| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Tomas Vondra <tv(at)fuzzy(dot)cz>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: bad estimation together with large work_mem generates terrible slow hash joins |
| Date: | 2014-09-11 14:11:45 |
| Message-ID: | 16746.1410444705@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Thu, Sep 11, 2014 at 9:59 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Robert Haas <robertmhaas(at)gmail(dot)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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