On Fri, Feb 28, 2020 at 2:35 PM Richard Guo <guofengli...@gmail.com> wrote:
> Hi All, > > Currently we will not consider EXPR_SUBLINK when pulling up sublinks and > this would cause performance issues for some queries with the form of: > 'a > (SELECT agg(b) from ...)' as described in [1]. > > So here is a patch as an attempt to pull up EXPR SubLinks. The idea, > which is based on Greenplum's implementation, is to perform the > following transformation. > > For query: > > select * from foo where foo.a > > (select avg(bar.a) from bar where foo.b = bar.b); > > we transform it to: > > select * from foo inner join > (select bar.b, avg(bar.a) as avg from bar group by bar.b) sub > on foo.b = sub.b and foo.a > sub.avg; > Glad to see this. I think the hard part is this transform is not *always* good. for example foo.a only has 1 rows, but bar has a lot of rows, if so the original would be the better one. doss this patch consider this problem? > Thanks > Richard >
