On 9/5/22 12:22, Richard Guo wrote:
On Fri, Sep 2, 2022 at 7:09 PM Andrey Lepikhov
Yeah, it's not easy-to-solve problem. If I correctly understand the
code, to fix this problem we must implement the same logic, as
pull_up_subqueries (lowest_outer_join/safe_upper_varnos).
Yeah, I think we'd have to consider the restrictions from lateral
references to guarantee correctness when we pull up subqueries. We need
to avoid the situation where quals need to be postponed past outer join.
However, even if we have taken care of that, there may be other issues
with flattening direct-correlated ANY SubLink. The constraints imposed
by LATERAL references may make it impossible for us to find any legal
join orders, as discussed in [1].
[1]
https://www.postgresql.org/message-id/CAMbWs49cvkF9akbomz_fCCKS=D5TY=4kgheqcfhpzcxs1gv...@mail.gmail.com <https://www.postgresql.org/message-id/CAMbWs49cvkF9akbomz_fCCKS=D5TY=4kgheqcfhpzcxs1gv...@mail.gmail.com>
The problem you mentioned under this link is about ineffective query
plan - as I understand it.
This is a problem, especially if we would think about more complex
pull-ups of subqueries - with aggregate functions in the target list.
I think about that problem as about next step - we already have an
example - machinery of alternative plans. This problem may be solved in
this way, or by a GUC, as usual.
--
Regards
Andrey Lepikhov
Postgres Professional
