"Tomas Vondra" <tv@fuzzy.cz> writes:
I've run into a strange plan difference on 9.1.9 - the first query does
"DISTINCT" by doing a GROUP BY on the columns (both INT). ...
Now, this takes ~45 seconds to execute, but after rewriting the query to
use the regular DISTINCT it suddenly switches to HashAggregate with ~1/3
the cost (although it produces the same output, AFAIK), and it executes in
~15 seconds.
[ scratches head... ] I guess you're running into some corner case where
choose_hashed_grouping and choose_hashed_distinct make different choices.
It's going to be tough to debug without a test case though. I couldn't
reproduce the behavior in a few tries here.
BTW I can't test this on 9.2 or 9.3 easily, as this is our production
environment and I can't just export the data. I've tried to simulate this
but so far no luck.
I suppose they won't yet you step through those two functions with a
debugger either ...

    regards, tom lane

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postedAug 14, '13 at 3:33p
activeAug 20, '13 at 11:32p
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