pgsql: Avoid unnecessary wrapping for more complex expressions

Avoid unnecessary wrapping for more complex expressions

When pulling up a subquery that is under an outer join, if the
subquery's target list contains a strict expression that uses a
subquery variable, it's okay to pull up the expression without
wrapping it in a PlaceHolderVar: if the subquery variable is forced to
NULL by the outer join, the expression result will come out as NULL
too.

If the strict expression does not contain any subquery variables, the
current code always wraps it in a PlaceHolderVar. While this is not
incorrect, the analysis could be tighter: if the strict expression
contains any variables of rels that are under the same lowest nulling
outer join as the subquery, we can also avoid wrapping it. This is
safe because if the subquery variable is forced to NULL by the outer
join, the variables of rels that are under the same lowest nulling
outer join will also be forced to NULL, resulting in the expression
evaluating to NULL as well. Therefore, it's not necessary to force
the expression to be evaluated below the outer join. It could be
beneficial to get rid of such PHVs because they could imply lateral
dependencies, which force us to resort to nestloop joins.

This patch checks if the lateral references in the strict expression
contain any variables of rels under the same lowest nulling outer join
as the subquery, and avoids wrapping the expression in that case.

This is fundamentally a generalization of the optimizations for bare
Vars and PHVs introduced in commit f64ec81a8.

No backpatch as this could result in plan changes.

Author: Richard Guo
Discussion: https://postgr.es/m/CAMbWs4_ENtfRdLaM_bXAxiKRYO7DmwDBDG4_2=VTDi0mJP-jAw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/60be3f9f0a64a071822c0711f69c7b5f6467d84f

Modified Files
--------------
src/backend/optimizer/prep/prepjointree.c | 67 ++++++++++++++-----
src/test/regress/expected/subselect.out | 103 ++++++++++++++++++++++++++++++
src/test/regress/sql/subselect.sql | 33 ++++++++++
3 files changed, 186 insertions(+), 17 deletions(-)