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

On Wed, Apr 08, 2020 at 04:08:39PM +0200, Tomas Vondra wrote:
>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.
>

I do think this does the trick by increasing the number of rows a bit
(from 100 to 1000) to make the Sort more expensive than Incremental
Sort, while still testing the transition points.

James, can you verify it that's still true?

regards

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