Re: accounting for memory used for BufFile during hash joins
От | Tomas Vondra |
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Тема | Re: accounting for memory used for BufFile during hash joins |
Дата | |
Msg-id | 20190507131742.nixczzyyyuuoig7m@development обсуждение исходный текст |
Ответ на | Re: accounting for memory used for BufFile during hash joins (Tom Lane <tgl@sss.pgh.pa.us>) |
Ответы |
Re: accounting for memory used for BufFile during hash joins
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Список | pgsql-hackers |
On Mon, May 06, 2019 at 11:18:28PM -0400, Tom Lane wrote: >Tomas Vondra <tomas.vondra@2ndquadrant.com> writes: >> Do we actually check how many duplicates are there during planning? > >Certainly that's part of the planner's cost estimates ... but it's >only as good as the planner's statistical knowledge. > I'm looking at the code, and the only place where I see code dealing with MCVs (probably the best place for info about duplicate values) is estimate_hash_bucketsize in final_cost_hashjoin. That's not quite what I had in mind - I was thinking more about something along the lines "See the larget group of duplicate values, disable hash join if it can't fit into work_mem at all." Of course, if the input estimates are off, that may not work too well. It would certainly not help the query failing with OOM, because that was a case of severe underestimate. Or did you mean some other piece of code that I have missed. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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