Hi Michal,
On Fri, Oct 6, 2023 at 9:34 PM Michał Kłeczek <mic...@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 try to 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
