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Document that SELECT ... ORDER BY .. FOR UPDATE/SHARE might return

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results out of order because of locking, per bug report 4593
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Bruce Momjian 2009-01-23 14:05:28 +00:00
parent 3b35a904aa
commit 4d65d2872b
1 changed files with 18 additions and 3 deletions
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doc/src/sgml/ref/select.sgml
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@ -1,5 +1,5 @@
<!-- <!--
$PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.118 2009/01/22 20:15:59 tgl Exp $ $PostgreSQL: pgsql/doc/src/sgml/ref/select.sgml,v 1.119 2009/01/23 14:05:28 momjian Exp $
PostgreSQL documentation PostgreSQL documentation
--> -->
@ -1163,16 +1163,31 @@ ROLLBACK TO s;
<caution> <caution>
<para> <para>
It is possible for a <command>SELECT</> command using both It is possible for a <command>SELECT</> command using both
<literal>LIMIT</literal> and <literal>FOR UPDATE/SHARE</literal> <literal>LIMIT</literal> and <literal>FOR UPDATE/SHARE</literal>
clauses to return fewer rows than specified by <literal>LIMIT</literal>. clauses to return fewer rows than specified by <literal>LIMIT</literal>.
This is because <literal>LIMIT</> is applied first. The command This is because <literal>LIMIT</> is applied first. The command
selects the specified number of rows, selects the specified number of rows,
but might then block trying to obtain lock on one or more of them. but might then block trying to obtain a lock on one or more of them.
Once the <literal>SELECT</> unblocks, the row might have been deleted Once the <literal>SELECT</> unblocks, the row might have been deleted
or updated so that it does not meet the query <literal>WHERE</> condition or updated so that it does not meet the query <literal>WHERE</> condition
anymore, in which case it will not be returned. anymore, in which case it will not be returned.
</para> </para>
</caution> </caution>
<caution>
<para>
Similarly, it is possible for a <command>SELECT</> command
using <literal>ORDER BY</literal> and <literal>FOR
UPDATE/SHARE</literal> to return rows out of order. This is
because <literal>ORDER BY</> is applied first. The command
orders the result, but might then block trying to obtain a lock
on one or more of the rows. Once the <literal>SELECT</>
unblocks, one of the ordered columns might have been modified
and be returned out of order. A workaround is to perform
<command>SELECT ... FOR UPDATE/SHARE</> and then <command>SELECT
... ORDER BY</>.
</para>
</caution>
</refsect2> </refsect2>
<refsect2 id="SQL-TABLE"> <refsect2 id="SQL-TABLE">