Horton

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

Alvaro Herrera <alvherre@2ndquadrant.com>

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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Cc: James Coleman <jtc331@gmail.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>, Tom Lane <tgl@sss.pgh.pa.us>, PostgreSQL Developers <pgsql-hackers@lists.postgresql.org>
Date: 2020-03-12T21:53:11Z
Lists: pgsql-hackers

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I gave this a very quick look; I don't claim to understand it or
anything, but I thought these trivial cleanups worthwhile.  The only
non-cosmetic thing is changing order of arguments to the SOn_printf()
calls in 0008; I think they are contrary to what the comment says.

I don't propose to commit 0003 of course, since it's not our policy;
that's just to allow running pgindent sanely, which gives you 0004
(though my local pgindent has an unrelated fix).  And after that you
notice the issue that 0005 fixes.

I did notice that show_incremental_sort_group_info() seems to be doing
things in a hard way, or something.  I got there because it throws this
warning:

/pgsql/source/master/src/backend/commands/explain.c: In function 'show_incremental_sort_group_info':
/pgsql/source/master/src/backend/commands/explain.c:2766:39: warning: passing argument 2 of 'lappend' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
    methodNames = lappend(methodNames, sortMethodName);
                                       ^~~~~~~~~~~~~~
In file included from /pgsql/source/master/src/include/access/xact.h:20,
                 from /pgsql/source/master/src/backend/commands/explain.c:16:
/pgsql/source/master/src/include/nodes/pg_list.h:509:14: note: expected 'void *' but argument is of type 'const char *'
 extern List *lappend(List *list, void *datum);
              ^~~~~~~
/pgsql/source/master/src/backend/commands/explain.c:2766:39: warning: passing 'const char *' to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
                        methodNames = lappend(methodNames, sortMethodName);
                                                           ^~~~~~~~~~~~~~
/pgsql/source/master/src/include/nodes/pg_list.h:509:40: note: passing argument to parameter 'datum' here
extern List *lappend(List *list, void *datum);
                                       ^
1 warning generated.

(Eh, it's funny that GCC reports two warnings about the same line, and
then says there's one warning.)

I suppose you could silence this by adding pstrdup(), and then use
list_free_deep (you have to put the sortMethodName declaration in the
inner scope for that, but seems fine).  Or maybe there's a clever way
around it.

But I hesitate to send a patch for that because I think the whole
function is written by handling text and the other outputs completely
separately -- but looking for example show_modifytable_info() it seems
you can do ExplainOpenGroup, ExplainPropertyText, ExplainPropertyList
etc in all explain output modes, and those routines will care about
emitting the data in the correct format, without having the
show_incremental_sort_group_info function duplicate everything.

HTH.  I would really like to get this patch done for pg13.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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.