Fix costing of parallel hash joins.

Peter Eisentraut <peter_e@gmx.net>

Commit: 5b804cc168a52039be02f392780b9e8c8db74412
Author: Peter Eisentraut <peter_e@gmx.net>
Date: 2018-03-07T02:54:37Z
Releases: 11.0
Fix costing of parallel hash joins.

Commit 1804284042e659e7d16904e7bbb0ad546394b6a3 established that single-batch
parallel-aware hash joins could create one large shared hash table using the
combined work_mem budget of all participants.  The costing accidentally
assumed that parallel-oblivious hash joins could also do that.  The
documentation for initial_cost_hashjoin() also failed to mention the new
argument.  Repair.

Author: Thomas Munro
Reported-By: Antonin Houska
Reviewed-By: Antonin Houska
Discussion: https://postgr.es/m/12441.1513935950%40localhost

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