| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | "Robert Haas" <robertmhaas(at)gmail(dot)com> |
| Cc: | "Gregory Stark" <stark(at)enterprisedb(dot)com>, "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>, "jd(at)commandprompt(dot)com" <jd(at)commandprompt(dot)com>, "Josh Berkus" <josh(at)agliodbs(dot)com>, "Greg Smith" <gsmith(at)gregsmith(dot)com>, pgsql-hackers(at)postgresql(dot)org |
| Subject: | Re: benchmarking the query planner |
| Date: | 2008-12-12 03:12:41 |
| Message-ID: | 21498.1229051561@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
"Robert Haas" <robertmhaas(at)gmail(dot)com> writes:
> I had this idle thought too, but I didn't write it down because...
>> ought to be, but it seems like it ought to be possible to determine
>> that given a desired maximum error in the overall estimate. I'm also
>> not very clear on what the "total frequency" computations (matchfreq2
>> and unmatchfreq2 in the current code) ought to look like if we are using
>> a variable subset of the inner list.
> ...of this exact concern, which I think is an insurmountable problem.
Maybe so. If we stick to the other design (end both lists at a preset
frequency threshold) then the math clearly goes through the same as
before, just with num_mcvs that are determined differently. But can
we prove anything about the maximum error added from that?
regards, tom lane
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