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.
