Horton

Re: [HACKERS] [PATCH] Incremental sort

Tomas Vondra <tomas.vondra@2ndquadrant.com>

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From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
To: Alexander Kuzmenkov <a.kuzmenkov@postgrespro.ru>, Alexander Korotkov <a.korotkov@postgrespro.ru>, Andres Freund <andres@anarazel.de>
Cc: Teodor Sigaev <teodor@sigaev.ru>, Darafei Komяpa Praliaskouski <me@komzpa.net>, Antonin Houska <ah@cybertec.at>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2018-03-31T21:06:54Z
Lists: pgsql-hackers 
On 03/31/2018 10:43 PM, Tomas Vondra wrote:
> ...
> But I'm pretty sure it may lead to surprising behavior - for example if
> you disable incremental sorts (enable_incrementalsort=off), the plan
> will switch to plain sort without the additional costs. So you'll get a
> cheaper plan by disabling some operation. That's surprising.
> 

To illustrate this is a valid issue, consider this trivial example:

create table t (a int, b int, c int);

insert into t select 10*random(), 10*random(), 10*random()
  from generate_series(1,1000000) s(i);

analyze t;

explain select * from (select * from t order by a,b) foo order by a,b,c;

                               QUERY PLAN
------------------------------------------------------------------------
 Incremental Sort  (cost=133100.48..264139.27 rows=1000000 width=12)
   Sort Key: t.a, t.b, t.c
   Presorted Key: t.a, t.b
   ->  Sort  (cost=132154.34..134654.34 rows=1000000 width=12)
         Sort Key: t.a, t.b
         ->  Seq Scan on t  (cost=0.00..15406.00 rows=1000000 width=12)
(6 rows)

set enable_incrementalsort = off;

explain select * from (select * from t order by a,b) foo order by a,b,c;
                               QUERY PLAN
------------------------------------------------------------------------
 Sort  (cost=261402.69..263902.69 rows=1000000 width=12)
   Sort Key: t.a, t.b, t.c
   ->  Sort  (cost=132154.34..134654.34 rows=1000000 width=12)
         Sort Key: t.a, t.b
         ->  Seq Scan on t  (cost=0.00..15406.00 rows=1000000 width=12)
(5 rows)

So the cost with incremental sort was 264139, and after disabling the
incremental cost it dropped to 263902. Granted, the difference is
negligible in this case, but it's still surprising.

Also, it can be made much more significant by reducing the number of
prefix groups in the data:

truncate t;

insert into t select 1,1,1 from generate_series(1,1000000) s(i);

analyze t;

set enable_incrementalsort = on;

explain select * from (select * from t order by a,b) foo order by a,b,c;

                               QUERY PLAN
------------------------------------------------------------------------
 Incremental Sort  (cost=324165.83..341665.85 rows=1000000 width=12)
   Sort Key: t.a, t.b, t.c
   Presorted Key: t.a, t.b
   ->  Sort  (cost=132154.34..134654.34 rows=1000000 width=12)
         Sort Key: t.a, t.b
         ->  Seq Scan on t  (cost=0.00..15406.00 rows=1000000 width=12)
(6 rows)

So that's 263902 vs. 341665, yet we still prefer the incremental mode.


regards

-- 
Tomas Vondra                  http://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.