pgsql: Use fuzzy not exact cost comparison for the final tie-breaker in

Use fuzzy not exact cost comparison for the final tie-breaker in add_path.

Instead of an exact cost comparison, use a fuzzy comparison with 1e-10
delta after all other path metrics have proved equal. This is to avoid
having platform-specific roundoff behaviors determine the choice when
two paths are really the same to our cost estimators. Adjust the
recently-added test case that made it obvious we had a problem here.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/33e99153e93b9accfa51ac036828144e1c2507b7

Modified Files
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src/backend/optimizer/util/pathnode.c | 44 ++++++++++++++++++++++----------
src/test/regress/expected/join.out | 16 ++++++------
2 files changed, 38 insertions(+), 22 deletions(-)