Re: [POC] Allow flattening of subquery with a link to upper query

On 9/1/22 17:24, Richard Guo wrote:
>
> On Wed, Aug 31, 2022 at 2:35 PM Andrey Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru <mailto:a(dot)lepikhov(at)postgrespro(dot)ru>> wrote:
>
> Before flattening procedure we just look through the quals of subquery,
> pull to the upper level OpExpr's containing variables from the upper
> relation and replace their positions in the quals with true expression.
> Further, the flattening machinery works as usual.
>
> Hmm, I'm not sure this patch works correctly in all cases. It seems to
> me this patch pulls up the subquery without checking the constraints
> imposed by lateral references. If its quals contain any lateral
> references to rels outside a higher outer join, we would need to
> postpone quals from below an outer join to above it, which is probably
> incorrect.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). It looks ugly.
But, more important, does this feature deserve such efforts/changes?

--
Regards
Andrey Lepikhov
Postgres Professional