Horton

Re: [PATCH] Incremental sort (was: PoC: Partial sort)

Alexander Korotkov <a.korotkov@postgrespro.ru>

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From: Alexander Korotkov <a.korotkov@postgrespro.ru>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: James Coleman <jtc331@gmail.com>, Shaun Thomas <shaun.thomas@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2019-07-08T22:31:29Z
Lists: pgsql-hackers 
On Mon, Jun 3, 2019 at 12:18 AM Tomas Vondra
<tomas.vondra@2ndquadrant.com> wrote:
> For a moment I thought we could/should look at the histogram, becase that
> could tell us if there are groups "before" the first MCV one, but I don't
> think we should do that, for two reasons. Firstly, rare values may not get
> to the histogram anyway, which makes this rather unreliable and might
> introduce sudden plan changes, because the cost would vary wildly
> depending on whether we happened to sample the rare row or not. And
> secondly, the rare row may be easily filtered out by a WHERE condition or
> something, at which point we'll have to deal with the large group anyway.

If first MCV is in the middle of first histogram bin, then it's
reasonable to think that it would fit to first group.  But if first
MCV is in the middle of histogram, such assumption would be
ridiculous.  Also, I'd like to note that with our
default_statistics_target == 100, non MCV values are not so rare.  So,
I'm +1 for taking histogram into account.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company

Commits

  1. Further adjustments to Hashagg EXPLAIN ANALYZE output

  2. Rework EXPLAIN format for incremental sort

  3. Fix typos and improve incremental sort comments

  4. Stabilize incremental_sort tests

  5. Minor improvements in Incremental Sort explain

  6. Consider Incremental Sort paths at additional places

  7. Fix representation of SORT_TYPE_STILL_IN_PROGRESS.

  8. Fix failures in incremental_sort due to number of workers

  9. Fix show_incremental_sort_info with force_parallel_mode

  10. Implement Incremental Sort

  11. Fix handling of "Subplans Removed" field in EXPLAIN output.

  12. Fix EXPLAIN (SETTINGS) to follow policy about when to print empty fields.

  13. Ensure plpgsql result tuples have the right composite type marking.

  14. Propagate sort instrumentation from workers back to leader.

  15. Make new regression test case parallel-safe, and improve its output.

  16. Push limit through subqueries to underlying sort, where possible.

  17. Fix inappropriate printing of never-measured times in EXPLAIN.

  18. Fix some infelicities in EXPLAIN output for parallel query plans.