| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
| Cc: | pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: The Future of Aggregation |
| Date: | 2015-06-09 15:27:21 |
| Message-ID: | 20150609152721.GA18593@alap3.anarazel.de |
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| Lists: | pgsql-hackers |
On 2015-06-09 17:19:33 +0200, Tomas Vondra wrote:
> ... and yet another use case for 'aggregate state combine' that I just
> remembered about is grouping sets. What GROUPING SET (ROLLUP, ...) do
> currently is repeatedly sorting the input, once for each grouping.
Actually, that's not really what happens. All aggregates that share a
sort order are computed in parallel. Only when sets do not share an
order additional sorts are required.
> What
> could happen in some cases is building the most detailed aggregation first,
> then repeatedly combine these partial states.
I'm not sure that'll routinely be beneficial, because it'd require
keeping track of all the individual "most detailed" results, no?
Greetings,
Andres Freund
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