Message-ID: <87a16031a58db0c4ba4bb506f2919dea8948d7b0.camel@j-davis.com>
Subject: Re: Default setting for enable_hashagg_disk
From: Jeff Davis <pgsql@j-davis.com>
To: Melanie Plageman <melanieplageman@gmail.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Tomas Vondra
	 <tomas.vondra@2ndquadrant.com>, Justin Pryzby <pryzby@telsasoft.com>,
	"pgsql-hackers@postgresql.org"
	 <pgsql-hackers@postgresql.org>
Date: Wed, 10 Jun 2020 10:39:08 -0700
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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