diff options
| author | Tom Lane | 2016-05-21 16:55:31 +0000 |
|---|---|---|
| committer | Tom Lane | 2016-05-21 16:55:31 +0000 |
| commit | 82eafabeaaf12231a85ed67bbf4eae698aacb1c9 (patch) | |
| tree | c339385837d7becaea12df5180dda4cb55013f19 /doc/src | |
| parent | 50e5315a58554735096f7530cb766fae2dd3b0c7 (diff) | |
Improve docs about using ORDER BY to control aggregate input order.
David Johnston pointed out that the original text here had been obsoleted
by SQL:2008, which allowed ORDER BY in subqueries. We could weaken the
text to describe ORDER-BY-in-subqueries as an optional SQL feature that's
possibly unportable; but then the exact same statements would apply to
the alternative it's being compared to (ORDER-BY-in-aggregate-calls).
So really that would be pretty useless; let's just take out the sentence
entirely. Instead, point out the hazard that any extra processing in the
upper query might cause the subquery output order to be destroyed.
Discussion: <CAKFQuwbAX=iO9QbpN7_jr+BnUWm9FYX8WbEPUvG0p+nZhp6TZg@mail.gmail.com>
Diffstat (limited to 'doc/src')
| -rw-r--r-- | doc/src/sgml/func.sgml | 5 |
1 files changed, 3 insertions, 2 deletions
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml index 3f627dc885f..ff7545de156 100644 --- a/doc/src/sgml/func.sgml +++ b/doc/src/sgml/func.sgml @@ -13102,8 +13102,9 @@ SELECT count(*) FROM sometable; SELECT xmlagg(x) FROM (SELECT x FROM test ORDER BY y DESC) AS tab; ]]></screen> - But this syntax is not allowed in the SQL standard, and is - not portable to other database systems. + Beware that this approach can fail if the outer query level contains + additional processing, such as a join, because that might cause the + subquery's output to be reordered before the aggregate is computed. </para> <para> |