Re: merge>hash>loop

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От Tom Lane
Тема Re: merge>hash>loop
Дата
Msg-id 10000.1145031205@sss.pgh.pa.us
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Ответ на merge>hash>loop  (Ian Westmacott <ianw@intellivid.com>)
Ответы Re: merge>hash>loop  (Ian Westmacott <ianw@intellivid.com>)
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Ian Westmacott <ianw@intellivid.com> writes:
> I have this query, where PG (8.1.2) prefers Merge Join over Hash Join
> over Nested Loop.  However, this order turns out to increase in
> performance.  I was hoping someone might be able to shed some light on
> why PG chooses the plans in this order, and what I might do to
> influence it otherwise.  Thanks,

Reducing random_page_cost would influence it to prefer the nestloop.
However, I doubt you're ever going to get really ideal results for
this query --- the estimated row counts are too far off, and the
WHERE conditions are sufficiently bizarre that there's not much hope
that they'd ever be much better.

            regards, tom lane

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