Horton

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

James Coleman <jtc331@gmail.com>

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From: James Coleman <jtc331@gmail.com>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Simon Riggs <simon@2ndquadrant.com>, Rafia Sabih <rafia.pghackers@gmail.com>, Shaun Thomas <shaun.thomas@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2019-06-25T18:03:28Z
Lists: pgsql-hackers 
On Tue, Jun 25, 2019 at 1:13 PM Peter Geoghegan <pg@bowt.ie> wrote:
>
> On Tue, Jun 25, 2019 at 9:53 AM James Coleman <jtc331@gmail.com> wrote:
> > Given the patch contents I don't see any obviously reason why either
> > of those possibilities (calling comparetup_heap less often, or it's
> > cheaper) are likely. Is that something we should investigate further?
> > Or is it just a nice happy accident that we should ignore since it's
> > dataset specific?
>
> Have you actually confirmed that comparetup_heap() is called less
> often, by instrumenting the number of individual calls specifically?
> If there has been a reduction in calls to comparetup_heap(), then
> that's weird, and needs to be explained. If it turns out that it isn't
> actually called less often, then I would suggest that the speedup
> might be related to memory fragmentation, which often matters a lot
> within tuplesort.c. (This is why external sort merging now uses big
> buffers, and double buffering.)

No, I haven't confirmed that it's called less frequently, and I'd be
extremely surprised if it were given the diff doesn't suggest any
changes to that at all.

If you think it's important enough to do so, I can instrument it to
confirm, but I was mostly wanting to know if there were any other
plausible explanations, and I think you've provided one: there *are*
changes in the patch to memory contexts in tuplesort.c, so if memory
fragmentation is a real concern this patch could definitely notice
changes in that regard.

James Coleman

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.