But I did not find a query that makes an incremental sort in this case.
After trying for a while it seems to me that we do not need to consider
incremental sort in this case, because for a partial path of a grouped
or partially grouped relation, it is either unordered (HashAggregate or
Append), or it has been ordered by the group_pathkeys (GroupAggregate).
It seems there is no case that we'd have a partial path that is
partially sorted.
Since now we'd try to reorder the group by keys (see 0452b461bc), it is
possible that we have a partial path that is partially sorted. So this
conclusion is not true any more. For instance,
create table t (a int, b int, c int, d int);
insert into t select i%10, i%10, i%10, i%10 from generate_series(1,1000000)i;
create index on t (a, b);
analyze t;
set enable_hashagg to off;
set enable_seqscan to off;
explain (costs off)
select count(*) from t group by a, c, b, parallel_safe_volatile(d);
QUERY PLAN
--------------------------------------------------------------------------
Finalize GroupAggregate
Group Key: a, c, b, (parallel_safe_volatile(d))
-> Gather Merge
Workers Planned: 2
-> Incremental Sort
Sort Key: a, c, b, (parallel_safe_volatile(d))
Presorted Key: a
-> Partial GroupAggregate
Group Key: a, b, c, (parallel_safe_volatile(d))
-> Incremental Sort
Sort Key: a, b, c, (parallel_safe_volatile(d))
Presorted Key: a, b
-> Parallel Index Scan using t_a_b_idx on t
(13 rows)
If we do not consider incremental sort on partial paths in
gather_grouping_paths(), we'd get a plan that looks like:
explain (costs off)
select count(*) from t group by a, c, b, parallel_safe_volatile(d);
QUERY PLAN
--------------------------------------------------------------------------------
Finalize GroupAggregate
Group Key: a, c, b, (parallel_safe_volatile(d))
-> Incremental Sort
Sort Key: a, c, b, (parallel_safe_volatile(d))
Presorted Key: a, c, b
-> Gather Merge
Workers Planned: 2
-> Incremental Sort
Sort Key: a, c, b
Presorted Key: a
-> Partial GroupAggregate
Group Key: a, b, c, (parallel_safe_volatile(d))
-> Incremental Sort
Sort Key: a, b, c, (parallel_safe_volatile(d))
Presorted Key: a, b
-> Parallel Index Scan using t_a_b_idx on t
(16 rows)
So in the v3 patch I've brought back the logic that considers
incremental sort on partial paths in gather_grouping_paths(). However,
I failed to compose a test case for this scenario without having to
generate a huge table. So in the v3 patch I did not include a test case
for this aspect.
Thanks
Richard