column default is evaluated once when the row is first
   inserted if no other value was provided; a generated column is updated
   whenever the row changes and cannot be overridden.  A column default may
   not refer to other columns of the table; a generation expression would
   normally do so.  A column default can use volatile functions, for example
   <literal>random()</literal> or functions referring to the current time;
   this is not allowed for generated columns.
  </para>

  <para>
   Several restrictions apply to the definition of generated columns and
   tables involving generated columns:

   <itemizedlist>
    <listitem>
     <para>
      The generation expression can only use immutable functions and cannot
      use subqueries or reference anything other than the current row in any
      way.
     </para>
    </listitem>
    <listitem>
     <para>
      A generation expression cannot reference another generated column.
     </para>
    </listitem>
    <listitem>
     <para>
      A generation expression cannot reference a system column, except
      <varname>tableoid</varname>.
     </para>
    </listitem>
    <listitem>
     <para>
      A generated column cannot have a column default or an identity definition.
     </para>
    </listitem>
    <listitem>
     <para>
      A generated column cannot be part of a partition key.
     </para>
    </listitem>
    <listitem>
     <para>
      Foreign tables can have generated columns.  See <xref
      linkend="sql-createforeigntable"/> for details.
     </para>
    </listitem>
    <listitem>
     <para>For inheritance and partitioning:</para>
     <itemizedlist>
      <listitem>
       <para>
        If a parent column is a generated column, its child column must also
        be a generated column of the same kind (stored or virtual); however,
        the child column can have a different generation expression.
       </para>

       <para>
        For stored generated columns, the generation expression that is
        actually applied during insert or update of a row is the one
        associated with the table that the row is physically in.  (This is
        unlike the behavior for column defaults: for those, the default value
        associated with the table named in the query applies.)  For virtual
        generated columns, the generation expression of the table named in the
        query applies when a table is read.
       </para>
      </listitem>
      <listitem>
       <para>
        If a parent column is not a generated column, its child column must
        not be generated either.
       </para>
      </listitem>
      <listitem>
       <para>
        For inherited tables, if you write a child column definition without
        any <literal>GENERATED</literal> clause in <command>CREATE TABLE
        ... INHERITS</command>, then its <literal>GENERATED</literal> clause
        will automatically be copied from the parent.  <command>ALTER TABLE
        ... INHERIT</command> will insist that parent and child columns
        already match as to generation status, but it will not require their
        generation expressions to match.
       </para>
      </listitem>
      <listitem>
       <para>
        Similarly for partitioned tables, if you write a child column
        definition without any <literal>GENERATED</literal> clause
        in <command>CREATE TABLE ... PARTITION OF</command>, then
        its <literal>GENERATED</literal> clause will automatically be copied
        from the parent.  <command>ALTER TABLE ... ATTACH PARTITION</command>
        will insist that parent and child columns already match as to
        generation status, but it will not require their generation
        expressions to match.
       </para>
      </listitem>
      <listitem>
       <para>
        In case of multiple inheritance, if one parent column is a generated
        column, then all parent columns must be generated columns.  If they
        do not all have the same generation