Re: planner support functions: handle GROUP BY estimates ?
От | Tomas Vondra |
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Тема | Re: planner support functions: handle GROUP BY estimates ? |
Дата | |
Msg-id | 20200114214521.lojkdad36rzmhj3y@development обсуждение исходный текст |
Ответ на | Re: planner support functions: handle GROUP BY estimates ? (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: planner support functions: handle GROUP BY estimates ?
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Список | pgsql-hackers |
On Tue, Jan 14, 2020 at 04:21:57PM -0500, Tom Lane wrote: >Tomas Vondra <tomas.vondra@2ndquadrant.com> writes: >> On Tue, Jan 14, 2020 at 03:12:21PM -0500, Tom Lane wrote: >>> cc'ing Tomas in case he has any thoughts about it. > >> Well, I certainly do thoughts about this - it's pretty much exactly what >> I proposed yesterday in this thread: >> https://www.postgresql.org/message-id/flat/20200113230008.g67iyk4cs3xbnjju@development >> The third part of that patch series is exactly about supporting extended >> statistics on expressions, about the way you described here. The current >> status of the WIP patch is that grammar + ANALYZE mostly works, but >> there is no support in the planner. It's obviously still very hackish. > >Cool. We should probably take the discussion to that thread, then. > >> I'm also wondering if we could/should 100% rely on extended statistics, >> because those are really meant to track correlations between columns, > >Yeah, it seems likely to me that the infrastructure for this would be >somewhat different --- the user-facing syntax could be basically the >same, but ultimately we want to generate entries in pg_statistic not >pg_statistic_ext_data. Or at least entries that look the same as what >you could find in pg_statistic. > Yeah. I think we could invent a new type of statistics "expressions" which would simply built this per-column stats. So for example CREATE STATISTICS s (expressions) ON (a*b), sqrt(c) FROM t; would build per-column stats stored in pg_statistics, while CREATE STATISTICS s (mcv) ON (a*b), sqrt(c) FROM t; would build the multi-column MCV list on expressions. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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