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<refentry id="SQL-ALTERTABLE">
 <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>

 <indexterm zone="sql-altertable">
  <primary>ALTER TABLE</primary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    <replaceable class="PARAMETER">action</replaceable> [, ... ]
ALTER TABLE [ ONLY ] <replaceable class="PARAMETER">name</replaceable> [ * ]
    RENAME [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> TO <replaceable class="PARAMETER">new_column</replaceable>
ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
    RENAME TO <replaceable class="PARAMETER">new_name</replaceable>
ALTER TABLE <replaceable class="PARAMETER">name</replaceable>
    SET SCHEMA <replaceable class="PARAMETER">new_schema</replaceable>

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

    ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">type</replaceable> [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
    DROP [ COLUMN ] [ IF EXISTS ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ SET DATA ] TYPE <replaceable class="PARAMETER">type</replaceable> [ USING <replaceable class="PARAMETER">expression</replaceable> ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET DEFAULT <replaceable class="PARAMETER">expression</replaceable>
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> DROP DEFAULT
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> { SET | DROP } NOT NULL
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STATISTICS <replaceable class="PARAMETER">integer</replaceable>
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET ( <replaceable class="PARAMETER">attribute_option</replaceable> = <replaceable class="PARAMETER">value</replaceable> [, ... ] )
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> RESET ( <replaceable class="PARAMETER">attribute_option</replaceable> [, ... ] )
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> SET STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
    ADD <replaceable class="PARAMETER">table_constraint</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>
    CLUSTER ON <replaceable class="PARAMETER">index_name</replaceable>
    SET WITHOUT CLUSTER
    SET WITH OIDS
    SET WITHOUT OIDS
    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>
    OWNER TO <replaceable class="PARAMETER">new_owner</replaceable>
    SET TABLESPACE <replaceable class="PARAMETER">new_tablespace</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER TABLE</command> changes the definition of an existing table.
   There are several subforms:

  <variablelist>
   <varlistentry>
    <term><literal>ADD COLUMN</literal></term>
    <listitem>
     <para>
      This form adds a new column to the table, using the same syntax as
      <xref linkend="SQL-CREATETABLE">.
     </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.  You will need to say <literal>CASCADE</> 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>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.
      The default values only apply to subsequent <command>INSERT</command>
      commands; they do not cause rows already in the table to change.
      Defaults can also be created for views, in which case they are
      inserted into <command>INSERT</> statements on the view before
      the view's <literal>ON INSERT</literal> rule is applied.
     </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</> when the column contains no null values.
     </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>
    </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 attribute-level options.  Currently, the only
      define attribute-level options are <literal>n_distinct</> and
      <literal>n_distinct_inherited</>, which override the
      number-of-distinct-values estimate made by subsequent
      <xref linkend="sql-analyze">
      operations.  <literal>n_distinct</> affects the statistics for the table
      itself, while <literal>n_distinct_inherited</> affects the statistics
      gathered for the table and its inheritance children.  When set to a
      positive value, <command>ANALYZE</> 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</> 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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <indexterm>
     <primary>TOAST</primary>
     <secondary>per-column storage settings</secondary>
    </indexterm>

    <term><literal>SET STORAGE</literal></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</> 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</> 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></literal></term>
    <listitem>
     <para>
      This form adds a new constraint to a table using the same syntax as
      <xref linkend="SQL-CREATETABLE">.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DROP CONSTRAINT [ IF EXISTS ]</literal></term>
    <listitem>
     <para>
      This form drops the specified constraint on a table.
      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.
      The trigger firing mechanism is also affected by the configuration
      variable <xref linkend="guc-session-replication-role">. Simply enabled
      triggers will fire when the replication role is <quote>origin</>
      (the default) or <quote>local</>. Triggers configured as <literal>ENABLE
      REPLICA</literal> will only fire if the session is in <quote>replica</>
      mode, and triggers configured as <literal>ENABLE ALWAYS</literal> will
      fire regardless of the current replication mode.
     </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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CLUSTER</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>
    </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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET WITH OIDS</literal></term>
    <listitem>
     <para>
      This form adds an <literal>oid</literal> system column to the
      table (see <xref linkend="ddl-system-columns">).
      It does nothing if the table already has OIDs.
     </para>

     <para>
      Note that this is not equivalent to <literal>ADD COLUMN oid oid</>;
      that would add a normal column that happened to be named
      <literal>oid</>, not a system column.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>SET WITHOUT OIDS</literal></term>
    <listitem>
     <para>
      This form removes the <literal>oid</literal> system column from the
      table.  This is exactly equivalent to
      <literal>DROP COLUMN oid RESTRICT</literal>,
      except that it will not complain if there is already no
      <literal>oid</literal> column.
     </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 <xref linkend="SQL-CLUSTER">
      or one of the forms of <command>ALTER
      TABLE</> that forces a table rewrite.
     </para>

     <note>
      <para>
       While <command>CREATE TABLE</> allows <literal>OIDS</> to be specified
       in the <literal>WITH (<replaceable
       class="PARAMETER">storage_parameter</>)</literal> syntax,
       <command>ALTER TABLE</> does not treat <literal>OIDS</> as a
       storage parameter.  Instead use the <literal>SET WITH OIDS</>
       and <literal>SET WITHOUT OIDS</> forms to change OID status.
      </para>
     </note>
    </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</>, 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. 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>OWNER</literal></term>
    <listitem>
     <para>
      This form changes the owner of the table, sequence, or view to the
      specified user.
     </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.
      See also
      <xref linkend="SQL-CREATETABLESPACE">.
     </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, or view) or the name of an individual column in
      a table. 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>

  </variablelist>
  </para>

  <para>
   All the actions except <literal>RENAME</literal> and <literal>SET SCHEMA</>
   can be combined into
   a list of multiple alterations to apply in parallel.  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</>.
   To change the schema of a table, you must also have
   <literal>CREATE</literal> privilege on the new schema.
   To add the table as a new child of a parent table, you must own the
   parent table as well.
   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.)
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

    <variablelist>

     <varlistentry>
      <term><replaceable class="PARAMETER">name</replaceable></term>
      <listitem>
       <para>
        The name (possibly schema-qualified) of an existing table to
        alter. If <literal>ONLY</> is specified, only that table is
        altered. If <literal>ONLY</> is not specified, the table and any
        descendant tables are altered.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">column</replaceable></term>
      <listitem>
       <para>
        Name of a new or existing column.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="PARAMETER">new_column</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">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 an existing constraint to drop.
       </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).
       </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 index name on which the table should be marked for clustering.
       </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>

    </variablelist>
 </refsect1>

 <refsect1>
  <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>, all existing
    rows in the table are initialized with the column's default value
    (NULL if no <literal>DEFAULT</> clause is specified).
   </para>

   <para>
    Adding a column with a non-null default or changing the type of an
    existing column will require the entire table to be rewritten.  This
    might take a significant amount of time for a large table; and it will
    temporarily require double the disk space.  Adding or removing a system
    <literal>oid</> column likewise requires rewriting the entire table.
   </para>

   <para>
    Adding a <literal>CHECK</> or <literal>NOT NULL</> constraint requires
    scanning the table to verify that existing rows meet the constraint.
   </para>

   <para>
    The main reason for providing the option to specify multiple changes
    in a single <command>ALTER TABLE</> is that multiple table scans or
    rewrites can thereby be combined into a single pass over the table.
   </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.  (These statements do
    not apply when dropping the system <literal>oid</> column; that is done
    with an immediate rewrite.)
   </para>

   <para>
    The fact that <literal>ALTER TYPE</> requires rewriting the whole table
    is sometimes an advantage, because the rewriting process eliminates
    any dead space in the table.  For example, to reclaim the space occupied
    by a dropped column immediately, the fastest way is:
<programlisting>
ALTER TABLE table ALTER COLUMN anycol TYPE anytype;
</programlisting>
    where <literal>anycol</> is any remaining table column and
    <literal>anytype</> is the same type that column already has.
    This results in no semantically-visible change in the table,
    but the command forces rewriting, which gets rid of no-longer-useful
    data.
   </para>

   <para>
    The <literal>USING</literal> option of <literal>ALTER TYPE</> 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>ALTER TYPE</>
    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>ALTER TYPE</> 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</>, perform the <literal>ALTER
    TYPE</>, and then use <literal>SET DEFAULT</> 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.  That is, <command>ALTER TABLE ONLY</command>
    will be rejected.  This ensures that the descendants always have
    columns matching the parent.
   </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.
   </para>

   <para>
    The <literal>TRIGGER</>, <literal>CLUSTER</>, <literal>OWNER</>,
    and <literal>TABLESPACE</> actions never recurse to descendant tables;
    that is, they always act as though <literal>ONLY</> were specified.
    Adding a constraint can recurse only for <literal>CHECK</> constraints,
    and is required to do so for such constraints.
   </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 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 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 a 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) MATCH FULL;
</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>

 </refsect1>

 <refsect1>
  <title>Compatibility</title>

  <para>
   The forms <literal>ADD</literal>, <literal>DROP</>, <literal>SET DEFAULT</>,
   and <literal>SET DATA TYPE</literal> (without <literal>USING</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 is an extension.
  </para>

  <para>
   <command>ALTER TABLE DROP COLUMN</> 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>
</refentry>
