Re: PATCH: add support for IN and @> in functional-dependencystatistics use
От | Tomas Vondra |
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Тема | Re: PATCH: add support for IN and @> in functional-dependencystatistics use |
Дата | |
Msg-id | 20200325002801.i45f34l45rqmik4v@development обсуждение исходный текст |
Ответ на | Re: PATCH: add support for IN and @> in functional-dependencystatistics use (Dean Rasheed <dean.a.rasheed@gmail.com>) |
Ответы |
Re: PATCH: add support for IN and @> in functional-dependencystatistics use
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Список | pgsql-hackers |
On Thu, Mar 19, 2020 at 07:53:39PM +0000, Dean Rasheed wrote: >On Wed, 18 Mar 2020 at 00:29, Tomas Vondra <tomas.vondra@2ndquadrant.com> wrote: >> >> OK, I took a look. I think from the correctness POV the patch is OK, but >> I think the dependencies_clauselist_selectivity() function now got a bit >> too complex. I've been able to parse it now, but I'm sure I'll have >> trouble in the future :-( >> >> Can we refactor / split it somehow and move bits of the logic to smaller >> functions, or something like that? >> > >Yeah, it has gotten a bit long. It's somewhat tricky splitting it up, >because of the number of shared variables used throughout the >function, but here's an updated patch splitting it into what seemed >like the 2 most logical pieces. The first piece (still in >dependencies_clauselist_selectivity()) works out what dependencies >can/should be applied, and the second piece in a new function does the >actual work of applying the list of functional dependencies to the >clause list. > >I think that has made it easier to follow, and it has also reduced the >complexity of the final "no applicable stats" branch. > Seems OK to me. I'd perhaps name deps_clauselist_selectivity differently, it's a bit too similar to dependencies_clauselist_selectivity. Perhaps something like clauselist_apply_dependencies? But that's a minor detail. >> Another thing I'd like to suggest is keeping the "old" formula, and >> instead of just replacing it with >> >> P(a,b) = f * Min(P(a), P(b)) + (1-f) * P(a) * P(b) >> >> but explaining how the old formula may produce nonsensical selectivity, >> and how the new formula addresses that issue. >> > >I think this is purely a comment issue? I've added some more extensive >comments attempting to justify the formulae. > Yes, it was purely a comment issue. Seems fine now. regards -- Tomas Vondra http://www.2ndQuadrant.com PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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