| 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>, "Greg Stark" <greg(dot)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: | 2008-12-10 17:58:52 |
| Message-ID: | 4313.1228931932@sss.pgh.pa.us |
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"Robert Haas" <robertmhaas(at)gmail(dot)com> writes:
> On Mon, Dec 8, 2008 at 10:24 AM, Gregory Stark <stark(at)enterprisedb(dot)com> wrote:
>> I tried a different query, trying to get quadratic growth and again failed. It
> The profiling results I sent the other day show an exactly-linear
> increase in the number of times eqjoinsel invokes FunctionCall2.
> Reading through the the eqjoinsel_inner loop in selfuncs.c beginning
> around line 2042, I think what is happening is this: since the two
> tables are really the same table, nvalues1 and nvalues2 are the same
> array, and therefore contain the same elements in the same order.
Yeah, that would be fast. To see a quadratic case you need MCV arrays
that have little or no overlap of common values --- then each element of
the first will be compared (in vain) to all or most of the elements in
the second.
regards, tom lane
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