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help / color / mirror / Atom feedFrom: Robert Haas <[email protected]>
To: Richard Guo <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: Tender Wang <[email protected]>
Cc: Paul George <[email protected]>
Cc: Andy Fan <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Cc: [email protected]
Cc: Matheus Alcantara <[email protected]>
Subject: Re: Eager aggregation, take 3
Date: Fri, 12 Sep 2025 14:47:48 -0400
Message-ID: <CA+TgmoaY6E5-UFTWp5BtAjBO=tDQd=UVAgeJ3dRbFFzhnP5NHg@mail.gmail.com> (raw)
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On Fri, Sep 12, 2025 at 5:34 AM Richard Guo <[email protected]> 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 = -aggtransspace *
rowcount, not just using a <0 value as a sentinel.
--
Robert Haas
EDB: http://www.enterprisedb.com
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Subject: Re: Eager aggregation, take 3
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