The optional <literal>USING</literal>
      clause specifies how to compute the new column value from the old;
      if omitted, the default conversion is the same as an assignment
      cast from old data type to new.  A  <literal>USING</literal>
      clause must be provided if there is no implicit or assignment
      cast from old to new type.
     </para>

     <para>
      When this form is used, the column's statistics are removed,
      so running <link linkend="sql-analyze"><command>ANALYZE</command></link>
      on the table afterwards is recommended.
     </para>
    </listitem>
   </varlistentry>

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

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

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

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

   <varlistentry id="sql-altertable-desc-set-expression">
    <term><literal>SET EXPRESSION AS</literal></term>
    <listitem>
     <para>
      This form replaces the expression of a generated column.  Existing data
      in a stored generated column is rewritten and all the future changes
      will apply the new generation expression.
     </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">