pgsql: Fix PlaceHolderVar mechanism's interaction with outer joins.

Fix PlaceHolderVar mechanism's interaction with outer joins.

The point of a PlaceHolderVar is to allow a non-strict expression to be
evaluated below an outer join, after which its value bubbles up like a Var
and can be forced to NULL when the outer join's semantics require that.
However, there was a serious design oversight in that, namely that we
didn't ensure that there was actually a correct place in the plan tree
to evaluate the placeholder :-(. It may be necessary to delay evaluation
of an outer join to ensure that a placeholder that should be evaluated
below the join can be evaluated there. Per recent bug report from Kirill
Simonov.

Back-patch to 8.4 where the PlaceHolderVar mechanism was introduced.

Branch
------
REL8_4_STABLE

Details
-------
http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=dc9cc887b74bfa0d40829c4df66dead509fdd8f6

Modified Files
--------------
src/backend/nodes/copyfuncs.c | 1 +
src/backend/nodes/equalfuncs.c | 1 +
src/backend/nodes/outfuncs.c | 1 +
src/backend/optimizer/plan/initsplan.c | 37 +++++
src/backend/optimizer/plan/planmain.c | 21 ++-
src/backend/optimizer/util/placeholder.c | 234 +++++++++++++++++++++++++++---
src/include/nodes/relation.h | 17 ++-
src/include/optimizer/placeholder.h | 5 +-
src/test/regress/expected/join.out | 45 ++++++
src/test/regress/sql/join.sql | 40 +++++
10 files changed, 369 insertions(+), 33 deletions(-)