From: | wickro <robwickert(at)gmail(dot)com> |
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To: | pgsql-general(at)postgresql(dot)org |
Subject: | Re: work_mem greater than 2GB issue |
Date: | 2009-05-15 16:13:38 |
Message-ID: | e3febfba-ac85-44aa-8949-9dff53892f85@21g2000vbk.googlegroups.com |
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Lists: | pgsql-general |
> HashAggregate doesn't have any ability to spill to disk. The planner
> will not select a HashAggregate if it thinks the required hash table
> would be larger than work_mem. What you've evidently got here is a
> misestimate of the required hash table size, which most likely is
> stemming from a bad estimate of the number of groups. How does that
> estimate (12617088 here) compare to reality? Have you tried increasing
> the statistics target for partner_id and keyword (or the whole table)?
Looking at the pg_statistics table the stats for one of the columns
(keyword) is seriously underestimated.
By increasing the stats target to 1000 it gets a bit better but still
underestimated by a a factor of about 10.
I guess that's why postgresql thinks it can fit it all into a hash.
Thanks for the help.
Cheers
Rob
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