From: Jeff Davis <pgsql@j-davis.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: Justin Pryzby <pryzby@telsasoft.com>
Cc: pgsql-hackers@postgresql.org <pgsql-hackers@postgresql.org>
Subject: Re: Default setting for enable_hashagg_disk
Date: Wed, 10 Jun 2020 10:39:08 -0700
Message-ID: <87a16031a58db0c4ba4bb506f2919dea8948d7b0.camel@j-davis.com> (raw)
In-Reply-To: <CAAKRu_aisiENMsPM2gC4oUY1hHG3yrCwY-fXUg22C6_MJUwQdA@mail.gmail.com>
References: <9d9d1e1252a52ea1bad84ea40dbebfd54e672a0f.camel@j-davis.com>
<20200407223900.GT2228@telsasoft.com>
<20200409114855.h4hh7e2wt77p56sv@development>
<CA+Tgmoa74xaiZTdLeQ_aiPE7-eJb_Pdkc_K_hv95PdaDjZnxKQ@mail.gmail.com>
<6292f5a766198d5745eb8cc17ac736366056671d.camel@j-davis.com>
<CAAKRu_aisiENMsPM2gC4oUY1hHG3yrCwY-fXUg22C6_MJUwQdA@mail.gmail.com>
On Tue, 2020-06-09 at 18:20 -0700, Melanie Plageman wrote:
> So, I was catching up on email and noticed the last email in this
> thread.
>
> I think I am not fully understanding what
> enable_groupingsets_hash_disk
> does. Is it only for testing?
It's mostly for testing. I could imagine cases where it would be useful
to force groupingsets to use the disk, but I mainly wanted the setting
there for testing the grouping sets hash disk code path.
> Using the tests you added to src/test/regress/sql/groupingsets.sql, I
> did get a plan that looks like hashagg is spilling to disk (goes
> through
I had something that worked as a test for a while, but then when I
tweaked the costing, it started using the Sort path (therefore not
testing my grouping sets hash disk code at all) and a bug crept in. So
I thought it would be best to have a more forceful knob.
Perhaps I should just get rid of that GUC and use the stats trick?
Regards,
Jeff Davis
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Subject: Re: Default setting for enable_hashagg_disk
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