| From: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> |
|---|---|
| To: | heikki(dot)linnakangas(at)enterprisedb(dot)com (Heikki Linnakangas), pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: Aggregate ORDER BY patch |
| Date: | 2009-11-13 16:32:41 |
| Message-ID: | 87skcipew7.fsf@news-spur.riddles.org.uk |
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| Lists: | pgsql-hackers |
>>>>> "Heikki" == Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> Herewith a patch to implement agg(foo ORDER BY bar) with or
>> without DISTINCT, etc.
Heikki> What does that mean? Aggregate functions are supposed to be
Heikki> commutative, right?
The SQL spec defines two non-commutative aggregates that we implement:
array_agg(x ORDER BY ...)
xmlagg(x ORDER BY ...)
In addition, of course, we allow user-defined aggregates, which are
perfectly free to be non-commutative.
--
Andrew (irc:RhodiumToad)
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