Hi Andrey:
> 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.
Looks like we are resolving the same problem, IIUC, great that more
people are interested in it!
We implemented flattening of correlated subqueries for simple N-J case,
I went through the code, and it looks like you tried to do the pull-up by
yourself, which would have many troubles to think about. but I just transformed
it into EXIST sublink after I distinguish it as the case I can improve.
> The only change is transforming the format of SUBLINK, so outer-join /
> pull-up as semi-join is unrelated, so the correctness should not be an
> issue.
That is just a difference, no matter which one is better.
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 ?
In my case, since aggregate function can't be handled by
covert_EXISTS_sublink_to_join, so it is not the target I want to optimize in
this patch. More testing/review on my method would be pretty appreciated.
but I'm not insisting on my method at all. Link [2] might be useful as well.
