Re: [PATCH] Incremental sort (was: PoC: Partial sort)

On Wed, Apr 08, 2020 at 09:54:42AM -0400, James Coleman wrote:
>On Wed, Apr 8, 2020 at 9:43 AM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>>
>> On Wed, Apr 08, 2020 at 12:51:05PM +0200, Tomas Vondra wrote:
>> >On Tue, Apr 07, 2020 at 11:54:23PM -0400, Tom Lane wrote:
>> >>hyrax is not too happy with this test:
>> >>
>> >>https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=hyrax&dt=2020-04-07%2004%3A55%3A15
>> >>
>> >>It's not too clear to me why CLOBBER_CACHE_ALWAYS would be breaking
>> >>EXPLAIN output, but it evidently is.
>> >>
>> >
>> >Thanks, I'll investigate. It's not clear to me either what might be
>> >causing this, but I guess something must have gone wrong in
>> >estimation/planning.
>> >
>>
>> OK, I know what's going on - it's a rather embarassing issue in the
>> regression test. There's no analyze on the test tables, so it uses
>> default estimates for number of groups etc. But with clobber cache the
>> test runs long enough for autoanalyze to kick in and collect stats, so
>> we generate better estimates which changes the plan.
>>
>> I'll get this fixed - explicit analyze and tweaking the data a bit
>> should do the trick.
>
>Looking at the tests that failed, I think we should consider just adding:
>set enable_sort = off;
>because several of those tests have very specific amounts of data to
>ensure we test the transition points around the different modes in the
>incremental sort node.
>

Maybe, but I'd much rather tweak the data so that we test both the
costing and execution part.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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