Re: queries with lots of UNIONed relations

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От Tom Lane
Тема Re: queries with lots of UNIONed relations
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Msg-id 11563.1294957582@sss.pgh.pa.us
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Ответ на Re: queries with lots of UNIONed relations  (Robert Haas <robertmhaas@gmail.com>)
Ответы Re: queries with lots of UNIONed relations  (Robert Haas <robertmhaas@gmail.com>)
Re: queries with lots of UNIONed relations  (Vitalii Tymchyshyn <tivv00@gmail.com>)
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Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Jan 13, 2011 at 3:12 PM, Jon Nelson <jnelson+pgsql@jamponi.net> wrote:
>> I still think that having UNION do de-duplication of each contributory
>> relation is a beneficial thing to consider -- especially if postgresql
>> thinks the uniqueness is not very high.

> This might be worth a TODO.

I don't believe there is any case where hashing each individual relation
is a win compared to hashing them all together.  If the optimizer were
smart enough to be considering the situation as a whole, it would always
do the latter.

            regards, tom lane

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