HI,
On Apr 10, 2023, 16:35 +0800, Andy Fan <zhihui.fan1...@gmail.com>, wrote: > When I am working on "Pushing limit into subqueries of a union" [1], I > found we already have a great infrastructure to support this. For a query > like > > subquery-1 UNION ALL subquery-2 LIMIT 3; > > We have considered the root->tuple_fraction when planning the subqueries > without an extra Limit node as an overhead. But the reason it doesn't work > in my real case is flatten_simple_union_all flat the union all subqueries > into append relation and we didn't handle the root->tuple_fraction during > add_paths_to_append_rel. > > Given the below query for example: > explain analyze > (select * from tenk1 order by hundred) > union all > (select * from tenk2 order by hundred) > limit 3; > > Without the patch: Execution Time: 7.856 ms > with the patch: Execution Time: 0.224 ms > > Any suggestion is welcome. > > [1] https://www.postgresql.org/message-id/11228.1118365833%40sss.pgh.pa.us > > -- > Best Regards > Andy Fan There is spare indent at else if. - if (childrel->pathlist != NIL && + if (cheapest_startup_path && cheapest_startup_path->param_info == NULL) + accumulate_append_subpath(cheapest_startup_path, + &subpaths, NULL); + else if (childrel->pathlist != NIL && childrel->cheapest_total_path->param_info == NULL) accumulate_append_subpath(childrel->cheapest_total_path, &subpaths, NULL); Could we also consider tuple_fraction in partial_pathlist for parallel append? Regards, Zhang Mingli
