Re: POC: GROUP BY optimization

From: Andrey 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: 2022-01-24 05:02:03
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On 22/1/2022 01:34, Tomas Vondra wrote:
> The other thing we could do is reduce the coefficient gradually - so
> it'd be 1.5 for the first pathkey, then 1.25 for the next one, and so
> on. But it seems somewhat arbitrary (I certainly don't have some sound
> theoretical justification ...).

I think, it hasn't a reason to increase complexity without any theory at
the bottom. Simple solution allows to realize problems much faster, if
they arise.

> ... I've skipped the path_save
> removal in planner.c, because that seems incorrect - if there are
> multiple pathkeys, we must start with the original path, not the
> modified one we built in the last iteration. Or am I missing something
You are right, I misunderstood the idea of path_save variable.

Andrey Lepikhov
Postgres Professional

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