Horton

Re: Add proper planner support for ORDER BY / DISTINCT aggregates

David Rowley <dgrowleyml@gmail.com>

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From: David Rowley <dgrowleyml@gmail.com>
To: Richard Guo <guofenglinux@gmail.com>
Cc: Ronan Dunklau <ronan.dunklau@aiven.io>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, Ranier Vilela <ranier.vf@gmail.com>
Date: 2022-07-25T23:38:44Z
Lists: pgsql-hackers

Attachments

On Fri, 22 Jul 2022 at 21:33, Richard Guo <guofenglinux@gmail.com> wrote:
> I can see this problem with
> the query below:
>
>     select max(b order by b), max(a order by a) from t group by a;
>
> When processing the first aggregate, we compose the 'currpathkeys' as
> {a, b} and mark this aggregate in 'aggindexes'. When it comes to the
> second aggregate, we compose its pathkeys as {a} and decide that it is
> not stronger than 'currpathkeys'. So the second aggregate is not
> recorded in 'aggindexes'. As a result, we fail to mark aggpresorted for
> the second aggregate.

Yeah, you're right. I have a missing check to see if currpathkeys are
better than the pathkeys for the current aggregate. In your example
case we'd have still processed the 2nd aggregate the old way instead
of realising we could take the new pre-sorted path for faster
processing.

I've adjusted that in the attached to make it properly include the
case where currpathkeys are better.

Thanks for the review.

David

Commits

  1. Don't presort ORDER BY/DISTINCT Aggrefs with volatile functions

  2. Add enable_presorted_aggregate GUC

  3. Remove pessimistic cost penalization from Incremental Sort

  4. Fix hypothetical problem passing the wrong GROUP BY pathkeys

  5. Remove unused fields from ExprEvalStep

  6. Improve performance of ORDER BY / DISTINCT aggregates

  7. Refactor function parse_subscription_options.