Horton

Re: Eager aggregation, take 3

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
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>
Date: 2025-09-05T13:12:44Z
Lists: pgsql-hackers 
On Wed, Aug 6, 2025 at 3:52 AM 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?

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

Commits

  1. Fix eager aggregation for semi/antijoin inner rels

  2. Cover additional errors and corner conditions in repack.c

  3. Fix volatile function evaluation in eager aggregation

  4. Fix collation handling for grouping keys in eager aggregation

  5. Rename apply_at to apply_agg_at for clarity

  6. Fix comment in eager_aggregate.sql

  7. Remove unnecessary include of "utils/fmgroids.h"

  8. Implement Eager Aggregation

  9. Allow negative aggtransspace to indicate unbounded state size

  10. Add macros for looping through a List without a ListCell.

  11. Account for the effect of lossy pages when costing bitmap scans.

  12. Fix a thinko in join_is_legal: when we decide we can implement a semijoin