Re: Some revises in adding sorting path - Mailing list pgsql-hackers

On Thu, Dec 28, 2023 at 4:00 PM Richard Guo <guofenglinux@gmail.com> wrote:
>
>
> On Wed, Mar 29, 2023 at 4:00 AM David Rowley <dgrowleyml@gmail.com> wrote:
>>
>> If you write some tests for this code, it will be useful to prove that
>> it actually does something, and also that it does not break again in
>> the future. I don't really want to just blindly copy the pattern used
>> in 3c6fc5820 for creating incremental sort paths if it's not useful
>> here. It would be good to see tests that make an Incremental Sort path
>> using the code you're changing.
>
>
> Thanks for the suggestion.  I've managed to come up with a query that
> gets the codes being changed here to perform an incremental sort.
>
> set min_parallel_index_scan_size to 0;
> set enable_seqscan to off;
>
> Without making those parallel paths:
>
> explain (costs off)
> select * from tenk1 where four = 2 order by four, hundred, parallel_safe_volatile(thousand);
>                           QUERY PLAN
> --------------------------------------------------------------
>  Incremental Sort
>    Sort Key: hundred, (parallel_safe_volatile(thousand))
>    Presorted Key: hundred
>    ->  Gather Merge
>          Workers Planned: 3
>          ->  Parallel Index Scan using tenk1_hundred on tenk1
>                Filter: (four = 2)
> (7 rows)
>
> and with those parallel paths:
>
> explain (costs off)
> select * from tenk1 where four = 2 order by four, hundred, parallel_safe_volatile(thousand);
>                           QUERY PLAN
> ---------------------------------------------------------------
>  Gather Merge
>    Workers Planned: 3
>    ->  Incremental Sort
>          Sort Key: hundred, (parallel_safe_volatile(thousand))
>          Presorted Key: hundred
>          ->  Parallel Index Scan using tenk1_hundred on tenk1
>                Filter: (four = 2)
> (7 rows)
>
> I've added two tests for the code changes in create_ordered_paths in the
> new patch.
>
>>
>> Same for the 0003 patch.
>
>
> For the code changes in gather_grouping_paths, I've managed to come up
> with a query that makes an explicit Sort atop cheapest partial path.
>
> explain (costs off)
> select count(*) from tenk1 group by twenty, parallel_safe_volatile(two);
>                              QUERY PLAN
> --------------------------------------------------------------------
>  Finalize GroupAggregate
>    Group Key: twenty, (parallel_safe_volatile(two))
>    ->  Gather Merge
>          Workers Planned: 4
>          ->  Sort
>                Sort Key: twenty, (parallel_safe_volatile(two))
>                ->  Partial HashAggregate
>                      Group Key: twenty, parallel_safe_volatile(two)
>                      ->  Parallel Seq Scan on tenk1
> (9 rows)
>
> Without this logic the plan would look like:
>
> explain (costs off)
> select count(*) from tenk1 group by twenty, parallel_safe_volatile(two);
>                              QUERY PLAN
> --------------------------------------------------------------------
>  Finalize GroupAggregate
>    Group Key: twenty, (parallel_safe_volatile(two))
>    ->  Sort
>          Sort Key: twenty, (parallel_safe_volatile(two))
>          ->  Gather
>                Workers Planned: 4
>                ->  Partial HashAggregate
>                      Group Key: twenty, parallel_safe_volatile(two)
>                      ->  Parallel Seq Scan on tenk1
> (9 rows)
>
> This test is also added in the new patch.
>
> 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.
>
I reviewed the Patch and it looks fine to me.

Thanks and Regards,
Shubham Khanna.