pgsql: Add support for deparsing semi-joins to contrib/postgres_fdw

Add support for deparsing semi-joins to contrib/postgres_fdw

SEMI-JOIN is deparsed as the EXISTS subquery. It references outer and inner
relations, so it should be evaluated as the condition in the upper-level WHERE
clause. The signatures of deparseFromExprForRel() and deparseRangeTblRef() are
revised so that they can add conditions to the upper level.

PgFdwRelationInfo now has a hidden_subquery_rels field, referencing the relids
used in the inner parts of semi-join. They can't be referred to from upper
relations and should be used internally for equivalence member searches.

The planner can create semi-join, which refers to inner rel vars in its target
list. However, we deparse semi-join as an exists() subquery. So we skip the
case when the target list references to inner rel of semi-join.

Author: Alexander Pyhalov
Reviewed-by: Ashutosh Bapat, Ian Lawrence Barwick, Yuuki Fujii, Tomas Vondra
Discussion: https://postgr.es/m/c9e2a757cf3ac2333714eaf83a9cc184@postgrespro.ru

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/824dbea3e41efa3b35094163c834988dea7eb139

Modified Files
--------------
contrib/postgres_fdw/deparse.c | 234 ++++++++++++++----
contrib/postgres_fdw/expected/postgres_fdw.out | 320 ++++++++++++++++++++++---
contrib/postgres_fdw/postgres_fdw.c | 94 +++++++-
contrib/postgres_fdw/postgres_fdw.h | 4 +
contrib/postgres_fdw/sql/postgres_fdw.sql | 126 +++++++++-
5 files changed, 697 insertions(+), 81 deletions(-)