Re: Group-count estimation statistics - Mailing list pgsql-hackers

From Tom Lane
Subject Re: Group-count estimation statistics
Date
Msg-id 26465.1106948476@sss.pgh.pa.us
Whole thread Raw
In response to Re: Group-count estimation statistics  (Kris Jurka <books@ejurka.com>)
List pgsql-hackers
Kris Jurka <books@ejurka.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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