pgsql: Fix costing of parallel hash joins.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5b804cc168a52039be02f392780b9e8c8db74412

Modified Files
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src/backend/optimizer/path/costsize.c | 2 ++
src/backend/optimizer/path/joinpath.c | 2 +-
2 files changed, 3 insertions(+), 1 deletion(-)