Re: Removing unneeded self joins

On Thu, Jun 26, 2025 at 08:54:55AM +0200, Andrei Lepikhov wrote:
> Before diving into the pg_hint_plan code, I wonder why you don't have
> similar issues with the remove_useless_joins. We intentionally designed SJE
> coupled with the left-join removal feature to avoid such type of complaints:
>
> CREATE TABLE test (x integer PRIMARY KEY);
> EXPLAIN (COSTS OFF)
> SELECT t1.* FROM test t1 LEFT JOIN test t2 ON (t1.x=t2.x);
>
> /*
>      QUERY PLAN
> ---------------------
>  Seq Scan on test t1
> (1 row)
> */
>
> It seems that this join removal is also beyond the pg_hint_plan control ...

Yeah, remove_useless_joins() has been around since the 9.0 ages as far
as I know, and it's not bothered the module much in the tests because
we have unlikely relied on it.

For now the trick I am going to rely on is just disable
enable_self_join_elimination in the test paths where I rely on the
same relation and self joins to keep the plans of the regression tests
stable.  This stuff could create separate relations, but then with
back-patching in mind that's going to be just extra conflict noise,
which is always annoying when dealing with plan outputs.  We've done
that in the past with max_parallel_workers_per_gather or jit, to keep
the expected plans stable.

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.
--
Michael