Horton

Re: [HACKERS] [PATCH] Incremental sort

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alexander Korotkov <a.korotkov@postgrespro.ru>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Teodor Sigaev <teodor@sigaev.ru>, Darafei Komяpa Praliaskouski <me@komzpa.net>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Antonin Houska <ah@cybertec.at>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-04-07T16:09:46Z
Lists: pgsql-hackers 
On 2018-04-07 12:06:52 -0400, Tom Lane wrote:
> Alexander Korotkov <a.korotkov@postgrespro.ru> writes:
> > On Wed, Mar 28, 2018 at 6:38 PM, Alvaro Herrera <alvherre@alvh.no-ip.org>
> > wrote:
> >> Can we have a recap on what the patch *does*?
> 
> > Ggeneral idea hasn't been changed much since first email.
> > Incremental sort gives benefit when you need to sort your dataset
> > by some list of columns while you alredy have input presorted
> > by some prefix of that list of columns.  Then you don't do full sort
> > of dataset, but rather sort groups where values of prefix columns
> > are equal (see header comment in nodeIncremenalSort.c).
> 
> I dunno, how would you estimate whether this is actually a win or not?
> I don't think our model of sort costs is anywhere near refined enough
> or accurate enough to reliably predict whether this is better than
> just doing it in one step.  Even if the cost model is good, it's not
> going to be better than our statistics about the number/size of the
> groups in the first column(s), and that's a notoriously unreliable stat.
> 
> Given that we already have more than enough dubious patches that have
> been shoved in in the last few days, I'd rather not pile on stuff that
> there's any question about.

I don't disagree with any of that. Just wanted to pipe up to say that
there's a fair argument to be made that this patch, which has lingered
for years, "deserves" more to mature in tree than some of the rest.

Greetings,

Andres Freund

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.