pgsql: Fix planning of parameterized appendrel paths with expensive joi

Fix planning of parameterized appendrel paths with expensive join quals.

The code in set_append_rel_pathlist() for building parameterized paths
for append relations (inheritance and UNION ALL combinations) supposed
that the cheapest regular path for a child relation would still be cheapest
when reparameterized.  Which might not be the case, particularly if the
added join conditions are expensive to compute, as in a recent example from
Jeff Janes.  Fix it to compare child path costs *after* reparameterizing.
We can short-circuit that if the cheapest pre-existing path is already
parameterized correctly, which seems likely to be true often enough to be
worth checking for.

Back-patch to 9.2 where parameterized paths were introduced.

Branch
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REL9_2_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/1bd25c03487dd2f96035f88c77fc0d89bb438fac

Modified Files
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src/backend/optimizer/path/allpaths.c |  108 ++++++++++++++++++++++++++-------
src/test/regress/expected/union.out   |   34 +++++++++++
src/test/regress/sql/union.sql        |   21 +++++++
3 files changed, 142 insertions(+), 21 deletions(-)