diff options
| author | Bruce Momjian <bruce@momjian.us> | 2009-01-23 14:05:28 +0000 |
|---|---|---|
| committer | Bruce Momjian <bruce@momjian.us> | 2009-01-23 14:05:28 +0000 |
| commit | 4d65d2872b534b9c5b313f7115208a998ad6bdb8 (patch) | |
| tree | d628109253fdeee8b7f9a22525d4a4745e23cbcc /doc/src | |
| parent | 3b35a904aab85b3e837dfb5f5a64375d2ab11da9 (diff) | |
Document that SELECT ... ORDER BY .. FOR UPDATE/SHARE might return
results out of order because of locking, per bug report 4593
Diffstat (limited to 'doc/src')
| -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"> |