pgsql: Fix another oversight in checking if a join with LATERAL refs is

Fix another oversight in checking if a join with LATERAL refs is legal.

It was possible for the planner to decide to join a LATERAL subquery to
the outer side of an outer join before the outer join itself is completed.
Normally that's fine because of the associativity rules, but it doesn't
work if the subquery contains a lateral reference to the inner side of the
outer join.  In such a situation the outer join *must* be done first.
join_is_legal() missed this consideration and would allow the join to be
attempted, but the actual path-building code correctly decided that no
valid join path could be made, sometimes leading to planner errors such as
"failed to build any N-way joins".

Per report from Andreas Seltenreich.  Back-patch to 9.3 where LATERAL
support was added.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7e19db0c09719d7919a8fdd96a1fffe7efd2df93

Modified Files
--------------
src/backend/optimizer/path/joinrels.c |   30 ++++++++++++++++++++++++
src/backend/optimizer/util/relnode.c  |   41 +++++++++++++++++++++++++++++++++
src/include/optimizer/pathnode.h      |    1 +
src/test/regress/expected/join.out    |   41 +++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql         |   19 +++++++++++++++
5 files changed, 132 insertions(+)