options.)
         </para>
        </listitem>
       </varlistentry>
      </variablelist>
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CONSTRAINT <replaceable class="parameter">constraint_name</replaceable></literal></term>
    <listitem>
     <para>
      An optional name for a column or table constraint.  If the
      constraint is violated, the constraint name is present in error messages,
      so constraint names like <literal>col must be positive</literal> can be used
      to communicate helpful constraint information to client applications.
      (Double-quotes are needed to specify constraint names that contain spaces.)
      If a constraint name is not specified, the system generates a name.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>NOT NULL</literal> [ NO INHERIT ]</term>
    <listitem>
     <para>
      The column is not allowed to contain null values.
     </para>

     <para>
      A constraint marked with <literal>NO INHERIT</literal> will not propagate to
      child tables.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>NULL</literal></term>
    <listitem>
     <para>
      The column is allowed to contain null values. This is the default.
     </para>

     <para>
      This clause is only provided for compatibility with
      non-standard SQL databases.  Its use is discouraged in new
      applications.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ] </literal></term>
    <listitem>
     <para>
      The <literal>CHECK</literal> clause specifies an expression producing a
      Boolean result which each row in the foreign table is expected
      to satisfy; that is, the expression should produce TRUE or UNKNOWN,
      never FALSE, for all rows in the foreign table.
      A check constraint specified as a column constraint should
      reference that column's value only, while an expression
      appearing in a table constraint can reference multiple columns.
     </para>

     <para>
      Currently, <literal>CHECK</literal> expressions cannot contain
      subqueries nor refer to variables other than columns of the
      current row.  The system column <literal>tableoid</literal>
      may be referenced, but not any other system column.
     </para>

     <para>
      A constraint marked with <literal>NO INHERIT</literal> will not propagate to
      child tables.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>DEFAULT
    <replaceable>default_expr</replaceable></literal></term>
    <listitem>
     <para>
      The <literal>DEFAULT</literal> clause assigns a default data value for
      the column whose column definition it appears within.  The value
      is any variable-free expression (subqueries and cross-references
      to other columns in the current table are not allowed).  The
      data type of the default expression must match the data type of the
      column.
     </para>

     <para>
      The default expression will be used in any insert operation that
      does not specify a value for the column.  If there is no default
      for a column, then the default is null.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><literal>GENERATED ALWAYS AS ( <replaceable>generation_expr</replaceable> ) [ STORED | VIRTUAL ]</literal><indexterm><primary>generated column</primary></indexterm></term>
    <listitem>
     <para>
      This clause creates the column as a <firstterm>generated
      column</firstterm>.  The column cannot be written to, and when read the
      result of the specified expression will be returned.
     </para>

     <para>
      When <literal>VIRTUAL</literal> is specified, the column will be
      computed when it is read.  (The foreign-data wrapper will see it as a