pgsql: Fix choose_bitmap_and() so that partial index predicates are

Log Message:
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Fix choose_bitmap_and() so that partial index predicates are considered when
deciding whether a potential additional indexscan is redundant or not. As now
coded, any use of a partial index that was already used in a previous AND arm
will be rejected as redundant. This might be overly restrictive, but not
considering the point at all is definitely bad, as per example in bug #2441
from Arjen van der Meijden. In particular, a clauseless scan of a partial
index was *never* considered redundant by the previous coding, and that's
surely wrong. Being more flexible would also require some consideration
of how not to double-count the index predicate's selectivity.

Modified Files:
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pgsql/src/backend/optimizer/path:
indxpath.c (r1.205 -> r1.206)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/indxpath.c.diff?r1=1.205&r2=1.206)