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From: Robert Haas <robertmhaas@gmail.com>
Date: Fri, 5 Sep 2025 09:12:44 -0400
Message-ID: 
 <CA+TgmobqbeJ9iRQO4ym6OiHt71sSv2eai=01kOZjxhdof9K4Mw@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 Wed, Aug 6, 2025 at 3:52=E2=80=AFAM Richard Guo <guofenglinux@gmail.com>=
 wrote:
> To avoid potential memory blowout risks from large partial aggregation
> values, v18 avoids applying eager aggregation if any aggregate uses an
> INTERNAL transition type, as this typically indicates a large internal
> data structure (as in string_agg or array_agg).  However, this also
> excludes aggregates like avg(numeric) and sum(numeric), which are
> actually safe to use with eager aggregation.
>
> What we really want to exclude are aggregate functions that can
> produce large transition values by accumulating or concatenating input
> rows.  So I'm wondering if we could instead check the transfn_oid
> directly and explicitly exclude only F_ARRAY_AGG_TRANSFN and
> F_STRING_AGG_TRANSFN.  We don't need to worry about json_agg,
> jsonb_agg, or xmlagg, since they don't support partial aggregation
> anyway.

This strategy seems fairly unfriendly towards out-of-core code. Can
you come up with something that allows the author of a SQL-callable
function to include or exclude the function by a choice that is under
their control, rather than hard-coding something in PostgreSQL itself?

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