| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Bruce Momjian <bruce(at)momjian(dot)us> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, 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: | 2008-12-11 23:57:47 |
| Message-ID: | 13996.1229039867@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Bruce Momjian <bruce(at)momjian(dot)us> writes:
> Why is selfuncs.c::var_eq_const() doing a linear scan over the MCV array
> instead of having the list sorted and doing a binary search on the
> array?
1. Keeping the MCV array sorted by frequency allows cheap extraction
of less-accurate MCV lists; you just take the first N entries.
2. Binary search presumes that you have a less-than operator, a concept
not implicit in the notion of MCVs (but it *is* implicit in a
histogram).
regards, tom lane
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