On 27/6/2025 02:26, Michael Paquier wrote:
The point regarding the search join hook may stand, though. Perhaps
somebody should check if we're still OK with this change in the
context of the self-join work. I tend to think that we are and I
agree that removing the joins when calling the hook can show benefits,
but it may be surprising and users tend to be very noisy with plan
stability, particularly if some of the FROM aliases get silenced by
the backend without the module knowing about that. At least there is
the trick with SET enable_self_join_elimination available as a last
resort method.
Parse tree transformation (pull-ups, flattenings) adds joins, constant
clause evaluation or partition pruning may smash whole subtrees - the
optimisation process is quite unstable from the query structure's point
of view.
If I understand correctly, pg_hint_plan needs to differentiate 'never
existed' relations and removed ones. In that sense, up to v.15, Postgres
saved RelOptInfo after removing the join (see e9a20e4).
--
regards, Andrei Lepikhov
