Re: Removing unneeded self joins

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