Re: bad estimation together with large work_mem generates terrible slow hash joins

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От Tom Lane
Тема Re: bad estimation together with large work_mem generates terrible slow hash joins
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Msg-id 16746.1410444705@sss.pgh.pa.us
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Ответ на Re: bad estimation together with large work_mem generates terrible slow hash joins  (Robert Haas <robertmhaas@gmail.com>)
Ответы Re: bad estimation together with large work_mem generates terrible slow hash joins  ("Tomas Vondra" <tv@fuzzy.cz>)
Re: bad estimation together with large work_mem generates terrible slow hash joins  (Robert Haas <robertmhaas@gmail.com>)
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Robert Haas <robertmhaas@gmail.com> writes:
> On Thu, Sep 11, 2014 at 9:59 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Robert Haas <robertmhaas@gmail.com> writes:
>>> (3) It allows the number of batches to increase on the fly while the
>>> hash join is in process.

>> Pardon me for not having read the patch yet, but what part of (3)
>> wasn't there already?

> EINSUFFICIENTCAFFEINE.

> It allows the number of BUCKETS to increase, not the number of
> batches.  As you say, the number of batches could already increase.

Ah.  Well, that would mean that we need a heuristic for deciding when to
increase the number of buckets versus the number of batches ... seems
like a difficult decision.
        regards, tom lane



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