pgsql: Further fixes for degenerate outer join clauses.

Further fixes for degenerate outer join clauses.

Further testing revealed that commit f69b4b9495269cc4 was still a few
bricks shy of a load: minor tweaking of the previous test cases resulted
in the same wrong-outer-join-order problem coming back.  After study
I concluded that my previous changes in make_outerjoininfo() were just
accidentally masking the problem, and should be reverted in favor of
forcing syntactic join order whenever an upper outer join's predicate
doesn't mention a lower outer join's LHS.  This still allows the
chained-outer-joins style that is the normally optimizable case.

I also tightened things up some more in join_is_legal().  It seems to me
on review that what's really happening in the exception case where we
ignore a mismatched special join is that we're allowing the proposed join
to associate into the RHS of the outer join we're comparing it to.  As
such, we should *always* insist that the proposed join be a left join,
which eliminates a bunch of rather dubious argumentation.  The case where
we weren't enforcing that was the one that was already known buggy anyway
(it had a violatable Assert before the aforesaid commit) so it hardly
deserves a lot of deference.

Back-patch to all active branches, like the previous patch.  The added
regression test case failed in all branches back to 9.1, and I think it's
only an unrelated change in costing calculations that kept 9.0 from
choosing a broken plan.

Branch
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REL9_3_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/3e79144a848e9e65d248d252488a37678407d6b3

Modified Files
--------------
src/backend/optimizer/README           |   23 +++++++---
src/backend/optimizer/path/joinrels.c  |   73 ++++++++++++--------------------
src/backend/optimizer/plan/initsplan.c |   28 ++++++------
src/test/regress/expected/join.out     |   72 ++++++++++++++++++++++++++++++-
src/test/regress/sql/join.sql          |   25 +++++++++++
5 files changed, 153 insertions(+), 68 deletions(-)