On 07.11.2022 20:30, Ronan Dunklau wrote:
What I meant here is that disabling seqscans, the planner still chooses a full
sort over a partial sort. The underlying index is the same, it is just a
matter of choosing a Sort node over an IncrementalSort node. This, I think, is
wrong: I can't see how it could be worse to use an incrementalsort in that
case.
I finally get your point. And I agree with you.
Maybe the original costing code for incremental sort was a bit too
pessimistic.
In this query, incremental sorting lost just a little bit in cost:
164468.95 vs 162504.23.
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=155002.98..162504.23 rows=100 width=34) (actual
time=296.591..568.270 rows=100 loops=1)
Group Key: a
-> Sort (cost=155002.98..157502.98 rows=1000000 width=4) (actual
time=293.810..454.170 rows=1000000 loops=1)
Sort Key: a, c
Sort Method: external merge Disk: 15560kB
-> Index Scan using t_a_b_idx on t (cost=0.42..41670.64
rows=1000000 width=4) (actual time=0.021..156.441 rows=1000000 loops=1)
Settings: enable_seqscan = 'off'
Planning Time: 0.074 ms
Execution Time: 569.957 ms
(9 rows)
set enable_sort=off;
SET
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------------
GroupAggregate (cost=1457.58..164468.95 rows=100 width=34) (actual
time=6.623..408.833 rows=100 loops=1)
Group Key: a
-> Incremental Sort (cost=1457.58..159467.70 rows=1000000 width=4)
(actual time=2.652..298.530 rows=1000000 loops=1)
Sort Key: a, c
Presorted Key: a
Full-sort Groups: 100 Sort Method: quicksort Average Memory:
27kB Peak Memory: 27kB
Pre-sorted Groups: 100 Sort Method: quicksort Average
Memory: 697kB Peak Memory: 697kB
-> Index Scan using t_a_b_idx on t (cost=0.42..41670.64
rows=1000000 width=4) (actual time=0.011..155.260 rows=1000000 loops=1)
Settings: enable_seqscan = 'off', enable_sort = 'off'
Planning Time: 0.044 ms
Execution Time: 408.867 ms
--
Pavel Luzanov
Postgres Professional: https://postgrespro.com
The Russian Postgres Company
