pgsql: Fix cost_rescan() to account for multi-batch hashing correctly.

Fix cost_rescan() to account for multi-batch hashing correctly.

cost_rescan assumed that we don't need to rebuild the hash table when
rescanning a hash join. However, that's currently only true for
single-batch joins; for a multi-batch join we must charge full freight.

This probably has escaped notice because we'd be unlikely to put a hash
join on the inside of a nestloop anyway. Nonetheless, it's wrong.
Fix in HEAD, but don't backpatch for fear of destabilizing plans in
stable releases.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/69995c3b3fd64361bb4d3938315f3e88ccc01e53

Modified Files
--------------
src/backend/optimizer/path/costsize.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

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