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From: Robert Haas <robertmhaas@gmail.com>
Date: Fri, 12 Sep 2025 14:47:48 -0400
Message-ID: 
 <CA+TgmoaY6E5-UFTWp5BtAjBO=tDQd=UVAgeJ3dRbFFzhnP5NHg@mail.gmail.com>
Subject: Re: Eager aggregation, take 3
To: Richard Guo <guofenglinux@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Tender Wang <tndrwang@gmail.com>,
	Paul George <p.a.george19@gmail.com>, Andy Fan <zhihuifan1213@163.com>,
	PostgreSQL-development <pgsql-hackers@postgresql.org>,
 pgsql-hackers@lists.postgresql.org,
	Matheus Alcantara <matheusssilv97@gmail.com>
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On Fri, Sep 12, 2025 at 5:34=E2=80=AFAM Richard Guo <guofenglinux@gmail.com=
> wrote:
> I really like this idea.  Currently, aggtransspace represents an
> estimate of the transition state size provided by the aggregate
> definition.  If it's set to zero, a default estimate based on the
> state data type is used.  Negative values currently have no defined
> meaning.  I think it makes perfect sense to reuse this field so that
> a negative value indicates that the transition state data can grow
> unboundedly in size.
>
> Attached 0002 implements this idea.  It requires fewer code changes
> than I expected.  This is mainly because that our current code uses
> aggtransspace in such a way that if it's a positive value, that value
> is used as it's provided by the aggregate definition; otherwise, some
> heuristics are applied to estimate the size.  For the aggregates that
> accumulate input rows (e.g., array_agg, string_agg), I don't currently
> have a better heuristic for estimating their size, so I've chosen to
> keep the current logic.  This won't regress anything in estimating
> transition state data size.

This might be OK, but it's not what I was suggesting: I was suggesting
trying to do a calculation like space_used =3D -aggtransspace *
rowcount, not just using a <0 value as a sentinel.

--=20
Robert Haas
EDB: http://www.enterprisedb.com