Re: A new strategy for pull-up correlated ANY_SUBLINK

On 2/11/2022 09:02, Andy Fan wrote:
> In the past we pull-up the ANY-sublink with 2 steps, the first step is to
> pull up the sublink as a subquery, and the next step is to pull up the
> subquery if it is allowed.  The benefits of this method are obvious,
> pulling up the subquery has more requirements, even if we can just finish
> the first step, we still get huge benefits. However the bad stuff happens
> if varlevelsup = 1 involves, things fail at step 1.
>
> convert_ANY_sublink_to_join ...
>
>     if (contain_vars_of_level((Node *) subselect, 1))
>         return NULL;
>
> In this patch we distinguish the above case and try to pull-up it within
> one step if it is helpful, It looks to me that what we need to do is just
> transform it to EXIST-SUBLINK.
Maybe code [1] would be useful for your purposes/tests.
We implemented flattening of correlated subqueries for simple N-J case,
but found out that in some cases the flattening isn't obvious the best
solution - we haven't info about cardinality/cost estimations and can do
worse.
I guess, for more complex flattening procedure (with aggregate function
in a targetlist of correlated subquery) situation can be even worse.
Maybe your idea has such corner cases too ?

[1]
https://www.postgresql.org/message-id/flat/CALNJ-vTa5VgvV1NPRHnypdnbx-fhDu7vWp73EkMUbZRpNHTYQQ%40mail.gmail.com

--
regards,
Andrey Lepikhov
Postgres Professional