Re: POC: GROUP BY optimization

From: "Andrey V(dot) Lepikhov" <a(dot)lepikhov(at)postgrespro(dot)ru>
To: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>, Teodor Sigaev <teodor(at)sigaev(dot)ru>
Cc: Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: POC: GROUP BY optimization
Date: 2022-03-18 07:06:57
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On 3/15/22 13:26, Tomas Vondra wrote:
> Thanks for the rebase. The two proposed changes (tweaked costing and
> simplified fake_var handling) seem fine to me. I think the last thing
> that needs to be done is cleanup of the debug GUCs, which I added to
> allow easier experimentation with the patch.
Thanks, I'm waiting for the last step.
> I probably won't remove the GUCs entirely, though. I plan to add a
> single GUC that would enable/disable this optimization. I'm not a huge
> fan of adding more and more GUCs, but in this case it's probably the
> right thing to do given the complexity of estimating cost with
> correlated columns etc.
Agree. Because it is a kind of automation we should allow user to switch
it off in the case of problems or manual tuning.

Also, I looked through this patch. It has some minor problems:
1. Multiple typos in the patch comment.
2. The term 'cardinality of a key' - may be replace with 'number of

Andrey Lepikhov
Postgres Professional

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