[Question] Similar Cost but variable execution time in sort

Hi,

This was noticed in
https://www.postgresql.org/message-id/CAApHDvo2y9S2AO-BPYo7gMPYD0XE2Lo-KFLnqX80fcftqBCcyw@mail.gmail.com

I am bringing it up again.

Consider the following example:

Setup (tuple should be in memory to avoid overshadowing of disk I/O in
the experimentation):

work_mem = 2048MB

create table abcd(a int, b int, c int, d int);
insert into abcd select x*random(), x*random(), x*random(), x*random()
from generate_series(1, 100000)x;

select pg_prewarm(abcd);

1. explain analyze select * from abcd order by a;

                                                    QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
 Sort  (cost=9845.82..10095.82 rows=100000 width=16) (actual
time=134.113..155.990 rows=100000 loops=1)
   Sort Key: a
   Sort Method: quicksort  Memory: 8541kB
   ->  Seq Scan on abcd  (cost=0.00..1541.00 rows=100000 width=16)
(actual time=0.013..28.418 rows=100000 loops=1)
 Planning Time: 0.392 ms
 Execution Time: 173.702 ms
(6 rows)

2. explain analyze select * from abcde order by a,b;

explain analyze select * from abcd order by a,b;
                                                    QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
 Sort  (cost=9845.82..10095.82 rows=100000 width=16) (actual
time=174.676..204.065 rows=100000 loops=1)
   Sort Key: a, b
   Sort Method: quicksort  Memory: 8541kB
   ->  Seq Scan on abcd  (cost=0.00..1541.00 rows=100000 width=16)
(actual time=0.018..29.213 rows=100000 loops=1)
 Planning Time: 0.055 ms
 Execution Time: 229.119 ms
(6 rows)

3. explain analyze select * from abcd order by a,b,c;

                                                    QUERY PLAN
-------------------------------------------------------------------------------------------------------------------
 Sort  (cost=9845.82..10095.82 rows=100000 width=16) (actual
time=159.829..179.675 rows=100000 loops=1)
   Sort Key: a, b, c
   Sort Method: quicksort  Memory: 8541kB
   ->  Seq Scan on abcd  (cost=0.00..1541.00 rows=100000 width=16)
(actual time=0.018..31.207 rows=100000 loops=1)
 Planning Time: 0.055 ms
 Execution Time: 195.393 ms
(6 rows)

In above queries, startup and total costs are same, yet execution time
varies wildly.

Question: If cost is same for similar query, shouldn't execution time be
similar as well?

From my observation, we only account for data in cost computation but
not number of

columns sorted.

Should we not account for number of columns in sort as well?

Relevant discussion:
https://www.postgresql.org/message-id/CAApHDvoc1m_vo1+XVpMUj+Mfy6rMiPQObM9Y-jZ=Xrwc1gkPFA@mail.gmail.com

Regards,

Ankit