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<sect1 id="release-9-0">

<title>Release 9.0</title>

<listitem>

<para>

<link linkend="SQL-CREATETABLE-compatibility">Deferrable

unique constraints, now permit mass updates to unique keys.</link>

unique constraints. Mass updates to unique keys are now possible

without trickery.</link>

</para>

</listitem>

</para>

<para>

This allows <command>UPDATE tab SET col = col + 1</> to work on

columns that have a unique indexes or are marked as primary key,

but <literal>DEFERRABLE INITIALLY DEFERRED</> must be used to mark

the constraint as deferred.

This allows <command>UPDATE tab SET col = col + 1</> to work reliably

on columns that have unique indexes or are marked as primary keys.

If the constraint is specified as <literal>DEFERRABLE</> it will be

checked at the end of the statement, rather than after each row is

updated. The constraint check may also be deferred until the end of the

current transaction, allowing updates to be spread over multiple SQL

commands.

</para>

</listitem>