Re: pull-up subquery if JOIN-ON contains refs to upper-query

Hello Alena!

Sorry for my long silence, if you are interested in my thoughts about
your patch then here they are:

1)  First of all, let's read the comment below.

/*
 * Separate out the WHERE clause.  (We could theoretically also remove
 * top-level plain JOIN/ON clauses, but it's probably not worth the
 * trouble.)
 */

We need to separate two things: the jointree and the WHERE clause, so
it's possible to do something like this

whereClause = subselect->jointree->quals;
subselect->jointree->quals = NULL;

jointree = subselect->jointree;
subselect->jointree = NULL;

if (contain_vars_of_level((Node *) subselect, 1))
    return NULL;

/* Do our checks in the jointree and stop if we can't do pullup */

/* Return the jointree back */
subselect->jointree = jointree;

I think it's more clear than in the patch right now.

2) We don't need to use get_relids_in_jointree() and nullable_above
since the tree's traversing is from the top to the bottom and we know that

     in LEFT JOIN and FULL JOIN LHS is nullable
     in RIGHT JOIN and FULL JOIN RHS  is nullable

So we can use a boolean variables like this

rarg_is_nullable = (is_nullable_side ||
                                  j->jointype == JOIN_FULL ||
                                  j->jointype == JOIN_LEFT);

larg_is_nullable = (is_nullable_side ||
                                 j->jointype == JOIN_FULL ||
                                 j->jointype == JOIN_RIGHT);

And then work with them. I suspect it will be much easier to follow that
in the patch right now.

I fear that you don't check FULL JOINS here.

3) To be honest, we just work with the top jointree, we don't descend to
subqueries, therefore, I am not sure that the mutator is a good name here.

    AFAICS, mutators were designed to modify something including some
parts of the subqueries but it's not the case here.

    It's a simple jointree traversal, we don't need
HoistJoinQualsContext as well.

    I think, we need three things here:

    Node *node - the node in the jointree we are working with

    bool is_nullable_side - are we on the nullable side of some outer join

    List **exprs - the list in which we collect JoinExpr and FromExpr
with outer references.

    I propose just traverse the jointree, make our checks and then
collect JoinExpr and FromExpr for the further processing if everything
is good.

4) After checking the WHERE clause and the jointree we can traverse our
list, make a new whereClause by appending quals with outer references
and then replace quals in JoinExprs to constant true.

    Something like make_and_qual()

5) I have also noticed that you are using canonicalize_qual()

    I suspect we don't need it either since it will be called later.
And here is the stack

    subquery_planner
       pull_up_sublinks(root)
       preprocess_qual_conditions(root, (Node *) parse->jointree)
           j->quals = preprocess_expression(root, j->quals, EXPRKIND_QUAL)
              expr = (Node *) canonicalize_qual((Expr *) expr, false)
                 find_duplicate_ors(qual, is_check)

   So we will flatten all nested BoolExpr with the type BOOL_AND. Let's
allow Postgres do its job.

6) I have noticed the new output from one regression test. The previous
output was

    Merge Anti Join
      Merge Cond: (t1.c1 = t2.c2)
       ->  Sort
               Sort Key: t1.c1
               ->  Seq Scan on tt4x t1
       ->  Sort

   and the new output is the Hash Anti Join

   Hash Anti Join
       Hash Cond: (t1.c1 = t2.c2)
        ->  Seq Scan on tt4x t1
        ->  Hash

  This is very suspicious, why does this patch cause such changes?

7) There is a SubLink which won't be pulled up, you can see it below

explain (COSTS OFF)
SELECT *
  FROM tenk1 A
 WHERE EXISTS (SELECT 1
                                FROM tenk2 B, (SELECT f.hundred
 FROM (SELECT A.hundred) AS f
                                                           ) AS v
                             WHERE B.hundred = v.hundred
              );

And the reason is the following. (SELECT A.hundred) is the subquery and
won't be processed until the function  pull_up_sublinks() finishes its job.

But it lead us to an interesting observation. Maybe we could process it
somewhere in the SS_process_sublinks().

Perhaps, f and v subqueries could be pulled up and then the EXISTS
sublink could be replaced with a SEMI JOIN.

I don't know whether we should solve this problem right now. I have
never seen such queries in real workloads but it doesn't mean that they
are not exists.

In short, I think your patch can be simplified.

On 6/26/25 18:40, solaimurugan vellaipandiyan wrote:
> In all tested cases, query results remained correct and I did not
> observe incorrect transformations during my testing.
>
> Overall the patch behavior looks good from my side and the planner now
> behaves consistently for these EXISTS pull-up scenarios.
>
> Regards,
> Solaimurugan V

Sincerely yours.

Peter Petrov.