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Explain automatic creation (or lack of it) of indexes for the various types

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of constraints.

Kevin Grittner
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Tom Lane 2010-08-26 21:08:35 +00:00
parent 16567b0581
commit 4832291655
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doc/src/sgml/ddl.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.96 2010/08/23 02:43:25 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/ddl.sgml,v 1.97 2010/08/26 21:08:35 tgl Exp $ -->
<chapter id="ddl">
<title>Data Definition</title>
@ -544,6 +544,11 @@ CREATE TABLE products (
</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>
@ -622,6 +627,11 @@ CREATE TABLE example (
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
@ -831,6 +841,16 @@ CREATE TABLE order_items (
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">.
@ -875,6 +895,11 @@ CREATE TABLE circles (
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>