On Apr 10, 2023, 16:35 +0800, Andy Fan <zhihui.fan1213@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