doc: Add Identity Column section under Data Definition chapter

This seems to be missing since identity column support was added.  Add
the same.  This gives a place to document various pieces of
information in one place that are currently distributed over several
command reference pages or just not documented (e.g., inheritance
behavior).

Author: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Author: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://www.postgresql.org/message-id/flat/CAExHW5uOykuTC+C6R1yDSp=o8Q83jr8xJdZxgPkxfZ1Ue5RRGg@mail.gmail.com

doc/src/sgml/datatype.sgml

@@ -955,7 +955,7 @@ FROM generate_series(-3.5, 3.5, 1) as x;
      <para>
       This section describes a PostgreSQL-specific way to create an
       autoincrementing column.  Another way is to use the SQL-standard
-      identity column feature, described at <xref linkend="sql-createtable"/>.
+      identity column feature, described at <xref linkend="ddl-identity-columns"/>.
      </para>
     </note>
 

doc/src/sgml/ddl.sgml

@@ -233,6 +233,116 @@ CREATE TABLE products (
   </para>
  </sect1>
 
+ <sect1 id="ddl-identity-columns">
+  <title>Identity Columns</title>
+
+  <indexterm zone="ddl-identity-columns">
+   <primary>identity column</primary>
+  </indexterm>
+
+  <para>
+   An identity column is a special column 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 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) VALUE ('A', 'foo');
+INSERT INTO people (name, address) VALUE ('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) VALUE (<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 are applied.
+  </para>
+
+  <para>
+   Identity columns are currently not supported for partitioned tables.
+  </para>
+ </sect1>
+
  <sect1 id="ddl-generated-columns">
   <title>Generated Columns</title>