a generated column.  Existing data
      in a stored generated column is rewritten and all the future changes
      will apply the new generation expression.
     </para>
    </listitem>
   </varlistentry>

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

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

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

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

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

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

   <varlistentry id="sql-altertable-desc-set-statistics">
    <term><literal>SET STATISTICS</literal></term>
    <listitem>
     <para>
      This form
      sets the per-column statistics-gathering target for subsequent
      <link linkend="sql-analyze"><command>ANALYZE</command></link> operations.
      The target can be set in the range 0 to 10000.  Set it
      to <literal>DEFAULT</literal> to revert to using the system default
      statistics target (<xref linkend="guc-default-statistics-target"/>).
      (Setting to a value of -1 is an obsolete way spelling to get the same
      outcome.)
      For more information on the use of statistics by the
      <productname>PostgreSQL</productname> query planner, refer to
      <xref linkend="planner-stats"/>.
     </para>
     <para>
      <literal>SET STATISTICS</literal> acquires a
      <literal>SHARE UPDATE EXCLUSIVE</literal> lock.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry id="sql-altertable-desc-set-attribute-option">
    <term><literal>SET ( <replaceable class="parameter">attribute_option</replaceable> = <replaceable class="parameter">value</replaceable> [, ... ] )</literal></term>
    <term><literal>RESET ( <replaceable class="parameter">attribute_option</replaceable> [, ... ] )</literal></term>
    <listitem>
     <para>
      This form sets or resets per-attribute options.  Currently, the only
      defined per-attribute options are <literal>n_distinct</literal> and