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.

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
                             );