[postgresql] / doc / src / sgml / ref / alter_table.sgml

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<refentry id="SQL-ALTERTABLE">
 <refmeta>
  <refentrytitle id="sql-altertable-title">ALTER TABLE</refentrytitle>
  <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>

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

    ADD [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> <replaceable class="PARAMETER">type</replaceable> [ <replaceable class="PARAMETER">column_constraint</replaceable> [ ... ] ]
    DROP [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> [ RESTRICT | CASCADE ]
    ALTER [ COLUMN ] <replaceable class="PARAMETER">column</replaceable> 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 STORAGE { PLAIN | EXTERNAL | EXTENDED | MAIN }
    ADD <replaceable class="PARAMETER">table_constraint</replaceable>
    DROP CONSTRAINT <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 ]
    CLUSTER ON <replaceable class="PARAMETER">index_name</replaceable>
    SET WITHOUT CLUSTER
    SET WITHOUT OIDS
    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" endterm="SQL-CREATETABLE-TITLE">.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DROP COLUMN</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.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>ALTER COLUMN 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 may 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" endterm="sql-analyze-title"> operations.
      The target can be set in the range 0 to 1000; 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>
    <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 supplementary 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 <type>text</type> and <type>bytea</type>
      columns 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" endterm="SQL-CREATETABLE-TITLE">. 
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DROP CONSTRAINT</literal></term>
    <listitem>
     <para>
      This form drops the specified constraint on a table.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DISABLE</literal>/<literal>ENABLE TRIGGER</literal></term>
    <listitem>
     <para>
      These forms disable or enable 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 may disable or enable a single
      trigger specified by name, or all triggers on the table, or only
      user triggers (this option excludes triggers that are used to implement
      foreign key constraints).  Disabling or enabling 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>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CLUSTER</literal></term>
    <listitem>
     <para>
      This form selects the default index for future 
      <xref linkend="SQL-CLUSTER" endterm="sql-cluster-title">
      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" endterm="sql-cluster-title">
      index specification from the table.  This affects
      future cluster operations that don't specify an index.
     </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>

     <para>
      Note that there is no variant of <command>ALTER TABLE</command>
      that allows OIDs to be restored to a table once they have been
      removed.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>INHERIT <replaceable class="PARAMETER">parent_table</replaceable></literal></term>
    <listitem>
     <para>

      This form adds a new parent table to the table. This won't add new
      columns to the child table, instead all columns of the parent table must
      already exist in the child table. They 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 table constraints for all
      <literal>CHECK</literal> table constraints of the parent. Currently
      <literal>UNIQUE</literal>, <literal>PRIMARY KEY</literal>, and
      <literal>FOREIGN KEY</literal> constraints are ignored however this may
      change in the future.

          </para>
          <para>

      The easiest way to create a suitable table is to create a table using
      <literal>INHERITS</literal> and then remove it via <literal>NO
      INHERIT</literal>. Alternatively create a table using
      <literal>LIKE</literal> however note that <literal>LIKE</literal> does
      not create the necessary constraints.

     </para>

    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>NO INHERIT <replaceable class="PARAMETER">parent_table</replaceable></literal></term>
    <listitem>
     <para>
         This form removes a parent table from the list of parents of the 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" endterm="sql-createtablespace-title">.
     </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 SERIAL-column sequences 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 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 all
        its descendant tables (if any) are updated. <literal>*</> can be
        appended to the table name to indicate that descendant tables are
        to be altered, but in the current version, this is the default
        behavior.  (In releases before 7.1, <literal>ONLY</> was the
        default behavior.  The default can be altered by changing the
        configuration parameter <xref linkend="guc-sql-inheritance">.)
       </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 for
        foreign key constraints.)
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><literal>USER</literal></term>
      <listitem>
       <para>
        Disable or enable all triggers belonging to the table except for
        foreign key constraint triggers.
       </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">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
    may take a significant amount of time for a large table; and it will
    temporarily require double the disk space.
   </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.
   </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</> may 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.
   </para>

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

   <para>
    Refer to <xref linkend="sql-createtable"
    endterm="sql-createtable-title"> 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 TYPE timestamp with time zone
    USING
        timestamp with time zone 'epoch' + foo_timestamp * interval '1 second';
</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:
<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 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 <literal>ADD</literal>, <literal>DROP</>, and <literal>SET DEFAULT</>
   forms 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>

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