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: jian he <jian.universality@gmail.com>, 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
Date: 2024-11-11T16:30:18Z
Lists: pgsql-hackers 
On Sun, Nov 10, 2024 at 7:52 PM Richard Guo <guofenglinux@gmail.com> wrote:
> > I have similar but weaker feelings about ordered aggregates. Consider:
> >
> > explain select t1.id, array_agg(t2.v order by t3.o) from t1, t2, t3
> > where t1.id = t2.id and t2.id = t3.id group by 1;
> >
>
> It seems to me that a partially aggregated row might need to be
> combined with other partially aggregated rows after the join, if they
> belong to the same t1.id group.  IIUC, this implies that we cannot
> perform partial aggregation on ordered input before the join,
> otherwise we may get incorrect results during the final aggregation
> phase.

Hmm, I think you're right. I think that if the t1.id=t2.id join is one
to one, then it would work out fine, but that need not be the case.

> Hmm, currently we only consider grouped aggregation for eager
> aggregation.  For grouped aggregation, the window function's
> arguments, as well as the PARTITION BY expressions, must appear in the
> GROUP BY clause.  That is to say, the depname column in the first
> query, or the n column in the second query, will not be aggregated
> into the partial groups.  Instead, they will remain as they are as
> input for the WindowAgg nodes.  It seems to me that this ensures
> that we're good with window functions.  But maybe I'm wrong.

Unfortunately, I don't know what you mean by grouped aggregation. I
think of grouping and aggregation as synonyms, pretty much.

-- 
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