Re: [HACKERS] [PATCH] Incremental sort - Mailing list 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.