<!--
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>

<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> 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>
    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 }
    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 ]
    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 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_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">numeric_literal</replaceable> | <replaceable class="parameter">string_literal</replaceable> | TRUE | FALSE | NULL } [, ...] ) |
FROM ( { <replaceable class="parameter">numeric_literal</replaceable> | <replaceable class="parameter">string_literal</replaceable> | TRUE | FALSE | MINVALUE | MAXVALUE } [, ...] )
  TO ( { <replaceable class="parameter">numeric_literal</replaceable> | <replaceable class="parameter">string_literal</replaceable> | TRUE | FALSE | 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 |
  NULL |
  CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] |
  DEFAULT <replaceable>default_expr</replaceable> |
  GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <replaceable>sequence_options</replaceable> ) ] |
  UNIQUE <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">action</replaceable> ] [ ON UPDATE <replaceable class="parameter">action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

<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 ] |
  UNIQUE ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <replaceable class="parameter">index_parameters</replaceable> |
  PRIMARY KEY ( <replaceable class="parameter">column_name</replaceable> [, ... ] ) <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> [, ... ] ) REFERENCES <replaceable class="parameter">reftable</replaceable> [ ( <replaceable class="parameter">refcolumn</replaceable> [, ... ] ) ]
    [ MATCH FULL | MATCH PARTIAL | MATCH SIMPLE ] [ ON DELETE <replaceable class="parameter">action</replaceable> ] [ ON UPDATE <replaceable class="parameter">action</replaceable> ] }
[ DEFERRABLE | NOT DEFERRABLE ] [ INITIALLY DEFERRED | INITIALLY IMMEDIATE ]

<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> ) } [ <replaceable class="parameter">opclass</replaceable> ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ]
</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 held
   unless explicitly noted. When multiple subcommands are listed, the lock
   held will be the strictest one required from any subcommand.

  <variablelist>
   <varlistentry>
    <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
      <xref linkend="sql-createtable"/>. If <literal>IF NOT EXISTS</literal>
      is specified and a column already exists with this name,
      no error is thrown.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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>
    <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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET</literal>/<literal>DROP DEFAULT</literal></term>
    <listitem>
     <para>
      These forms set or remove the default value for a column.
      Default values only apply in subsequent <command>INSERT</command>
      or <command>UPDATE</command> commands; they do not cause rows already in the
      table to change.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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.  You can only use <literal>SET
      NOT NULL</literal> when the column contains no null values.
     </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.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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 <xref linkend="sql-createtable"/> for details.
     </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>
    <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 <xref linkend="sql-altersequence"/> such
      as <literal>INCREMENT BY</literal>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET STATISTICS</literal></term>
    <listitem>
     <para>
      This form
      sets the per-column statistics-gathering target for subsequent
      <xref linkend="sql-analyze"/> operations.
      The target can be set in the range 0 to 10000; alternatively, set it
      to -1 to revert to using the system default statistics
      target (<xref linkend="guc-default-statistics-target"/>).
      For more information on the use of statistics by the
      <productname>PostgreSQL</productname> query planner, refer to
      <xref linkend="planner-stats"/>.
     </para>
     <para>
      <literal>SET STATISTICS</literal> acquires a
      <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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
      <xref linkend="sql-analyze"/>
      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.  When set to a
      positive value, <command>ANALYZE</command> will assume that the column contains
      exactly the specified number of distinct nonnull values.  When set to a
      negative value, which must be greater
      than or equal to -1, <command>ANALYZE</command> will assume that the number of
      distinct nonnull values in the column is linear in the size of the
      table; the exact count is to be computed by multiplying the estimated
      table size by the absolute value of the given number.  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 the average.
      This can be useful when the size of the table changes over time, since
      the multiplication by the number of rows in the table is not performed
      until query planning time.  Specify a value of 0 to revert to estimating
      the number of distinct values normally.  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>
    <term>
     <literal>SET STORAGE</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.  <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>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>
    <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 syntax as
      <xref linkend="sql-createtable"/>, plus the option <literal>NOT
      VALID</literal>, which is currently only allowed for foreign key
      and CHECK 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
      enforced 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"
      endterm="sql-altertable-notes-title"/> below for more information
      about using the <literal>NOT VALID</literal> option.
     </para>

     <para>
      Addition of a foreign key constraint requires a
      <literal>SHARE ROW EXCLUSIVE</literal> lock on the referenced table,
      in addition to the lock on the table receiving the constraint.
     </para>

     <para>
      Additional restrictions apply when unique or primary key constraints
      are added to partitioned tables; see <xref linkend="sql-createtable"/>.
      Also, foreign key constraints on partitioned
      tables may not be declared <literal>NOT VALID</literal> at present.
     </para>

    </listitem>
   </varlistentry>

   <varlistentry>
    <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 INDEX CONCURRENTLY</command>, and then install it as an
       official constraint using this syntax.  See the example below.
      </para>
     </note>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ALTER CONSTRAINT</literal></term>
    <listitem>
     <para>
      This form alters the attributes of a constraint that was previously
      created. Currently only foreign key constraints may be altered.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>VALIDATE CONSTRAINT</literal></term>
    <listitem>
     <para>
      This form validates a foreign key or check 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.  Nothing happens if the constraint is already marked valid.
      (See <xref linkend="sql-altertable-notes"
      endterm="sql-altertable-notes-title"/> below for an explanation of the
      usefulness of this command.)
     </para>
    </listitem>
   </varlistentry>

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

   <varlistentry>
    <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>
    <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
      NOT be applied and the policies will be ignored.
      See also
      <xref linkend="sql-createpolicy"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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
      <xref linkend="sql-createpolicy"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CLUSTER ON</literal></term>
    <listitem>
     <para>
      This form selects the default index for future
      <xref linkend="sql-cluster"/>
      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>
    <term><literal>SET WITHOUT CLUSTER</literal></term>
    <listitem>
     <para>
      This form removes the most recently used
      <xref linkend="sql-cluster"/>
      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>
    <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>
    <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.
      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
      <xref linkend="sql-createtablespace"/>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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"
      endterm="sql-createtable-storage-parameters-title"/>
      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">VACUUM
      FULL</link>, <xref linkend="sql-cluster"/> 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
      following planner related parameters:
      <varname>effective_io_concurrency</varname>, <varname>parallel_workers</varname>, <varname>seq_page_cost</varname>,
      <varname>random_page_cost</varname>, <varname>n_distinct</varname> and <varname>n_distinct_inherited</varname>.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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>
    <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,
      and if they have <literal>NOT NULL</literal> constraints in the parent
      then they must also have <literal>NOT NULL</literal> constraints in the
      child.
     </para>

     <para>
      There must also be matching child-table constraints for all
      <literal>CHECK</literal> constraints of the parent, except those
      marked non-inheritable (that is, created with <literal>ALTER TABLE ... ADD CONSTRAINT ... NO INHERIT</literal>)
      in the parent, 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>
    <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>
    <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>
    <term><literal>NOT OF</literal></term>
    <listitem>
     <para>
      This form dissociates a typed table from its type.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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-createtable-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.  This option has no effect
      except when logical replication is in use.  <literal>DEFAULT</literal>
      (the default for non-system tables) records the
      old values of the columns of the primary key, if any.  <literal>USING INDEX</literal>
      records the old values of the columns covered by the named index, which
      must be unique, not partial, not deferrable, and include only columns marked
      <literal>NOT NULL</literal>.  <literal>FULL</literal> records the old values of all columns
      in the row.  <literal>NOTHING</literal> records no information about the old row.
      (This is the default for system tables.)
      In all cases, no old values are logged unless at least one of the columns
      that would be logged differs between the old and new versions of the row.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <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>
    <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>
    <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, a corresponding
      one will be created in the attached table; or, if an equivalent
      index already exists, will be attached to the target table's index,
      as if <command>ALTER INDEX ATTACH PARTITION</command> had been executed.
     </para>

     <para>
      A partition using <literal>FOR VALUES</literal> uses same syntax for
      <replaceable class="parameter">partition_bound_spec</replaceable> as
      <xref linkend="sql-createtable"/>.  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.  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.
      If any of the <literal>CHECK</literal> constraints of the table being
      attached is marked <literal>NO INHERIT</literal>, the command will fail;
      such a constraint must be recreated without the <literal>NO INHERIT</literal>
      clause.
     </para>

     <para>
      If the new partition is a regular table, a full table scan is performed
      to check that no existing row in the table violates the partition
      constraint.  It is possible to avoid this scan by adding a valid
      <literal>CHECK</literal> constraint to the table that would allow only
      the rows satisfying the desired partition constraint before running this
      command.  It will be determined using such a constraint 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 <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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DETACH PARTITION</literal> <replaceable class="parameter">partition_name</replaceable></term>
    <listitem>
     <para>
      This form detaches 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.
     </para>
    </listitem>
   </varlistentry>

  </variablelist>
  </para>

  <para>
   All the forms of ALTER TABLE that act on a single table, except
   <literal>RENAME</literal>, <literal>SET SCHEMA</literal>,
   <literal>ATTACH PARTITION</literal>, and
   <literal>DETACH 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 need 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 also be a direct or indirect member of 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>
      <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>
      <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>
      <term><replaceable class="parameter">column_name</replaceable></term>
      <listitem>
       <para>
        Name of a new or existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">new_column_name</replaceable></term>
      <listitem>
       <para>
        New name for an existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">new_name</replaceable></term>
      <listitem>
       <para>
        New name for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <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>
      <term><replaceable class="parameter">table_constraint</replaceable></term>
      <listitem>
       <para>
        New table constraint for the table.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">constraint_name</replaceable></term>
      <listitem>
       <para>
        Name of a new or existing constraint.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <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>
      <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>
      <term><replaceable class="parameter">trigger_name</replaceable></term>
      <listitem>
       <para>
        Name of a single trigger to disable or enable.
       </para>
      </listitem>
     </varlistentry>

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

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

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

     <varlistentry>
      <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>
      <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>
      <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.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <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">
  <title id="sql-altertable-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 is
    evaluated at the time of the statement and the result stored in the
    table's metadata.  That value will be used for the column for all existing
    rows.  If no <literal>DEFAULT</literal> is specified, NULL is used.  In
    neither case is a rewrite of the table required.
   </para>

   <para>
    Adding a column with a volatile <literal>DEFAULT</literal> or
    changing the type of an existing column will require 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; but any indexes on the affected columns must still be rebuilt.
    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.
   </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 a new foreign key or check constraint
    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>TRIGGER</literal>, <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.
    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>
  </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>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The forms <literal>ADD</literal> (without <literal>USING INDEX</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 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>
