From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | David Rowley <dgrowleyml(at)gmail(dot)com> |
Cc: | mba(dot)ogolny(at)gmail(dot)com, PostgreSQL mailing lists <pgsql-bugs(at)lists(dot)postgresql(dot)org> |
Subject: | Re: BUG #16558: `AND FALSE` increases planning time of query on 2 tables with 1000 partitions to more than 7 seconds |
Date: | 2020-07-31 01:13:45 |
Message-ID: | 1659523.1596158025@sss.pgh.pa.us |
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Lists: | pgsql-bugs |
David Rowley <dgrowleyml(at)gmail(dot)com> writes:
> hmm ok. FWIW, I did have in mind that the FALSE had been constant
> folded from something more complex.
Hm. Maybe that's a legitimate argument, not sure. But I'd still
rather find someplace that's less critical performance-wise if we're
going to try to hack up this case.
I also wonder about cases like
select * from (t1 join t2 on false) join t3 on t1.x=t3.y;
If we were taking this seriously, it'd be nice to deduce that
(1) the t1/t2 join is empty and (2) therefore so is the join
to t3, so (3) we need not build paths for any of these base rels.
I think the syntactically-driven method you're proposing would
not catch that, which'd be problematic if t3 is the giant
partitioned rel.
[ wanders away wondering if reduce_outer_joins could be taught
to do this... ]
regards, tom lane
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