="sql-createtable"><command>CREATE TABLE</command></link>. If <literal>IF NOT EXISTS</literal>
      is specified and a column already exists with this name,
      no error is thrown.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-drop-column">
    <term><literal>DROP [ COLUMN ] [ IF EXISTS ]</literal></term>
    <listitem>
     <para>
      This form drops a column from a table.  Indexes and
      table constraints involving the column will be automatically
      dropped as well.
      Multivariate statistics referencing the dropped column will also be
      removed if the removal of the column would cause the statistics to
      contain data for only a single column.
      You will need to say <literal>CASCADE</literal> if anything outside the table
      depends on the column, for example, foreign key references or views.
      If <literal>IF EXISTS</literal> is specified and the column
      does not exist, no error is thrown. In this case a notice
      is issued instead.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-data-type">
    <term><literal>SET DATA TYPE</literal></term>
    <listitem>
     <para>
      This form changes the type of a column of a table. Indexes and
      simple table constraints involving the column will be automatically
      converted to use the new column type by reparsing the originally
      supplied expression.
      The optional <literal>COLLATE</literal> clause specifies a collation
      for the new column; if omitted, the collation is the default for the
      new column type.
      The optional <literal>USING</literal>
      clause specifies how to compute the new column value from the old;
      if omitted, the default conversion is the same as an assignment
      cast from old data type to new.  A  <literal>USING</literal>
      clause must be provided if there is no implicit or assignment
      cast from old to new type.
     </para>

     <para>
      When this form is used, the column's statistics are removed,
      so running <link linkend="sql-analyze"><command>ANALYZE</command></link>
      on the table afterwards is recommended.
      For a virtual generated column, <command>ANALYZE</command>
      is not necessary unless extended statistics are defined on it,
      since such columns do not have statistics by default.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-drop-default">
    <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
    <listitem>
     <para>
      These forms set or remove the default value for a column (where
      removal is equivalent to setting the default value to NULL).  The new
      default value will only apply in subsequent <command>INSERT</command>
      or <command>UPDATE</command> commands; it does not cause rows already
      in the table to change.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-drop-not-null">
    <term><literal>SET</literal>/<literal>DROP NOT NULL</literal></term>
    <listitem>
     <para>
      These forms change whether a column is marked to allow null
      values or to reject null values.
     </para>

     <para>
      <literal>SET NOT NULL</literal> may only be applied to a column
      provided none of the records in the table contain a
      <literal>NULL</literal> value for the column.  Ordinarily this is
      checked during the <literal>ALTER TABLE</literal> by scanning the
      entire table, unless <literal>NOT VALID</literal> is specified;
      however, if a valid <literal>CHECK</literal> constraint exists
      (and is not dropped in the same command) which proves no
      <literal>NULL</literal> can exist, then the table scan is skipped.
      If a column has an invalid not-null constraint,
      <literal>SET NOT NULL</literal> validates it.
     </para>

     <para>
      If this table is a partition, one cannot perform <literal>DROP NOT NULL</literal>
      on a column if it is marked <literal>NOT NULL</literal> in the parent
      table.  To drop the <literal>NOT NULL</literal> constraint from all the
      partitions, perform <literal>DROP NOT NULL</literal> on the parent
      table.  Even if there is no <literal>NOT NULL</literal> constraint on the
      parent, such a constraint can still be added to individual partitions,
      if desired; that is, the children can disallow nulls even if the parent
      allows them, but not the other way around.  It is also possible to drop
      the <literal>NOT NULL</literal> constraint from <literal>ONLY</literal>
      the parent table, which does not remove it from the children.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-expression">
    <term><literal>SET EXPRESSION AS</literal></term>
    <listitem>
     <para>
      This form replaces the expression of a generated column.  Existing data
      in a stored generated column is rewritten and all the future changes
      will apply the new generation expression.
      Replacing the expression of a virtual generated column does not require a
      table rewrite, but if the column is used in a constraint, the table will
      be scanned to check that existing rows meet the constraint.
     </para>

     <para>
      When this form is used on a stored generated column, its statistics
      are removed, so running
      <link linkend="sql-analyze"><command>ANALYZE</command></link>
      on the table afterwards is recommended.
      For a virtual generated column, <command>ANALYZE</command>
      is not necessary unless extended statistics are defined on it,
      since such columns do not have statistics by default.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-drop-expression">
    <term><literal>DROP EXPRESSION [ IF EXISTS ]</literal></term>
    <listitem>
     <para>
      This form turns a stored generated column into a normal base column.
      Existing data in the columns is retained, but future changes will no
      longer apply the generation expression.
     </para>

     <para>
      This form is currently only supported for stored generated columns (not
      virtual ones).
     </para>

     <para>
      If <literal>DROP EXPRESSION IF EXISTS</literal> is specified and the
      column is not a generated column, no error is thrown.  In this case a
      notice is issued instead.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-generated-identity">
    <term><literal>ADD GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY</literal></term>
    <term><literal>SET GENERATED { ALWAYS | BY DEFAULT }</literal></term>
    <term><literal>DROP IDENTITY [ IF EXISTS ]</literal></term>
    <listitem>
     <para>
      These forms change whether a column is an identity column or change the
      generation attribute of an existing identity column.
      See <link linkend="sql-createtable"><command>CREATE TABLE</command></link> for details.
      Like <literal>SET DEFAULT</literal>, these forms only affect the
      behavior of subsequent <command>INSERT</command>
      and <command>UPDATE</command> commands; they do not cause rows
      already in the table to change.
     </para>

     <para>
      If <literal>DROP IDENTITY IF EXISTS</literal> is specified and the
      column is not an identity column, no error is thrown.  In this case a
      notice is issued instead.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-sequence-option">
    <term><literal>SET <replaceable>sequence_option</replaceable></literal></term>
    <term><literal>RESTART</literal></term>
    <listitem>
     <para>
      These forms alter the sequence that underlies an existing identity
      column.  <replaceable>sequence_option</replaceable> is an option
      supported by <link linkend="sql-altersequence"><command>ALTER SEQUENCE</command></link> such
      as <literal>INCREMENT BY</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-statistics">
    <term><literal>SET STATISTICS</literal></term>
    <listitem>
     <para>
      This form
      sets the per-column statistics-gathering target for subsequent
      <link linkend="sql-analyze"><command>ANALYZE</command></link> operations.
      The target can be set in the range 0 to 10000.  Set it
      to <literal>DEFAULT</literal> to revert to using the system default
      statistics target (<xref linkend="guc-default-statistics-target"/>).
      (Setting to a value of -1 is an obsolete way spelling to get the same
      outcome.)
      For more information on the use of statistics by the
      <productname>PostgreSQL</productname> query planner, refer to
      <xref linkend="planner-stats"/>.
     </para>
     <para>
      This form is not supported on virtual generated columns, since such
      columns do not have statistics by default.  If extended statistics are
      defined on such columns, the statistics-gathering target may be set on
      the extended statistics object using <xref linkend="sql-alterstatistics"/>.
     </para>
     <para>
      <literal>SET STATISTICS</literal> acquires a
      <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-attribute-option">
    <term><literal>SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )</literal></term>
    <term><literal>RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] )</literal></term>
    <listitem>
     <para>
      This form sets or resets per-attribute options.  Currently, the only
      defined per-attribute options are <literal>n_distinct</literal> and
      <literal>n_distinct_inherited</literal>, which override the
      number-of-distinct-values estimates made by subsequent
      <link linkend="sql-analyze"><command>ANALYZE</command></link>
      operations. <literal>n_distinct</literal> affects the statistics for the
      table itself, while <literal>n_distinct_inherited</literal> affects the
      statistics gathered for the table plus its inheritance children, and for
      the statistics gathered for partitioned tables.  When the value
      specified is a positive value, the query planner will assume that the
      column contains exactly the specified number of distinct nonnull values.
      Fractional values may also be specified by using values below 0 and
      above or equal to -1.  This instructs the query planner to estimate the
      number of distinct values by multiplying the absolute value of the
      specified number by the estimated number of rows in the table.  For
      example, a value of -1 implies that all values in the column are
      distinct, while a value of -0.5 implies that each value appears twice on
      average.  This can be useful when the size of the table changes over
      time.  For more information on the use of statistics by the
      <productname>PostgreSQL</productname> query planner, refer to
      <xref linkend="planner-stats"/>.
     </para>
     <para>
      Changing per-attribute options acquires a
      <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-storage">
    <term>
     <literal>SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN | DEFAULT }</literal>
     <indexterm>
      <primary>TOAST</primary>
      <secondary>per-column storage settings</secondary>
     </indexterm>
    </term>
    <listitem>
     <para>
      This form sets the storage mode for a column. This controls whether this
      column is held inline or in a secondary <acronym>TOAST</acronym> table,
      and whether the data
      should be compressed or not. <literal>PLAIN</literal> must be used
      for fixed-length values such as <type>integer</type> and is
      inline, uncompressed. <literal>MAIN</literal> is for inline,
      compressible data. <literal>EXTERNAL</literal> is for external,
      uncompressed data, and <literal>EXTENDED</literal> is for external,
      compressed data.
      Writing <literal>DEFAULT</literal> sets the storage mode to the default
      mode for the column's data type.  <literal>EXTENDED</literal> is the
      default for most data types that support non-<literal>PLAIN</literal>
      storage.
      Use of <literal>EXTERNAL</literal> will make substring operations on
      very large <type>text</type> and <type>bytea</type> values run faster,
      at the penalty of increased storage space.
      Note that <literal>ALTER TABLE ... SET STORAGE</literal> doesn't itself
      change anything in the table; it just sets the strategy to be pursued
      during future table updates.
      See <xref linkend="storage-toast"/> for more information.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-compression">
    <term>
     <literal>SET COMPRESSION <replaceable class="parameter">compression_method</replaceable></literal>
    </term>
    <listitem>
     <para>
      This form sets the compression method for a column, determining how
      values inserted in future will be compressed (if the storage mode
      permits compression at all).
      This does not cause the table to be rewritten, so existing data may still
      be compressed with other compression methods.  If the table is restored
      with <application>pg_restore</application>, then all values are rewritten
      with the configured compression method.
      However, when data is inserted from another relation (for example,
      by <command>INSERT ... SELECT</command>), values from the source table are
      not necessarily detoasted, so any previously compressed data may retain
      its existing compression method, rather than being recompressed with the
      compression method of the target column.
      The supported compression
      methods are <literal>pglz</literal> and <literal>lz4</literal>.
      (<literal>lz4</literal> is available only if <option>--with-lz4</option>
      was used when building <productname>PostgreSQL</productname>.)  In
      addition, <replaceable class="parameter">compression_method</replaceable>
      can be <literal>default</literal>, which selects the default behavior of
      consulting the <xref linkend="guc-default-toast-compression"/> setting
      at the time of data insertion to determine the method to use.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-add-table-constraint">
    <term><literal>ADD <replaceable class="parameter">table_constraint</replaceable> [ NOT VALID ]</literal></term>
    <listitem>
     <para>
      This form adds a new constraint to a table using the same constraint
      syntax as <link linkend="sql-createtable"><command>CREATE TABLE</command></link>, plus the option <literal>NOT
      VALID</literal>, which is currently only allowed for foreign-key,
      <literal>CHECK</literal>, and not-null constraints.
     </para>

     <para>
      Normally, this form will cause a scan of the table to verify that all
      existing rows in the table satisfy the new constraint.  But if
      the <literal>NOT VALID</literal> option is used, this
      potentially-lengthy scan is skipped.  The constraint will still be
      applied against subsequent inserts or updates (that is, they'll fail
      unless there is a matching row in the referenced table, in the case
      of foreign keys, or they'll fail unless the new row matches the
      specified check condition).  But the
      database will not assume that the constraint holds for all rows in
      the table, until it is validated by using the <literal>VALIDATE
      CONSTRAINT</literal> option.
      See <xref linkend="sql-altertable-notes"/> below for more information
      about using the <literal>NOT VALID</literal> option.
     </para>

     <para>
      Although most forms of <literal>ADD
      <replaceable class="parameter">table_constraint</replaceable></literal>
      require an <literal>ACCESS EXCLUSIVE</literal> lock, <literal>ADD
      FOREIGN KEY</literal> requires only a <literal>SHARE ROW
      EXCLUSIVE</literal> lock.  Note that <literal>ADD FOREIGN KEY</literal>
      also acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock on the
      referenced table, in addition to the lock on the table on which the
      constraint is declared.
     </para>

     <para>
      Additional restrictions apply when unique or primary key constraints
      are added to partitioned tables; see <link linkend="sql-createtable"><command>CREATE TABLE</command></link>.
     </para>

    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-add-table-constraint-using-index">
    <term><literal>ADD <replaceable class="parameter">table_constraint_using_index</replaceable></literal></term>
    <listitem>
     <para>
      This form adds a new <literal>PRIMARY KEY</literal> or <literal>UNIQUE</literal>
      constraint to a table based on an existing unique index.  All the
      columns of the index will be included in the constraint.
     </para>

     <para>
      The index cannot have expression columns nor be a partial index.
      Also, it must be a b-tree index with default sort ordering.  These
      restrictions ensure that the index is equivalent to one that would be
      built by a regular <literal>ADD PRIMARY KEY</literal> or <literal>ADD UNIQUE</literal>
      command.
     </para>

     <para>
      If <literal>PRIMARY KEY</literal> is specified, and the index's columns are not
      already marked <literal>NOT NULL</literal>, then this command will attempt to
      do <literal>ALTER COLUMN SET NOT NULL</literal> against each such column.
      That requires a full table scan to verify the column(s) contain no
      nulls.  In all other cases, this is a fast operation.
     </para>

     <para>
      If a constraint name is provided then the index will be renamed to match
      the constraint name.  Otherwise the constraint will be named the same as
      the index.
     </para>

     <para>
      After this command is executed, the index is <quote>owned</quote> by the
      constraint, in the same way as if the index had been built by
      a regular <literal>ADD PRIMARY KEY</literal> or <literal>ADD UNIQUE</literal>
      command.  In particular, dropping the constraint will make the index
      disappear too.
     </para>

     <para>
      This form is not currently supported on partitioned tables.
     </para>

     <note>
      <para>
       Adding a constraint using an existing index can be helpful in
       situations where a new constraint needs to be added without blocking
       table updates for a long time.  To do that, create the index using
       <command>CREATE UNIQUE INDEX CONCURRENTLY</command>, and then convert it to a
       constraint using this syntax.  See the example below.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-alter-constraint">
    <term><literal>ALTER CONSTRAINT</literal></term>
    <listitem>
     <para>
      This form alters the attributes of a constraint that was previously
      created.  Currently, the deferrability attributes can be altered only
      for <literal>FOREIGN KEY</literal> constraints.  The enforceability
      attribute can be altered for <literal>FOREIGN KEY</literal> and
      <literal>CHECK</literal> constraints.
     </para>

     <para>
      Setting a constraint to <literal>NOT ENFORCED</literal> causes the
      database system to stop checking it for new or updated rows.  Setting
      a constraint to <literal>ENFORCED</literal> causes the database system
      to verify that existing rows satisfy the constraint and to check it
      for new or updated rows.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-alter-constraint-inherit">
    <term><literal>ALTER CONSTRAINT ... INHERIT</literal></term>
    <term><literal>ALTER CONSTRAINT ... NO INHERIT</literal></term>
    <listitem>
     <para>
      These forms modify a inheritable constraint so that it becomes not
      inheritable, or vice-versa. Only not-null constraints may be altered
      in this fashion at present.
      In addition to changing the inheritability status of the constraint,
      in the case where a non-inheritable constraint is being marked
      inheritable, if the table has children, an equivalent constraint
      will be added to them. If marking an inheritable constraint as
      non-inheritable on a table with children, then the corresponding
      constraint on children will be marked as no longer inherited,
      but not removed.
      </para>
     </listitem>
    </varlistentry>

   <varlistentry id="sql-altertable-desc-validate-constraint">
    <term><literal>VALIDATE CONSTRAINT</literal></term>
    <listitem>
     <para>
      This form validates a foreign key, check, or not-null constraint that was
      previously created as <literal>NOT VALID</literal>, by scanning the
      table to ensure there are no rows for which the constraint is not
      satisfied.  If the constraint was set to <literal>NOT ENFORCED</literal>, an error is thrown.
      Nothing happens if the constraint is already marked valid.
      (See <xref linkend="sql-altertable-notes"/> below for an explanation
      of the usefulness of this command.)
     </para>
     <para>
      This command acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-drop-constraint">
    <term><literal>DROP CONSTRAINT [ IF EXISTS ]</literal></term>
    <listitem>
     <para>
      This form drops the specified constraint on a table, along with
      any index underlying the constraint.
      If <literal>IF EXISTS</literal> is specified and the constraint
      does not exist, no error is thrown. In this case a notice is issued instead.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-disable-enable-trigger">
    <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] TRIGGER</literal></term>
    <listitem>
     <para>
      These forms configure the firing of trigger(s) belonging to the table.
      A disabled trigger is still known to the system, but is not executed
      when its triggering event occurs.  (For a deferred trigger, the enable
      status is checked when the event occurs, not when the trigger function
      is actually executed.)  One can disable or enable a single
      trigger specified by name, or all triggers on the table, or only
      user triggers (this option excludes internally generated constraint
      triggers, such as those that are used to implement foreign key
      constraints or deferrable uniqueness and exclusion constraints).
      Disabling or enabling internally generated constraint triggers
      requires superuser privileges; it should be done with caution since
      of course the integrity of the constraint cannot be guaranteed if the
      triggers are not executed.
     </para>

     <para>
      The trigger firing mechanism is also affected by the configuration
      variable <xref linkend="guc-session-replication-role"/>. Simply enabled
      triggers (the default) will fire when the replication role is <quote>origin</quote>
      (the default) or <quote>local</quote>. Triggers configured as <literal>ENABLE
      REPLICA</literal> will only fire if the session is in <quote>replica</quote>
      mode, and triggers configured as <literal>ENABLE ALWAYS</literal> will
      fire regardless of the current replication role.
     </para>

     <para>
      The effect of this mechanism is that in the default configuration,
      triggers do not fire on replicas.  This is useful because if a trigger
      is used on the origin to propagate data between tables, then the
      replication system will also replicate the propagated data; so the
      trigger should not fire a second time on the replica, because that would
      lead to duplication.  However, if a trigger is used for another purpose
      such as creating external alerts, then it might be appropriate to set it
      to <literal>ENABLE ALWAYS</literal> so that it is also fired on
      replicas.
     </para>

     <para>
      When this command is applied to a partitioned table, the states of
      corresponding clone triggers in the partitions are updated too,
      unless <literal>ONLY</literal> is specified.
     </para>

     <para>
      This command acquires a <literal>SHARE ROW EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-disable-enable-rule">
    <term><literal>DISABLE</literal>/<literal>ENABLE [ REPLICA | ALWAYS ] RULE</literal></term>
    <listitem>
     <para>
      These forms configure the firing of rewrite rules belonging to the table.
      A disabled rule is still known to the system, but is not applied
      during query rewriting. The semantics are as for disabled/enabled
      triggers. This configuration is ignored for <literal>ON SELECT</literal> rules, which
      are always applied in order to keep views working even if the current
      session is in a non-default replication role.
     </para>

     <para>
      The rule firing mechanism is also affected by the configuration variable
      <xref linkend="guc-session-replication-role"/>, analogous to triggers as
      described above.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-disable-enable-row-level-security">
    <term><literal>DISABLE</literal>/<literal>ENABLE ROW LEVEL SECURITY</literal></term>
    <listitem>
     <para>
      These forms control the application of row security policies belonging
      to the table.  If enabled and no policies exist for the table, then a
      default-deny policy is applied.  Note that policies can exist for a table
      even if row-level security is disabled.  In this case, the policies will
      <emphasis>not</emphasis> be applied and the policies will be ignored.
      See also
      <link linkend="sql-createpolicy"><command>CREATE POLICY</command></link>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-force-row-level-security">
    <term><literal>NO FORCE</literal>/<literal>FORCE ROW LEVEL SECURITY</literal></term>
    <listitem>
     <para>
      These forms control the application of row security policies belonging
      to the table when the user is the table owner.  If enabled, row-level
      security policies will be applied when the user is the table owner.  If
      disabled (the default) then row-level security will not be applied when
      the user is the table owner.
      See also
      <link linkend="sql-createpolicy"><command>CREATE POLICY</command></link>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-cluster-on">
    <term><literal>CLUSTER ON</literal></term>
    <listitem>
     <para>
      This form selects the default index for future
      <link linkend="sql-cluster"><command>CLUSTER</command></link>
      operations.  It does not actually re-cluster the table.
     </para>
     <para>
      Changing cluster options acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-without-cluster">
    <term><literal>SET WITHOUT CLUSTER</literal></term>
    <listitem>
     <para>
      This form removes the most recently used
      <link linkend="sql-cluster"><command>CLUSTER</command></link>
      index specification from the table.  This affects
      future cluster operations that don't specify an index.
     </para>
     <para>
      Changing cluster options acquires a <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-without-oids">
    <term><literal>SET WITHOUT OIDS</literal></term>
    <listitem>
     <para>
      Backward-compatible syntax for removing the <literal>oid</literal>
      system column.  As <literal>oid</literal> system columns cannot be
      added anymore, this never has an effect.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-access-method">
    <term><literal>SET ACCESS METHOD</literal></term>
    <listitem>
     <para>
      This form changes the access method of the table by rewriting it
      using the indicated access method; specifying
      <literal>DEFAULT</literal> selects the access method set as the
      <xref linkend="guc-default-table-access-method"/> configuration
      parameter.
      See <xref linkend="tableam"/> for more information.
     </para>
     <para>
      When applied to a partitioned table, there is no data to rewrite,
      but partitions created afterwards will default to the given access
      method unless overridden by a <literal>USING</literal> clause.
      Specifying <varname>DEFAULT</varname> removes a previous value,
      causing future partitions to default to
      <varname>default_table_access_method</varname>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-tablespace">
    <term><literal>SET TABLESPACE</literal></term>
    <listitem>
     <para>
      This form changes the table's tablespace to the specified tablespace and
      moves the data file(s) associated with the table to the new tablespace.
      Indexes on the table, if any, are not moved; but they can be moved
      separately with additional <literal>SET TABLESPACE</literal> commands.
      When applied to a partitioned table, nothing is moved, but any
      partitions created afterwards with
      <command>CREATE TABLE PARTITION OF</command> will use that tablespace,
      unless overridden by a <literal>TABLESPACE</literal> clause.
     </para>

     <para>
      All tables in the current database in a tablespace can be moved by using
      the <literal>ALL IN TABLESPACE</literal> form, which will lock all tables
      to be moved first and then move each one.  This form also supports
      <literal>OWNED BY</literal>, which will only move tables owned by the
      roles specified.  If the <literal>NOWAIT</literal> option is specified
      then the command will fail if it is unable to acquire all of the locks
      required immediately.  Note that system catalogs are not moved by this
      command; use <command>ALTER DATABASE</command> or explicit
      <command>ALTER TABLE</command> invocations instead if desired.  The
      <literal>information_schema</literal> relations are not considered part
      of the system catalogs and will be moved.
      See also
      <link linkend="sql-createtablespace"><command>CREATE TABLESPACE</command></link>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-logged-unlogged">
    <term><literal>SET { LOGGED | UNLOGGED }</literal></term>
    <listitem>
     <para>
      This form changes the table from unlogged to logged or vice-versa
      (see <xref linkend="sql-createtable-unlogged"/>).  It cannot be applied
      to a temporary table.
     </para>

     <para>
      This also changes the persistence of any sequences linked to the table
      (for identity or serial columns).  However, it is also possible to
      change the persistence of such sequences separately.
     </para>

     <para>
      This form is not supported for partitioned tables.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-storage-parameter">
    <term><literal>SET ( <replaceable class="parameter">storage_parameter</replaceable> [= <replaceable class="parameter">value</replaceable>] [, ... ] )</literal></term>
    <listitem>
     <para>
      This form changes one or more storage parameters for the table.  See
      <xref linkend="sql-createtable-storage-parameters"/> in the
      <link linkend="sql-createtable"><command>CREATE TABLE</command></link> documentation
      for details on the available parameters.  Note that the table contents
      will not be modified immediately by this command; depending on the
      parameter you might need to rewrite the table to get the desired effects.
      That can be done with <link linkend="sql-vacuum"><command>VACUUM
      FULL</command></link>, <link linkend="sql-cluster"><command>CLUSTER</command></link> or one of the forms
      of <command>ALTER TABLE</command> that forces a table rewrite.
      For planner related parameters, changes will take effect from the next
      time the table is locked so currently executing queries will not be
      affected.
     </para>

     <para>
      <literal>SHARE UPDATE EXCLUSIVE</literal> lock will be taken for
      fillfactor, TOAST and autovacuum storage parameters, as well as the
      planner parameter <varname>parallel_workers</varname>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-reset-storage-parameter">
    <term><literal>RESET ( <replaceable class="parameter">storage_parameter</replaceable> [, ... ] )</literal></term>
    <listitem>
     <para>
      This form resets one or more storage parameters to their
      defaults.  As with <literal>SET</literal>, a table rewrite might be
      needed to update the table entirely.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-inherit">
    <term><literal>INHERIT <replaceable class="parameter">parent_table</replaceable></literal></term>
    <listitem>
     <para>
      This form adds the target table as a new child of the specified parent
      table.  Subsequently, queries against the parent will include records
      of the target table.  To be added as a child, the target table must
      already contain all the same columns as the parent (it could have
      additional columns, too).  The columns must have matching data types.
     </para>

     <para>
      In addition, all <literal>CHECK</literal> and <literal>NOT NULL</literal>
      constraints on the parent must also exist on the child, except those
      marked non-inheritable (that is, created with
      <literal>ALTER TABLE ... ADD CONSTRAINT ... NO INHERIT</literal>), which
      are ignored.  All child-table constraints matched must not be marked
      non-inheritable.  Currently
      <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and
      <literal>FOREIGN KEY</literal> constraints are not considered, but
      this might change in the future.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-no-inherit">
    <term><literal>NO INHERIT <replaceable class="parameter">parent_table</replaceable></literal></term>
    <listitem>
     <para>
      This form removes the target table from the list of children of the
      specified parent table.
      Queries against the parent table will no longer include records drawn
      from the target table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-of">
    <term><literal>OF <replaceable class="parameter">type_name</replaceable></literal></term>
    <listitem>
     <para>
      This form links the table to a composite type as though <command>CREATE
      TABLE OF</command> had formed it.  The table's list of column names and types
      must precisely match that of the composite type.  The table must
      not inherit from any other table.  These restrictions ensure
      that <command>CREATE TABLE OF</command> would permit an equivalent table
      definition.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-not-of">
    <term><literal>NOT OF</literal></term>
    <listitem>
     <para>
      This form dissociates a typed table from its type.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-owner-to">
    <term><literal>OWNER TO</literal></term>
    <listitem>
     <para>
      This form changes the owner of the table, sequence, view, materialized view,
      or foreign table to the specified user.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-replica-identity">
    <term><literal>REPLICA IDENTITY</literal></term>
    <listitem>
     <para>
      This form changes the information which is written to the write-ahead log
      to identify rows which are updated or deleted.
      In most cases, the old value of each column is only logged if it differs
      from the new value; however, if the old value is stored externally, it is
      always logged regardless of whether it changed.
      This option has no effect except when logical replication is in use.
     <variablelist>
      <varlistentry id="sql-altertable-replica-identity-default">
       <term><literal>DEFAULT</literal></term>
       <listitem>
        <para>
         Records the old values of the columns of the primary key.
         This is the default for non-system tables.
         When there is no primary key, the behavior is the same as <literal>NOTHING</literal>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="sql-altertable-replica-identity-using-index">
       <term><literal>USING INDEX <replaceable class="parameter">index_name</replaceable></literal></term>
       <listitem>
        <para>
         Records the old values of the columns covered by the named index,
         that must be unique, not partial, not deferrable, and include only
         columns marked <literal>NOT NULL</literal>. If this index is
         dropped, the behavior is the same as <literal>NOTHING</literal>.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="sql-altertable-replica-identity-full">
       <term><literal>FULL</literal></term>
       <listitem>
        <para>
         Records the old values of all columns in the row.
        </para>
       </listitem>
      </varlistentry>

      <varlistentry id="sql-altertable-replica-identity-nothing">
       <term><literal>NOTHING</literal></term>
       <listitem>
        <para>
         Records no information about the old row. This is the default for
         system tables.
        </para>
       </listitem>
      </varlistentry>
     </variablelist></para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-rename">
    <term><literal>RENAME</literal></term>
    <listitem>
     <para>
      The <literal>RENAME</literal> forms change the name of a table
      (or an index, sequence, view, materialized view, or foreign table), the
      name of an individual column in a table, or the name of a constraint of
      the table.  When renaming a constraint that has an underlying index,
      the index is renamed as well.
      There is no effect on the stored data.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-schema">
    <term><literal>SET SCHEMA</literal></term>
    <listitem>
     <para>
      This form moves the table into another schema.  Associated indexes,
      constraints, and sequences owned by table columns are moved as well.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-attach-partition">
    <term><literal>ATTACH PARTITION <replaceable class="parameter">partition_name</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }</literal></term>
    <listitem>
     <para>
      This form attaches an existing table (which might itself be partitioned)
      as a partition of the target table. The table can be attached
      as a partition for specific values using <literal>FOR VALUES</literal>
      or as a default partition by using <literal>DEFAULT</literal>.
      For each index in the target table, if a valid equivalent index
      already exists in the partition, it will be attached to the target
      table's index, as if ALTER INDEX ATTACH PARTITION had been executed;
      otherwise, a new corresponding index will be created.  Invalid indexes
      on the partition are skipped.
      Note that if the existing table is a foreign table, it is currently not
      allowed to attach the table as a partition of the target table if there
      are <literal>UNIQUE</literal> indexes on the target table.  (See also
      <xref linkend="sql-createforeigntable"/>.)  For each user-defined
      row-level trigger that exists in the target table, a corresponding one
      is created in the attached table.
     </para>

     <para>
      A partition using <literal>FOR VALUES</literal> uses same syntax for
      <replaceable class="parameter">partition_bound_spec</replaceable> as
      <link linkend="sql-createtable"><command>CREATE TABLE</command></link>.
      The partition bound specification
      must correspond to the partitioning strategy and partition key of the
      target table.  The table to be attached must have all the same columns
      as the target table and no more; moreover, the column types must also
      match.  Also, it must have all the <literal>NOT NULL</literal> and
      <literal>CHECK</literal> constraints of the target table, not marked
      <literal>NO INHERIT</literal>.  Currently
      <literal>FOREIGN KEY</literal> constraints are not considered.
      <literal>UNIQUE</literal> and <literal>PRIMARY KEY</literal> constraints
      from the parent table will be created in the partition, if they don't
      already exist.
     </para>

     <para>
      If the new partition is a regular table, a full table scan is performed
      to check that existing rows in the table do not violate the partition
      constraint. It is possible to avoid this scan by adding a valid
      <literal>CHECK</literal> constraint to the table that allows only
      rows satisfying the desired partition constraint before running this
      command. The <literal>CHECK</literal> constraint will be used to
      determine that the table need not be scanned to validate the partition
      constraint. This does not work, however, if any of the partition keys
      is an expression and the partition does not accept
      <literal>NULL</literal> values. If attaching a list partition that will
      not accept <literal>NULL</literal> values, also add a
      <literal>NOT NULL</literal> constraint to the partition key column,
      unless it's an expression.
     </para>

     <para>
      If the new partition is a foreign table, nothing is done to verify
      that all the rows in the foreign table obey the partition constraint.
      (See the discussion in <xref linkend="sql-createforeigntable"/> about
      constraints on the foreign table.)
     </para>

     <para>
      When a table has a default partition, defining a new partition changes
      the partition constraint for the default partition. The default
      partition can't contain any rows that would need to be moved to the new
      partition, and will be scanned to verify that none are present. This
      scan, like the scan of the new partition, can be avoided if an
      appropriate <literal>CHECK</literal> constraint is present. Also like
      the scan of the new partition, it is always skipped when the default
      partition is a foreign table.
     </para>

     <para>
      Attaching a partition acquires a
      <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the parent table,
      in addition to the <literal>ACCESS EXCLUSIVE</literal> locks on the table
      being attached and on the default partition (if any).
     </para>

     <para>
      Further locks must also be held on all sub-partitions if the table being
      attached is itself a partitioned table.  Likewise if the default
      partition is itself a partitioned table.  The locking of the
      sub-partitions can be avoided by adding a <literal>CHECK</literal>
      constraint as described in
      <xref linkend="ddl-partitioning-declarative-maintenance"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-detach-partition">
    <term><literal>DETACH PARTITION <replaceable class="parameter">partition_name</replaceable> [ CONCURRENTLY | FINALIZE ]</literal></term>

    <listitem>
     <para>
      This form detaches the specified partition of the target table.  The detached
      partition continues to exist as a standalone table, but no longer has any
      ties to the table from which it was detached.  Any indexes that were
      attached to the target table's indexes are detached.  Any triggers that
      were created as clones of those in the target table are removed.
      <literal>SHARE</literal> lock is obtained on any tables that reference
      this partitioned table in foreign key constraints.
     </para>
     <para>
      If <literal>CONCURRENTLY</literal> is specified, it runs using a reduced
      lock level to avoid blocking other sessions that might be accessing the
      partitioned table.  In this mode, two transactions are used internally.
      During the first transaction, a <literal>SHARE UPDATE EXCLUSIVE</literal>
      lock is taken on both parent table and partition, and the partition is
      marked as undergoing detach; at that point, the transaction is committed
      and all other transactions using the partitioned table are waited for.
      Once all those transactions have completed, the second transaction
      acquires <literal>SHARE UPDATE EXCLUSIVE</literal> on the partitioned
      table and <literal>ACCESS EXCLUSIVE</literal> on the partition,
      and the detach process completes.  A <literal>CHECK</literal> constraint
      that duplicates the partition constraint is added to the partition.
      <literal>CONCURRENTLY</literal> cannot be run in a transaction block and
      is not allowed if the partitioned table contains a default partition.
     </para>
     <para>
      If <literal>FINALIZE</literal> is specified, a previous
      <literal>DETACH CONCURRENTLY</literal> invocation that was canceled or
      interrupted is completed.
      At most one partition in a partitioned table can be pending detach at
      a time.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-merge-partitions">
    <term><literal>MERGE PARTITIONS (<replaceable class="parameter">partition_name1</replaceable>, <replaceable class="parameter">partition_name2</replaceable> [, ...]) INTO <replaceable class="parameter">partition_name</replaceable></literal></term>

    <listitem>
     <para>
      This form merges several partitions of the target table into a new partition.
      Hash-partitioned target table is not supported.
      Only simple, non-partitioned partitions can be merged.
      The new partition (<replaceable class="parameter">partition_name</replaceable>)
      can have the same name as one of the merged partitions
      (<literal><replaceable class="parameter">partition_name1</replaceable>,
      <replaceable class="parameter">partition_name2</replaceable> [, ...]</literal>).
     </para>

     <para>
      If the <literal>DEFAULT</literal> partition is not in the
      list of merged partitions:
      <itemizedlist>
       <listitem>
        <para>
         For range-partitioned tables, the ranges of merged partitions
         must be adjacent; this applies even if the partitioned table
         has no default partition.
         The partition bounds of merged partitions are combined to form the new partition bound for
         <replaceable class="parameter">partition_name</replaceable>.
        </para>
       </listitem>
       <listitem>
        <para>
         For list-partitioned tables, the partition bounds of
         merged partitions are combined to form the new partition bound for
         <replaceable class="parameter">partition_name</replaceable>.
        </para>
       </listitem>
      </itemizedlist>
      If the <literal>DEFAULT</literal> partition is in the list of merged partitions:
      <itemizedlist>
       <listitem>
        <para>
         The partition <replaceable class="parameter">partition_name</replaceable>
         will be the new <literal>DEFAULT</literal> partition of the target table.
        </para>
       </listitem>
       <listitem>
        <para>
         The partition bound specifications for merged partitions can be arbitrary.
        </para>
       </listitem>
      </itemizedlist>
     </para>
     <para>
      All merged partitions must have the same owner.
      The owner of merged partitions will be the owner of the new partition.
      It is the user's responsibility to setup <acronym>ACL</acronym> on
      the new partition.
     </para>

     <para>
       <command>ALTER TABLE MERGE PARTITION</command> uses the partitioned
       table itself as the template to construct the new partition.
       The new partition inherits the table access method and persistence type
       of the partitioned table.  Its tablespace is selected as for a
       <command>CREATE TABLE ... PARTITION OF</command> command issued
       without a <literal>TABLESPACE</literal> clause: if the partitioned
       table has an explicit tablespace, the new partition uses it;
       otherwise the value of <xref linkend="guc-default-tablespace"/> is
       taken into account, falling back to the database's default tablespace.

       Constraints, column defaults, column generation expressions, identity
       columns, indexes, and triggers are copied from the partitioned table to
       the new partition.  But extended statistics, security policies, etc,
       won't be copied from the partitioned table.
       Indexes and identity columns copied from the partitioned table will be
       created afterward, once the data has been moved into the new partition.
     </para>

     <para>
      When partitions are merged, any objects depending on this partition,
      such as constraints, triggers, extended statistics, etc, will be
      dropped.
      Eventually, we will drop all the merged partitions
      (using <literal>RESTRICT</literal> mode) too;  therefore, if any objects
      are still dependent on them,
      <command>ALTER TABLE MERGE PARTITION</command> would fail.
      (see <xref linkend="ddl-depend"/>).
     </para>

     <para>
      Extension dependencies on partition indexes (created via
      <link linkend="sql-alterindex"><command>ALTER INDEX ... DEPENDS ON
      EXTENSION</command></link>) are preserved during merge operations.
      All source partition indexes must have the same extension dependencies;
      if they differ, an error is raised. This ensures that extension
      dependencies are not silently lost during merge.
     </para>

     <note>
      <para>
       Merging partitions acquires an <literal>ACCESS EXCLUSIVE</literal> lock on
       the parent table, in addition to the <literal>ACCESS EXCLUSIVE</literal>
       locks on the tables being merged and on the default partition (if any).
      </para>
     </note>
     <note>
      <para>
       <command>ALTER TABLE MERGE PARTITIONS</command> creates a new partition and
       moves data from all merging partitions into it, which can take a long time.
       So it is not recommended to use the command to merge very big partitions
       with small ones.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-split-partition">
    <term>
    <literal>SPLIT PARTITION <replaceable class="parameter">partition_name</replaceable> INTO (
      PARTITION <replaceable class="parameter">partition_name1</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT },
      PARTITION <replaceable class="parameter">partition_name2</replaceable> { FOR VALUES <replaceable class="parameter">partition_bound_spec</replaceable> | DEFAULT }
      [, ...])</literal>
    </term>

    <listitem>
     <para>
      This form splits a single partition of the target table into new
      partitions.  Hash-partitioned target tables are not supported.
      Only a simple, non-partitioned partition can be split.
      If the split partition is the <literal>DEFAULT</literal> partition,
      one of the new partitions must be <literal>DEFAULT</literal>.
      If the partitioned table does not have a <literal>DEFAULT</literal>
      partition, a <literal>DEFAULT</literal> partition can be defined as one
      of the new partitions.
     </para>

     <para>
      The bounds of new non-<literal>DEFAULT</literal> partitions must not
      overlap with those of new or existing partitions, except
      <replaceable class="parameter">partition_name</replaceable>, and must be
      contained within the bounds of the split partition
      <replaceable class="parameter">partition_name</replaceable>.
      If no new <literal>DEFAULT</literal> partition is specified, the
      combined bounds of the new partitions
      <literal>
      <replaceable class="parameter">partition_name1</replaceable>,
      <replaceable class="parameter">partition_name2</replaceable>[, ...]
      </literal> must exactly match the bounds of the split partition
      <replaceable class="parameter">partition_name</replaceable>.
      One of the new partitions can have the same name as the split partition
      <replaceable class="parameter">partition_name</replaceable>.
      This is useful when splitting the <literal>DEFAULT</literal> partition,
      so that after the split, the <literal>DEFAULT</literal> partition
      keeps the same name but its partition bound changes.
     </para>

     <para>
      New partitions will have the same owner as the parent partition.
      It is the user's responsibility to setup <acronym>ACL</acronym> on new
      partitions.
     </para>

     <para>
       <command>ALTER TABLE SPLIT PARTITION</command> uses the partitioned
       table itself as the template to construct new partitions.
       New partitions inherit the table access method and persistence type of
       the partitioned table.  Their tablespace is selected as for a
       <command>CREATE TABLE ... PARTITION OF</command> command issued
       without a <literal>TABLESPACE</literal> clause: if the partitioned
       table has an explicit tablespace, the new partitions use it;
       otherwise the value of <xref linkend="guc-default-tablespace"/> is
       taken into account, falling back to the database's default tablespace.
     </para>

     <para>
       Constraints, column defaults, column generation expressions,
       identity columns, indexes, and triggers are copied from the partitioned
       table to the new partitions.  But extended statistics, security
       policies, etc, won't be copied from the partitioned table.
       Indexes and identity columns copied from the partitioned table will be
       created afterward, once the data has been moved into the new partitions.
     </para>

     <para>
      When a partition is split, any objects that depend on this partition,
      such as constraints, triggers, extended statistics, etc, will be dropped.
      This occurs because <command>ALTER TABLE SPLIT PARTITION</command> uses
      the partitioned table itself as the template to reconstruct these
      objects later.
      Eventually, we will drop the split partition
      (using <literal>RESTRICT</literal> mode) too;  therefore, if any objects
      are still dependent on it, <command>ALTER TABLE SPLIT PARTITION</command>
      would fail (see <xref linkend="ddl-depend"/>).
     </para>

     <para>
      Extension dependencies on partition indexes (created via
      <link linkend="sql-alterindex"><command>ALTER INDEX ... DEPENDS ON
      EXTENSION</command></link>) are preserved during split operations.
      The new partitions' indexes will inherit the extension dependencies
      from the source partition's indexes.
     </para>

     <note>
      <para>
       Split partition acquires an <literal>ACCESS EXCLUSIVE</literal> lock on
       the parent table, in addition to the <literal>ACCESS EXCLUSIVE</literal>
       lock on the table being split.
      </para>
     </note>
     <note>

      <para>
       <command>ALTER TABLE SPLIT PARTITION</command> creates new partitions and
       moves data from the split partition into them, which can take a long
       time.  So it is not recommended to use the command for splitting a
       small fraction of rows out of a very big partition.
      </para>
     </note>
    </listitem>
   </varlistentry>

  </variablelist>
  </para>

  <para>
   All the forms of <command>ALTER TABLE</command> that act on a single table,
   except <literal>RENAME</literal>, <literal>SET SCHEMA</literal>,
   <literal>ATTACH PARTITION</literal>, <literal>DETACH PARTITION</literal>,
   <literal>MERGE PARTITIONS</literal>, and <literal>SPLIT PARTITION</literal>
   can be combined into
   a list of multiple alterations to be applied together.  For example, it
   is possible to add several columns and/or alter the type of several
   columns in a single command.  This is particularly useful with large
   tables, since only one pass over the table needs to be made.
  </para>

  <para>
   You must own the table to use <command>ALTER TABLE</command>.
   To change the schema or tablespace of a table, you must also have
   <literal>CREATE</literal> privilege on the new schema or tablespace.
   To add the table as a new child of a parent table, you must own the parent
   table as well.  Also, to attach a table as a new partition of the table,
   you must own the table being attached.
   To alter the owner, you must be able to <literal>SET ROLE</literal> to the
   new owning role, and that role must have <literal>CREATE</literal>
   privilege on the table's schema.
   (These restrictions enforce that altering the owner
   doesn't do anything you couldn't do by dropping and recreating the table.
   However, a superuser can alter ownership of any table anyway.)
   To add a column or alter a column type or use the <literal>OF</literal>
   clause, you must also have <literal>USAGE</literal> privilege on the data
   type.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>

     <varlistentry id="sql-altertable-parms-if-exists">
      <term><literal>IF EXISTS</literal></term>
      <listitem>
       <para>
        Do not throw an error if the table does not exist. A notice is issued
        in this case.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-name">
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
        The name (optionally schema-qualified) of an existing table to
        alter. If <literal>ONLY</literal> is specified before the table name, only
        that table is altered. If <literal>ONLY</literal> is not specified, the table
        and all its descendant tables (if any) are altered.  Optionally,
        <literal>*</literal> can be specified after the table name to explicitly
        indicate that descendant tables are included.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-column-name">
      <term><replaceable class="parameter">column_name</replaceable></term>
      <listitem>
       <para>
        Name of a new or existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-new-column-name">
      <term><replaceable class="parameter">new_column_name</replaceable></term>
      <listitem>
       <para>
        New name for an existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-new-name">
      <term><replaceable class="parameter">new_name</replaceable></term>
      <listitem>
       <para>
        New name for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-data-type">
      <term><replaceable class="parameter">data_type</replaceable></term>
      <listitem>
       <para>
        Data type of the new column, or new data type for an existing
        column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-table-constraint">
      <term><replaceable class="parameter">table_constraint</replaceable></term>
      <listitem>
       <para>
        New table constraint for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-constraint-name">
      <term><replaceable class="parameter">constraint_name</replaceable></term>
      <listitem>
       <para>
        Name of a new or existing constraint.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-cascade">
      <term><literal>CASCADE</literal></term>
      <listitem>
       <para>
        Automatically drop objects that depend on the dropped column
        or constraint (for example, views referencing the column),
        and in turn all objects that depend on those objects
        (see <xref linkend="ddl-depend"/>).
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-restrict">
      <term><literal>RESTRICT</literal></term>
      <listitem>
       <para>
        Refuse to drop the column or constraint if there are any dependent
        objects. This is the default behavior.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-trigger-name">
      <term><replaceable class="parameter">trigger_name</replaceable></term>
      <listitem>
       <para>
        Name of a single trigger to disable or enable.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-all">
      <term><literal>ALL</literal></term>
      <listitem>
       <para>
        Disable or enable all triggers belonging to the table.
        (This requires superuser privilege if any of the triggers are
        internally generated constraint triggers, such as those that are used
        to implement foreign key constraints or deferrable uniqueness and
        exclusion constraints.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-user">
      <term><literal>USER</literal></term>
      <listitem>
       <para>
        Disable or enable all triggers belonging to the table except for
        internally generated constraint triggers, such as those that are used
        to implement foreign key constraints or deferrable uniqueness and
        exclusion constraints.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-index-name">
      <term><replaceable class="parameter">index_name</replaceable></term>
      <listitem>
       <para>
        The name of an existing index.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-storage-parameter">
      <term><replaceable class="parameter">storage_parameter</replaceable></term>
      <listitem>
       <para>
        The name of a table storage parameter.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-value">
      <term><replaceable class="parameter">value</replaceable></term>
      <listitem>
       <para>
        The new value for a table storage parameter.
        This might be a number or a word depending on the parameter.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-parent-table">
      <term><replaceable class="parameter">parent_table</replaceable></term>
      <listitem>
       <para>
        A parent table to associate or de-associate with this table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-new-owner">
      <term><replaceable class="parameter">new_owner</replaceable></term>
      <listitem>
       <para>
        The user name of the new owner of the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-new-access-method">
      <term><replaceable class="parameter">new_access_method</replaceable></term>
      <listitem>
       <para>
        The name of the access method to which the table will be converted.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-new-tablespace">
      <term><replaceable class="parameter">new_tablespace</replaceable></term>
      <listitem>
       <para>
        The name of the tablespace to which the table will be moved.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-new-schema">
      <term><replaceable class="parameter">new_schema</replaceable></term>
      <listitem>
       <para>
        The name of the schema to which the table will be moved.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-partition-name">
      <term><replaceable class="parameter">partition_name</replaceable></term>
      <listitem>
       <para>
        The name of the table to attach as a new partition or to detach from this table,
        or the name of split partition, or the name of the new merged partition.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-partition-name1">
      <term><replaceable class="parameter">partition_name1</replaceable></term>
      <term><replaceable class="parameter">partition_name2</replaceable></term>
      <listitem>
       <para>
        The names of the tables being merged into the new partition or split into
        new partitions.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry id="sql-altertable-parms-partition-bound-spec">
      <term><replaceable class="parameter">partition_bound_spec</replaceable></term>
      <listitem>
       <para>
        The partition bound specification for a new partition.  Refer to
        <xref linkend="sql-createtable"/> for more details on the syntax of the same.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>
 </refsect1>

 <refsect1 id="sql-altertable-notes" xreflabel="Notes">
  <title>Notes</title>

   <para>
    The key word <literal>COLUMN</literal> is noise and can be omitted.
   </para>

   <para>
    When a column is added with <literal>ADD COLUMN</literal> and a
    non-volatile <literal>DEFAULT</literal> is specified, the default value is
    evaluated at the time of the statement and the result stored in the
    table's metadata, where it will be returned when any existing rows are
    accessed.  The value will be only applied when the table is rewritten,
    making the <command>ALTER TABLE</command> very fast even on large tables.
    If no column constraints are specified, NULL is used as the
    <literal>DEFAULT</literal>.  In neither case is a rewrite of the table
    required.
   </para>

   <para>
    Adding a column with a volatile <literal>DEFAULT</literal>
    (e.g., <function>clock_timestamp()</function>), a stored generated column,
    an identity column, or a column with a domain data type that has
    constraints will cause the entire table and its indexes to be rewritten.
    Adding a virtual generated column never requires a rewrite.
   </para>

   <para>
    Changing the type of an existing column will normally cause the entire table
    and its indexes to be rewritten.
    As an exception, when changing the type of an existing column,
    if the <literal>USING</literal> clause does not change the column
    contents and the old type is either binary coercible to the new type
    or an unconstrained domain over the new type, a table rewrite is not
    needed.  However, indexes will still be rebuilt unless the system
    can verify that the new index would be logically equivalent to the
    existing one.  For example, if the collation for a column has been
    changed, an index rebuild is required because the new sort
    order might be different.  However, in the absence of a collation
    change, a column can be changed from <type>text</type> to
    <type>varchar</type> (or vice versa) without rebuilding the indexes
    because these data types sort identically.
   </para>

   <para>
    Table and/or index
    rebuilds may take a significant amount of time for a large table,
    and will temporarily require as much as double the disk space.
   </para>

   <para>
    Adding a <literal>CHECK</literal> or <literal>NOT NULL</literal>
    constraint requires scanning the table to verify that existing rows meet the
    constraint, but does not require a table rewrite.  If a <literal>CHECK</literal>
    constraint is added as <literal>NOT ENFORCED</literal>, no verification will
    be performed.
   </para>

   <para>
    Similarly, when attaching a new partition it may be scanned to verify that
    existing rows meet the partition constraint.
   </para>

   <para>
    The main reason for providing the option to specify multiple changes
    in a single <command>ALTER TABLE</command> is that multiple table scans or
    rewrites can thereby be combined into a single pass over the table.
   </para>

   <para>
    Scanning a large table to verify new foreign-key, check, or not-null constraints
    can take a long time, and other updates to the table are locked out
    until the <command>ALTER TABLE ADD CONSTRAINT</command> command is
    committed.  The main purpose of the <literal>NOT VALID</literal>
    constraint option is to reduce the impact of adding a constraint on
    concurrent updates.  With <literal>NOT VALID</literal>,
    the <command>ADD CONSTRAINT</command> command does not scan the table
    and can be committed immediately.  After that, a <literal>VALIDATE
    CONSTRAINT</literal> command can be issued to verify that existing rows
    satisfy the constraint.  The validation step does not need to lock out
    concurrent updates, since it knows that other transactions will be
    enforcing the constraint for rows that they insert or update; only
    pre-existing rows need to be checked.  Hence, validation acquires only
    a <literal>SHARE UPDATE EXCLUSIVE</literal> lock on the table being
    altered.  (If the constraint is a foreign key then a <literal>ROW
    SHARE</literal> lock is also required on the table referenced by the
    constraint.)  In addition to improving concurrency, it can be useful to
    use <literal>NOT VALID</literal> and <literal>VALIDATE
    CONSTRAINT</literal> in cases where the table is known to contain
    pre-existing violations.  Once the constraint is in place, no new
    violations can be inserted, and the existing problems can be corrected
    at leisure until <literal>VALIDATE CONSTRAINT</literal> finally
    succeeds.
   </para>

   <para>
    The <literal>DROP COLUMN</literal> form does not physically remove
    the column, but simply makes it invisible to SQL operations.  Subsequent
    insert and update operations in the table will store a null value for the
    column. Thus, dropping a column is quick but it will not immediately
    reduce the on-disk size of your table, as the space occupied
    by the dropped column is not reclaimed.  The space will be
    reclaimed over time as existing rows are updated.
   </para>

   <para>
    To force immediate reclamation of space occupied by a dropped column,
    you can execute one of the forms of <command>ALTER TABLE</command> that
    performs a rewrite of the whole table.  This results in reconstructing
    each row with the dropped column replaced by a null value.
   </para>

   <para>
    The rewriting forms of <command>ALTER TABLE</command> are not MVCC-safe.
    After a table rewrite, the table will appear empty to concurrent
    transactions, if they are using a snapshot taken before the rewrite
    occurred.  See <xref linkend="mvcc-caveats"/> for more details.
   </para>

   <para>
    The <literal>USING</literal> option of <literal>SET DATA TYPE</literal> can actually
    specify any expression involving the old values of the row; that is, it
    can refer to other columns as well as the one being converted.  This allows
    very general conversions to be done with the <literal>SET DATA TYPE</literal>
    syntax.  Because of this flexibility, the <literal>USING</literal>
    expression is not applied to the column's default value (if any); the
    result might not be a constant expression as required for a default.
    This means that when there is no implicit or assignment cast from old to
    new type, <literal>SET DATA TYPE</literal> might fail to convert the default even
    though a <literal>USING</literal> clause is supplied.  In such cases,
    drop the default with <literal>DROP DEFAULT</literal>, perform the <literal>ALTER
    TYPE</literal>, and then use <literal>SET DEFAULT</literal> to add a suitable new
    default.  Similar considerations apply to indexes and constraints involving
    the column.
   </para>

   <para>
    If a table has any descendant tables, it is not permitted to add,
    rename, or change the type of a column in the parent table without doing
    the same to the descendants.  This ensures that the descendants always
    have columns matching the parent.  Similarly, a <literal>CHECK</literal>
    constraint cannot be renamed in the parent without also renaming it in
    all descendants, so that <literal>CHECK</literal> constraints also match
    between the parent and its descendants.  (That restriction does not apply
    to index-based constraints, however.)
    Also, because selecting from the parent also selects from its descendants,
    a constraint on the parent cannot be marked valid unless it is also marked
    valid for those descendants.  In all of these cases, <command>ALTER TABLE
    ONLY</command> will be rejected.
   </para>

   <para>
    A recursive <literal>DROP COLUMN</literal> operation will remove a
    descendant table's column only if the descendant does not inherit
    that column from any other parents and never had an independent
    definition of the column.  A nonrecursive <literal>DROP
    COLUMN</literal> (i.e., <command>ALTER TABLE ONLY ... DROP
    COLUMN</command>) never removes any descendant columns, but
    instead marks them as independently defined rather than inherited.
    A nonrecursive <literal>DROP COLUMN</literal> command will fail for a
    partitioned table, because all partitions of a table must have the same
    columns as the partitioning root.
   </para>

   <para>
    The actions for identity columns (<literal>ADD
    GENERATED</literal>, <literal>SET</literal> etc., <literal>DROP
    IDENTITY</literal>), as well as the actions
    <literal>CLUSTER</literal>, <literal>OWNER</literal>,
    and <literal>TABLESPACE</literal> never recurse to descendant tables;
    that is, they always act as though <literal>ONLY</literal> were specified.
    Actions affecting trigger states recurse to partitions of partitioned
    tables (unless <literal>ONLY</literal> is specified), but never to
    traditional-inheritance descendants.
    Adding a constraint recurses only for <literal>CHECK</literal> constraints
    that are not marked <literal>NO INHERIT</literal>.
   </para>

   <para>
    Changing any part of a system catalog table is not permitted.
   </para>

   <para>
    Refer to <xref linkend="sql-createtable"/> for a further description of valid
    parameters. <xref linkend="ddl"/> has further information on
    inheritance.
   </para>
 </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   To add a column of type <type>varchar</type> to a table:
<programlisting>
ALTER TABLE distributors ADD COLUMN address varchar(30);
</programlisting>
   That will cause all existing rows in the table to be filled with null
   values for the new column.
  </para>

  <para>
   To add a column with a non-null default:
<programlisting>
ALTER TABLE measurements
  ADD COLUMN mtime timestamp with time zone DEFAULT now();
</programlisting>
   Existing rows will be filled with the current time as the value of the
   new column, and then new rows will receive the time of their insertion.
  </para>

  <para>
   To add a column and fill it with a value different from the default to
   be used later:
<programlisting>
ALTER TABLE transactions
  ADD COLUMN status varchar(30) DEFAULT 'old',
  ALTER COLUMN status SET DEFAULT 'current';
</programlisting>
   Existing rows will be filled with <literal>old</literal>, but then
   the default for subsequent commands will be <literal>current</literal>.
   The effects are the same as if the two sub-commands had been issued
   in separate <command>ALTER TABLE</command> commands.
  </para>

  <para>
   To drop a column from a table:
<programlisting>
ALTER TABLE distributors DROP COLUMN address RESTRICT;
</programlisting>
  </para>

  <para>
   To change the types of two existing columns in one operation:
<programlisting>
ALTER TABLE distributors
    ALTER COLUMN address TYPE varchar(80),
    ALTER COLUMN name TYPE varchar(100);
</programlisting>
  </para>

  <para>
   To change an integer column containing Unix timestamps to <type>timestamp
   with time zone</type> via a <literal>USING</literal> clause:
<programlisting>
ALTER TABLE foo
    ALTER COLUMN foo_timestamp SET DATA TYPE timestamp with time zone
    USING
        timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';
</programlisting>
  </para>

  <para>
   The same, when the column has a default expression that won't automatically
   cast to the new data type:
<programlisting>
ALTER TABLE foo
    ALTER COLUMN foo_timestamp DROP DEFAULT,
    ALTER COLUMN foo_timestamp TYPE timestamp with time zone
    USING
        timestamp with time zone 'epoch' + foo_timestamp * interval '1 second',
    ALTER COLUMN foo_timestamp SET DEFAULT now();
</programlisting>
  </para>

  <para>
   To rename an existing column:
<programlisting>
ALTER TABLE distributors RENAME COLUMN address TO city;
</programlisting>
  </para>

  <para>
   To rename an existing table:
<programlisting>
ALTER TABLE distributors RENAME TO suppliers;
</programlisting>
  </para>

  <para>
   To rename an existing constraint:
<programlisting>
ALTER TABLE distributors RENAME CONSTRAINT zipchk TO zip_check;
</programlisting>
  </para>

  <para>
   To add a not-null constraint to a column:
<programlisting>
ALTER TABLE distributors ALTER COLUMN street SET NOT NULL;
</programlisting>
   To remove a not-null constraint from a column:
<programlisting>
ALTER TABLE distributors ALTER COLUMN street DROP NOT NULL;
</programlisting>
  </para>

  <para>
   To add a check constraint to a table and all its children:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5);
</programlisting>
  </para>

  <para>
   To add a check constraint only to a table and not to its children:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT zipchk CHECK (char_length(zipcode) = 5) NO INHERIT;
</programlisting>
   (The check constraint will not be inherited by future children, either.)
  </para>

  <para>
   To remove a check constraint from a table and all its children:
<programlisting>
ALTER TABLE distributors DROP CONSTRAINT zipchk;
</programlisting>
  </para>

  <para>
   To remove a check constraint from one table only:
<programlisting>
ALTER TABLE ONLY distributors DROP CONSTRAINT zipchk;
</programlisting>
   (The check constraint remains in place for any child tables.)
  </para>

  <para>
   To add a foreign key constraint to a table:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address);
</programlisting>
  </para>

  <para>
   To add a foreign key constraint to a table with the least impact on other work:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT distfk FOREIGN KEY (address) REFERENCES addresses (address) NOT VALID;
ALTER TABLE distributors VALIDATE CONSTRAINT distfk;
</programlisting>
  </para>

  <para>
   To add a (multicolumn) unique constraint to a table:
<programlisting>
ALTER TABLE distributors ADD CONSTRAINT dist_id_zipcode_key UNIQUE (dist_id, zipcode);
</programlisting>
  </para>

  <para>
   To add an automatically named primary key constraint to a table, noting
   that a table can only ever have one primary key:
<programlisting>
ALTER TABLE distributors ADD PRIMARY KEY (dist_id);
</programlisting>
  </para>

  <para>
   To move a table to a different tablespace:
<programlisting>
ALTER TABLE distributors SET TABLESPACE fasttablespace;
</programlisting>
  </para>

  <para>
   To move a table to a different schema:
<programlisting>
ALTER TABLE myschema.distributors SET SCHEMA yourschema;
</programlisting>
  </para>

  <para>
   To recreate a primary key constraint, without blocking updates while the
   index is rebuilt:
<programlisting>
CREATE UNIQUE INDEX CONCURRENTLY dist_id_temp_idx ON distributors (dist_id);
ALTER TABLE distributors DROP CONSTRAINT distributors_pkey,
    ADD CONSTRAINT distributors_pkey PRIMARY KEY USING INDEX dist_id_temp_idx;
</programlisting></para>

  <para>
   To attach a partition to a range-partitioned table:
<programlisting>
ALTER TABLE measurement
    ATTACH PARTITION measurement_y2016m07 FOR VALUES FROM ('2016-07-01') TO ('2016-08-01');
</programlisting></para>

  <para>
   To attach a partition to a list-partitioned table:
<programlisting>
ALTER TABLE cities
    ATTACH PARTITION cities_ab FOR VALUES IN ('a', 'b');
</programlisting></para>

  <para>
   To attach a partition to a hash-partitioned table:
<programlisting>
ALTER TABLE orders
    ATTACH PARTITION orders_p4 FOR VALUES WITH (MODULUS 4, REMAINDER 3);
</programlisting></para>

  <para>
   To attach a default partition to a partitioned table:
<programlisting>
ALTER TABLE cities
    ATTACH PARTITION cities_partdef DEFAULT;
</programlisting></para>

  <para>
   To detach a partition from a partitioned table:
<programlisting>
ALTER TABLE measurement
    DETACH PARTITION measurement_y2015m12;
</programlisting></para>

  <para>
   To split a single partition of the range-partitioned table:
<programlisting>
ALTER TABLE sales_range SPLIT PARTITION sales_feb_mar_apr2023 INTO
   (PARTITION sales_feb2023 FOR VALUES FROM ('2023-02-01') TO ('2023-03-01'),
    PARTITION sales_mar2023 FOR VALUES FROM ('2023-03-01') TO ('2023-04-01'),
    PARTITION sales_apr2023 FOR VALUES FROM ('2023-04-01') TO ('2023-05-01'));
</programlisting></para>

  <para>
   To split a single partition of the list-partitioned table:
<programlisting>
ALTER TABLE sales_list SPLIT PARTITION sales_all INTO
   (PARTITION sales_west FOR VALUES IN ('Lisbon', 'New York', 'Madrid'),
    PARTITION sales_east FOR VALUES IN ('Beijing', 'Delhi', 'Vladivostok'),
    PARTITION sales_central FOR VALUES IN ('Warsaw', 'Berlin', 'Kyiv'));
</programlisting></para>

  <para>
   To merge several partitions into one partition of the target table:
<programlisting>
ALTER TABLE sales_list MERGE PARTITIONS (sales_west, sales_east, sales_central)
    INTO sales_all;
</programlisting></para>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The forms <literal>ADD [COLUMN]</literal>,
   <literal>DROP [COLUMN]</literal>, <literal>DROP IDENTITY</literal>, <literal>RESTART</literal>,
   <literal>SET DEFAULT</literal>, <literal>SET DATA TYPE</literal> (without <literal>USING</literal>),
   <literal>SET GENERATED</literal>, and <literal>SET <replaceable>sequence_option</replaceable></literal>
   conform with the SQL standard.
   The form <literal>ADD <replaceable>table_constraint</replaceable></literal>
   conforms with the SQL standard when the <literal>USING INDEX</literal> and
   <literal>NOT VALID</literal> clauses are omitted and the constraint type is
   one of <literal>CHECK</literal>, <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>,
   or <literal>REFERENCES</literal>.
   The other forms are
   <productname>PostgreSQL</productname> extensions of the SQL standard.
   Also, the ability to specify more than one manipulation in a single
   <command>ALTER TABLE</command> command is an extension.
  </para>

  <para>
   <command>ALTER TABLE DROP COLUMN</command> can be used to drop the only
   column of a table, leaving a zero-column table.  This is an
   extension of SQL, which disallows zero-column tables.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-createtable"/></member>
  </simplelist>
 </refsect1>
</refentry>