-<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.93.2.2 2010/08/23 02:43:35 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.93.2.3 2010/08/26 21:08:43 tgl Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
</programlisting>
</para>
+ <para>
+ Adding a unique constraint will automatically create a unique btree
+ index on the column or group of columns used in the constraint.
+ </para>
+
<indexterm>
<primary>null value</primary>
<secondary sortas="unique constraints">with unique constraints</secondary>
</para>
<para>
+ Adding a primary key will automatically create a unique btree index
+ on the column or group of columns used in the primary key.
+ </para>
+
+ <para>
A table can have at most one primary key. (There can be any number
of unique and not-null constraints, which are functionally the same
thing, but only one can be identified as the primary key.)
</para>
<para>
+ Since a <command>DELETE</command> of a row from the referenced table
+ or an <command>UPDATE</command> of a referenced column will require
+ a scan of the referencing table for rows matching the old value, it
+ is often a good idea to index the referencing columns. Because this
+ is not always needed, and there are many choices available on how
+ to index, declaration of a foreign key constraint does not
+ automatically create an index on the referencing columns.
+ </para>
+
+ <para>
More information about updating and deleting data is in <xref
linkend="dml">.
</para>
See also <link linkend="SQL-CREATETABLE-EXCLUDE"><command>CREATE
TABLE ... CONSTRAINT ... EXCLUDE</></link> for details.
</para>
+
+ <para>
+ Adding an exclusion constraint will automatically create an index
+ of the type specified in the constraint declaration.
+ </para>
</sect2>
</sect1>