pgsql: Fix planning of star-schema-style queries.

Fix planning of star-schema-style queries.

Part of the intent of the parameterized-path mechanism was to handle
star-schema queries efficiently, but some overly-restrictive search
limiting logic added in commit e2fa76d80ba571d4de8992de6386536867250474
prevented such cases from working as desired. Fix that and add a
regression test about it. Per gripe from Marc Cousin.

This is arguably a bug rather than a new feature, so back-patch to 9.2
where parameterized paths were introduced.

Branch
------
REL9_4_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/fdacbf9e89267a3c3eb382b285bdb724ad5c6e44

Modified Files
--------------
src/backend/optimizer/README | 34 +++++++++++++++++++++------
src/backend/optimizer/path/joinpath.c | 41 +++++++++++++++++++++++++--------
src/test/regress/expected/join.out | 19 +++++++++++++++
src/test/regress/sql/join.sql | 9 ++++++++
4 files changed, 86 insertions(+), 17 deletions(-)