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| -rw-r--r-- | doc/src/sgml/ref/select.sgml | 21 |
1 files changed, 18 insertions, 3 deletions
diff --git a/doc/src/sgml/ref/select.sgml b/doc/src/sgml/ref/select.sgml index 5cedb1cf318..a33a537bd48 100644 --- a/doc/src/sgml/ref/select.sgml +++ b/doc/src/sgml/ref/select.sgml @@ -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 --> @@ -1163,16 +1163,31 @@ ROLLBACK TO s; <caution> <para> 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>. This is because <literal>LIMIT</> is applied first. The command 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 or updated so that it does not meet the query <literal>WHERE</> condition anymore, in which case it will not be returned. </para> </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 id="SQL-TABLE"> |