COUNT(*) returns the number of rows, and COUNT(foo) returns the number of rows with non-null foo. But there are lots of myths regarding COUNT(*), mostly coming from its syntax resembling SELECT *. Let’s bust them!
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Double negative and query performance
In certain situations, a query can be optimised using double negation (NOT NOT).
PostgreSQL has two implementations of hash join: Hash Join itself and Hashed Subplan, which may serve anti- and semi-joins.
Alexey Bashtanov PostgreSQL Blog 