Teach the planner to treat a partial unique index as proving a variable is

Tom Lane <tgl@sss.pgh.pa.us>

Commit: ce6e31de9c99430256468d059eb226ea7a267376
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2009-02-15T20:16:21Z
Releases: 8.4.0
Teach the planner to treat a partial unique index as proving a variable is
unique for a particular query, if the index predicate is satisfied.  This
requires a bit of reordering of operations so that we check the predicates
before doing any selectivity estimates, but shouldn't really cause any
noticeable slowdown.  Per a comment from Michal Politowski.

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