William Bernbaum <[email protected]> writes:
> Recently I improved a dynamic SQL compiler (an ORM-like layer) using a
> pattern that seems to generalize to the Postgres planner. I taught the
> layer to emit an EXISTS subquery when the target relation only served to
> filter, rather than fanning out a join whose extra rows were discarded.
> That led me into the join elimination machinery, and to the attached draft
> patch. It adds convert_joins_to_semijoins(), which recognizes an inner
> join that only restricts which rows survive and adds a JOIN_SEMI
> SpecialJoinInfo for the filtering relation.
I find myself a bit astonished by this proposal. Normally we try
to convert semijoins to inner joins in order to gain join-order
flexibility. I can't see a good reason to go in the other direction.
There might be a reason to install some special case for the query
pattern you have in mind, but I really doubt that this particular
special case is the best option.
regards, tom lane
