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
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REL9_0_STABLE

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=4ec15077b2eaa1f7713e4fff87e220a23c772688

Modified Files
--------------
src/backend/nodes/copyfuncs.c             |    1 +
src/backend/nodes/equalfuncs.c            |    1 +
src/backend/nodes/outfuncs.c              |    1 +
src/backend/optimizer/plan/analyzejoins.c |    2 +
src/backend/optimizer/plan/initsplan.c    |   37 +++++
src/backend/optimizer/plan/planmain.c     |   23 ++-
src/backend/optimizer/util/placeholder.c  |  238 +++++++++++++++++++++++++---
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 +++++
11 files changed, 374 insertions(+), 36 deletions(-)