Re: Why is that index not used?

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От Tom Lane
Тема Re: Why is that index not used?
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Msg-id 2612.1232553100@sss.pgh.pa.us
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Ответ на Why is that index not used?  (Michael Monnerie <michael.monnerie@is.it-management.at>)
Ответы Re: Why is that index not used?  (Michael Monnerie <michael.monnerie@is.it-management.at>)
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Michael Monnerie <michael.monnerie@is.it-management.at> writes:
> Can somebody explain me why on the last line, "dbmail_physmessage",
> there is a seq. scan going on and no index used?

Because it chose to use a hash instead.  Given the rowcounts I don't
think that's wrong, but if you want to see the other plan you can try
setting enable_hashjoin = off (and maybe enable mergejoin = off if
it goes to a merge join next).

As for your other question, the reason it likes the dbmail_messages_1
index is probably that it's a lot smaller than dbmail_messages_7.
The only thing I'm seeing that seems curious is that when forced to
use dbmail_messages_7, it isn't using what seems to be an available
index condition.

            regards, tom lane

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