Horton

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

Justin Pryzby <pryzby@telsasoft.com>

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From: Justin Pryzby <pryzby@telsasoft.com>
To: James Coleman <jtc331@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Tom Lane <tgl@sss.pgh.pa.us>, Alvaro Herrera <alvherre@2ndquadrant.com>, Michael Paquier <michael@paquier.xyz>, Rafia Sabih <rafia.pghackers@gmail.com>, Peter Geoghegan <pg@bowt.ie>, Simon Riggs <simon@2ndquadrant.com>, Shaun Thomas <shaun.thomas@2ndquadrant.com>, Dmitry Dolgov <9erthalion6@gmail.com>, Alexander Korotkov <a.korotkov@postgrespro.ru>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>, Peter Eisentraut <peter.eisentraut@2ndquadrant.com>, Andreas Karlsson <andreas@proxel.se>
Date: 2020-04-07T15:53:05Z
Lists: pgsql-hackers 
On Tue, Apr 07, 2020 at 08:40:30AM -0400, James Coleman wrote:
> On Tue, Apr 7, 2020 at 12:25 AM Justin Pryzby <pryzby@telsasoft.com> wrote:
> > On Mon, Apr 06, 2020 at 09:57:22PM +0200, Tomas Vondra wrote:
> > > I've pushed the fist part of this patch series - I've reorganized it a
> >
> > I scanned through this again post-commit.  Find attached some suggestions.
> >
> > Shouldn't non-text explain output always show both disk *and* mem, including
> > zeros ?
> 
> Could you give more context on this? Is there a standard to follow?
> Regular sort nodes only ever report one type, so there's not a good
> parallel there.

Right, I'm not sure either, since it seems to be a new case.  Maybe Tomas has a
strong intuition.

See at least the commit messages here:
3ec20c7091e97a554e7447ac2b7f4ed795631395
7d91b604d9b5d6ec8c19c57a9ffd2f27129cdd94
8ebb69f85445177575684a0ba5cfedda8d840a91

Maybe this one suggests that it should *not* be present unconditionally, but
only when that sort type is used?
4b234fd8bf21cd6f5ff44f1f1c613bf40860998d

Another thought: is checking if bytes>0 really a good way to determine if a
sort type was used ?  It seems like checking a bit or a pointer would be
better.  I guess a size of 0 is unlikely, and it's ok at least in text mode.

        if (groupInfo->maxMemorySpaceUsed > 0)

On Tue, Apr 07, 2020 at 08:40:30AM -0400, James Coleman wrote:
> > And, should it use two spaces before "Sort Method", "Memory" and "Pre-sorted
...
> I read through that subthread, and the ending seemed to be Peter
> wanting things to be unified. Was there a conclusion beyond that?

This discussion is ongoing.  I think let's wait until that's settled before
addressing this more complex and even newer case.  We can add "explain, two
spaces and equals vs colon" to the "Open items" list if need be - I hope the
discussion will not delay the release.

-- 
Justin

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.