Re: About reducing EXISTS sublink

On Friday, May 22, 2020, Richard Guo <guofenglinux(at)gmail(dot)com> wrote:

> Hi hackers,
>
> For EXISTS SubLink, in some cases the subquery can be reduced to
> constant TRUE or FALSE, based on the knowledge that it's being used in
> EXISTS(). One such case is when the subquery has aggregates without
> GROUP BY or HAVING, and we know its result is exactly one row, unless
> that row is discarded by LIMIT/OFFSET. (Greenplum does this.)
>
> Is it worthwhile to add some codes for such optimization? If so, I can
> try to propose a patch
>

While the examples clearly demonstrate what you are saying they don’t
provide any motivation to do anything about it - adding aggregates and
offset to an exists subquery seems like poor query design that should be
fixed by the query writer not by spending hacker cycles optimizing it.

David J.