| From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
|---|---|
| To: | Robert Haas <robertmhaas(at)gmail(dot)com> |
| Cc: | Serge Rielau <serge(at)rielau(dot)com>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Rushabh Lathia <rushabh(dot)lathia(at)gmail(dot)com>, Prabhat Sahu <prabhat(dot)sahu(at)enterprisedb(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Rafia Sabih <rafia(dot)sabih(at)enterprisedb(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>, Oleg Golovanov <rentech(at)mail(dot)ru> |
| Subject: | Re: Treating work_mem as a shared resource (Was: Parallel Hash take II) |
| Date: | 2017-11-18 00:09:45 |
| Message-ID: | CAH2-WznNquzj3+Qod1DKsaGsP-Q_dLSA2MvNUWbFj9OqTyai0A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Fri, Nov 17, 2017 at 3:23 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> Hmm. I wonder if you are correct that hashing is the special case.
> Hashing and sorting are of course the two main operations -- but
> there's materialize and anything else that uses a CTE, and maybe other
> stuff I'm not thinking about right now. You might be right that hash
> is the one where it really matters, but it's probably worth a bit more
> reflection on where it matters most and for what reasons.
I'd rather be approximately correct than precisely wrong. I think that
the current work_mem model is precisely wrong. I'm conscious of the
fact that we are loathe to create new GUCs (I sometimes think that
we're a bit too averse to doing so), but maybe there is room for
adding a second work_mem-alike GUC.
For now, I admit that I am applying fuzzy criteria, and that I could
easily have missed an important subtlety. Creating hash_mem instead of
sort_mem is a direction that is entirely debatable, and should
actually be debated. OTOH, it seems like a real problem that we don't
allow hashing to take full advantage of available main memory, and
*some* interim solution seems preferable to what we have now.
--
Peter Geoghegan
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