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
------
REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/df3b0f47b9766ff14f50c3e381d8c00a9c2b7a4f

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(-)