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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master

Details
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http://git.postgresql.org/pg/commitdiff/5372275b4b5fc183c6c6dd4517cfd74d5b641446

Modified Files
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src/backend/optimizer/path/allpaths.c | 104 +++++++++++++++++++++++++++------
src/test/regress/expected/union.out | 34 +++++++++++
src/test/regress/sql/union.sql | 21 +++++++
3 files changed, 140 insertions(+), 19 deletions(-)