From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Kris Jurka <books(at)ejurka(dot)com> |
Cc: | Greg Stark <gsstark(at)mit(dot)edu>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Group-count estimation statistics |
Date: | 2005-01-28 21:41:16 |
Message-ID: | 26465.1106948476@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Kris Jurka <books(at)ejurka(dot)com> writes:
> The proposed change biases towards a hash plan which has no provision for
> spilling to disk. Slow is one thing, but excessive memory usage and
> possibly failing is another thing.
Keep in mind that we are replacing 7.4 code that had a serious tendency
to select hash plans when it really shouldn't, because of underestimated
table sizes. Now that we have the physical-size-driven estimate of
table rowcounts, I think we've gone too far over in the other direction.
Which is not to say that spill-to-disk logic wouldn't be a nice thing to
add, but I'm not going to panic about its not being there today. 7.4
presented a much greater hazard than we have now, but we got through
that cycle without a large number of complaints. There's always the
enable_hashagg switch if you really find yourself backed into a corner.
regards, tom lane
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