PostgreSQL and NOT IN

PostgreSQL treats LEFT JOIN and NOT EXISTS equally, using same execution plan for both of them (namely a Hash Anti Join for the example above).

As for NOT IN, which is semantically different since its logic is trivalent and it can return NULL, PostgreSQL tries to take this into account and limits itself to using a filter against a subplan (a hashed subplan for a hashable resultset like in example above).

Since it need to search the hash table for each missing value twice (first time to find the value, second time to find a NULL), this method is a little less efficient.

LEFT JOIN

SELECT  l.*
FROM    t_left l
LEFT JOIN
        t_right r
ON      r.value = l.value
WHERE   r.value IS NULL

NOT IN

SELECT  l.*
FROM    t_left l
WHERE   l.value NOT IN
        (
        SELECT  value
        FROM    t_right r
        )

NOT EXISTS

SELECT  l.*
FROM    t_left l
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    t_right r
        WHERE   r.value = l.value
        )