Re: FDW LIM IT pushdown

Hi Michal,


On Fri, Oct 6, 2023 at 9:34 PM Michał Kłeczek <michal@kleczek.org> wrote:
>
> Hello hackers,
>
> First timer here with a question:
>
> I’ve searched through various e-mail threads and documents and could not find if pushdown of LIMIT clauses to FDW is
implemented.
> Seen some conversation about that a couple of years ago when that was treated as a feature request but nothing  since
then.
> Citus does that according to its documentation but for now we are trying to base our solution on “plain” Postgres.
>
> If it is not implemented - could you, guys, provide me with some pointers to source code where I could look at and
tryto implement that? 
>

I started looking at code from grouping_planner and reached
create_ordinary_grouping_paths(). It calls
create_partitionwise_grouping_paths() to push aggregate and grouping
down into partitions and from there it is pushed down into FDW.
Looking at create_limit_path(), I don't similar treatment. So there
may be some cases where we are not pushing down final LIMIT.

However create_append_path() uses PlannerInfo::limit_tuples or
root->limit_tuples when creating append path node. So either it's
being used for costing or for pushing it down to the partitions.

This isn't a full answer, but I hope these pointers would help you.

--
Best Wishes,
Ashutosh Bapat