From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com> |
Cc: | pgsql-hackers(at)postgresql(dot)org, Rushabh Lathia <rushabh(dot)lathia(at)enterprisedb(dot)com> |
Subject: | Re: Query ending up with hitting all the partition with sub-query in the projection list |
Date: | 2012-10-26 17:09:12 |
Message-ID: | 24797.1351271352@sss.pgh.pa.us |
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Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com> writes:
> *-- Hitting all the partition table*
> postgres=# explain select a , ( select city_id from measurement where *logdate
> = test.b and logdate = '2006-02-02')* xyz from test;
Hm. What's happening here is that you get an equivalence class
containing logdate, test.b, and '2006-02-02', where test.b is actually
a Param supplied from the outer query level. So the equivclass.c
machinery considers that both test.b and '2006-02-02' are constants,
and it just picks the first one to construct derived equalities from.
So what comes out is "test.b = logdate and test.b = '2006-02-02'", and
neither of those can be used by the constraint-exclusion machinery to
prove that some partitions of "measurement" can be skipped.
What we need is to teach generate_base_implied_equalities_const() to
prefer an actual constant to pseudo-constants. Will fix, thanks for the
example!
regards, tom lane
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