that is generated automatically from
   an implicit sequence.  It can be used to generate key values.
  </para>

  <para>
   To create an identity column, use the <literal>GENERATED ...
   AS IDENTITY</literal> clause in <command>CREATE TABLE</command>, for example:
<programlisting>
CREATE TABLE people (
    id bigint <emphasis>GENERATED ALWAYS AS IDENTITY</emphasis>,
    ...,
);
</programlisting>
   or alternatively
<programlisting>
CREATE TABLE people (
    id bigint <emphasis>GENERATED BY DEFAULT AS IDENTITY</emphasis>,
    ...,
);
</programlisting>
   See <xref linkend="sql-createtable"/> for more details.
  </para>

  <para>
   If an <command>INSERT</command> command is executed on the table with the
   identity column and no value is explicitly specified for the identity
   column, then a value generated by the implicit sequence is inserted.  For
   example, with the above definitions and assuming additional appropriate
   columns, writing
<programlisting>
INSERT INTO people (name, address) VALUES ('A', 'foo');
INSERT INTO people (name, address) VALUES ('B', 'bar');
</programlisting>
   would generate values for the <literal>id</literal> column starting at 1
   and result in the following table data:
<screen>
 id | name | address
----+------+---------
  1 | A    | foo
  2 | B    | bar
</screen>
   Alternatively, the keyword <literal>DEFAULT</literal> can be specified in
   place of a value to explicitly request the sequence-generated value, like
<programlisting>
INSERT INTO people (id, name, address) VALUES (<emphasis>DEFAULT</emphasis>, 'C', 'baz');
</programlisting>
   Similarly, the keyword <literal>DEFAULT</literal> can be used in
   <command>UPDATE</command> commands.
  </para>

  <para>
   Thus, in many ways, an identity column behaves like a column with a default
   value.
  </para>

  <para>
   The clauses <literal>ALWAYS</literal> and <literal>BY DEFAULT</literal> in
   the column definition determine how explicitly user-specified values are
   handled in <command>INSERT</command> and <command>UPDATE</command>
   commands.  In an <command>INSERT</command> command, if
   <literal>ALWAYS</literal> is selected, a user-specified value is only
   accepted if the <command>INSERT</command> statement specifies
   <literal>OVERRIDING SYSTEM VALUE</literal>.  If <literal>BY
   DEFAULT</literal> is selected, then the user-specified value takes
   precedence.  Thus, using <literal>BY DEFAULT</literal> results in a
   behavior more similar to default values, where the default value can be
   overridden by an explicit value, whereas <literal>ALWAYS</literal> provides
   some more protection against accidentally inserting an explicit value.
  </para>

  <para>
   The data type of an identity column must be one of the data types supported
   by sequences.  (See <xref linkend="sql-createsequence"/>.)  The properties
   of the associated sequence may be specified when creating an identity
   column (see <xref linkend="sql-createtable"/>) or changed afterwards (see
   <xref linkend="sql-altertable"/>).
  </para>

  <para>
   An identity column is automatically marked as <literal>NOT NULL</literal>.
   An identity column, however, does not guarantee uniqueness.  (A sequence
   normally returns unique values, but a sequence could be reset, or values
   could be inserted manually into the identity column, as discussed above.)
   Uniqueness would need to be enforced using a <literal>PRIMARY KEY</literal>
   or <literal>UNIQUE</literal> constraint.
  </para>

  <para>
   In table inheritance hierarchies, identity columns and their properties in
   a child table are independent of those in its parent tables.  A child table
   does not inherit identity columns or their properties automatically from
   the parent. During <command>INSERT</command> or <command>UPDATE</command>,
   a column is treated as an identity column if that column is an identity
   column in the table named in the statement, and the corresponding identity
   properties