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<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>

uniquely.

</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

column is changed (updated). The possible actions are the same.

</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">.

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>