From: | Peter Geoghegan <pg(at)bowt(dot)ie> |
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To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Jeff Davis <pgsql(at)j-davis(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, David Rowley <dgrowleyml(at)gmail(dot)com>, Stephen Frost <sfrost(at)snowman(dot)net>, Andres Freund <andres(at)anarazel(dot)de>, Bruce Momjian <bruce(at)momjian(dot)us>, Justin Pryzby <pryzby(at)telsasoft(dot)com>, Melanie Plageman <melanieplageman(at)gmail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: Default setting for enable_hashagg_disk |
Date: | 2020-07-26 00:13:00 |
Message-ID: | CAH2-Wzn_Zh9L-NsBmMDbpLTJnOq_9iuUc7+2DAER=cD3qiPzhg@mail.gmail.com |
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On Sat, Jul 25, 2020 at 5:05 PM Tomas Vondra
<tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
> I'm not sure what you mean by "reported memory usage doesn't reflect the
> space used for transition state"? Surely it does include that, we've
> built the memory accounting stuff pretty much exactly to do that.
>
> I think it's pretty clear what's happening - in the sorted case there's
> only a single group getting new values at any moment, so when we decide
> to spill we'll only add rows to that group and everything else will be
> spilled to disk.
Right.
> In the unsorted case however we manage to initialize all groups in the
> hash table, but at that point the groups are tiny an fit into work_mem.
> As we process more and more data the groups grow, but we can't evict
> them - at the moment we don't have that capability. So we end up
> processing everything in memory, but significantly exceeding work_mem.
work_mem was set to 200MB, which is more than the reported "Peak
Memory Usage: 1605334kB". So either the random case significantly
exceeds work_mem and the "Peak Memory Usage" accounting is wrong
(because it doesn't report this excess), or the random case really
doesn't exceed work_mem but has a surprising advantage over the sorted
case.
--
Peter Geoghegan
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