From: David Rowley <drowley@postgresql.org>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: Allow Gather Merge in more cases for parallel DISTINCT
Date: Fri, 02 Feb 2024 11:20:45 +0000
Message-ID: <E1rVrbB-004Ktn-Fh@gemulon.postgresql.org> (raw)
Allow Gather Merge in more cases for parallel DISTINCT
Here we adjust the partial path generation for parallel DISTINCT queries
to add Sort nodes on top of any unsorted partial distinct paths.
This increases the likelihood of the planner pushing a Sort below a Gather
Merge which enables the final phase of the parallel distinct to be
implemented using a Unique node in more cases.
Sorting the partial distinct paths is particularly useful when the
DISTINCT query has an ORDER BY and LIMIT clause as this can allow cheaper
plans by having the workers Hash Aggregate then Sort before feeding the
results into the Gather Merge. The non-parallel portion of the plan then
becomes very cheap as it leaves only Unique and Limit to do in the leader
process.
Author: Richard Guo
Reviewed-by: David Rowley
Discussion: https://postgr.es/m/CAMbWs48u9VoVOouJsys1qOaC9WVGVmBa+wT1dx8KvxF5GPzezA@mail.gmail.com
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/7e0ade0ffe0a76b1926a4af39ecdf799c96ef1ba
Modified Files
--------------
src/backend/optimizer/plan/planner.c | 2 +-
src/test/regress/expected/select_distinct.out | 8 ++++----
2 files changed, 5 insertions(+), 5 deletions(-)
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Reply to all the recipients using the --to and --cc options:
reply via email
To: pgsql-committers@postgresql.org
Cc: drowley@postgresql.org, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: Allow Gather Merge in more cases for parallel DISTINCT
In-Reply-To: <E1rVrbB-004Ktn-Fh@gemulon.postgresql.org>
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox