Fix cost_nestloop and cost_hashjoin to model the behavior of semi and anti

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

Commit: 8dcf18414ba3fd68970fb5095f26bb9f358dedfe
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2009-05-09T22:51:41Z
Releases: 8.4.0
Fix cost_nestloop and cost_hashjoin to model the behavior of semi and anti
joins a bit better, ie, understand the differing cost functions for matched
and unmatched outer tuples.  There is more that could be done in cost_hashjoin
but this already helps a great deal.  Per discussions with Robert Haas.

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