pgsql: Fix nested PlaceHolderVar expressions that appear only in target

Fix nested PlaceHolderVar expressions that appear only in targetlists.

A PlaceHolderVar's expression might contain another, lower-level
PlaceHolderVar. If the outer PlaceHolderVar is used, the inner one
certainly will be also, and so we have to make sure that both of them get
into the placeholder_list with correct ph_may_need values during the
initial pre-scan of the query (before deconstruct_jointree starts).
We did this correctly for PlaceHolderVars appearing in the query quals,
but overlooked the issue for those appearing in the top-level targetlist;
with the result that nested placeholders referenced only in the targetlist
did not work correctly, as illustrated in bug #6154.

While at it, add some error checking to find_placeholder_info to ensure
that we don't try to create new placeholders after it's too late to do so;
they have to all be created before deconstruct_jointree starts.

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/pg/commitdiff/8a14bdb10f0a9b123e5c6f70f33334b132b3da29

Modified Files
--------------
src/backend/optimizer/path/costsize.c | 2 +-
src/backend/optimizer/path/equivclass.c | 2 +-
src/backend/optimizer/plan/initsplan.c | 27 ++++++----
src/backend/optimizer/util/placeholder.c | 86 ++++++++++++++++++++---------
src/include/optimizer/placeholder.h | 4 +-
src/include/optimizer/planmain.h | 2 +-
src/test/regress/expected/join.out | 45 ++++++++++++++++
src/test/regress/sql/join.sql | 37 +++++++++++++
8 files changed, 164 insertions(+), 41 deletions(-)