Re: Question about antijoin

"dandl" <david@andl.org> writes:
> This got my interest! It's of great interest to me to know how and when Postgres performs an anti-join (this being a
significantomission from SQL). 
> Is this a reliable trigger: (NOT EXISTS <subselect>)?

That's one case; see convert_EXISTS_sublink_to_join() for the full set
of conditions involved.  There is also a relevant transformation in
reduce_outer_joins():

 * Another transformation we apply here is to recognize cases like
 *        SELECT ... FROM a LEFT JOIN b ON (a.x = b.y) WHERE b.y IS NULL;
 * If the join clause is strict for b.y, then only null-extended rows could
 * pass the upper WHERE, and we can conclude that what the query is really
 * specifying is an anti-semijoin.  We change the join type from JOIN_LEFT
 * to JOIN_ANTI.  The IS NULL clause then becomes redundant, and must be
 * removed to prevent bogus selectivity calculations, but we leave it to
 * distribute_qual_to_rels to get rid of such clauses.

            regards, tom lane