Fix two latent(?) bugs in equivclass.c.

get_eclass_for_sort_expr() computes expr_relids and nullable_relids
early on, even though they won't be needed unless we make a new
EquivalenceClass, which we often don't.  Aside from the probably-minor
inefficiency, there's a memory management problem: these bitmapsets will
be built in the caller's context, leading to dangling pointers if that
is shorter-lived than root->planner_cxt.  This would be a live bug if
get_eclass_for_sort_expr() could be called with create_it = true during
GEQO join planning.  So far as I can find, the core code never does
that, but it's hard to be sure that no extensions do, especially since
the comments make it clear that that's supposed to be a supported case.
Fix by not computing these values until we've switched into planner_cxt
to build the new EquivalenceClass.

generate_join_implied_equalities() uses inner_rel->relids to look up
relevant eclasses, but it ought to be using nominal_inner_relids.
This is presently harmless because a child RelOptInfo will always have
exactly the same eclass_indexes as its topmost parent; but that might
not be true forever, and anyway it makes the code confusing.

The first of these is old (introduced by me in f3b3b8d5), so back-patch
to all supported branches.  The second only dates to v13, but we might
as well back-patch it to keep the code looking similar across branches.

Discussion: https://postgr.es/m/1508010.1601832581@sss.pgh.pa.us

src/backend/optimizer/path/equivclass.c

@@ -634,12 +634,6 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 	 */
 	expr = canonicalize_ec_expression(expr, opcintype, collation);
 
-	/*
-	 * Get the precise set of nullable relids appearing in the expression.
-	 */
-	expr_relids = pull_varnos((Node *) expr);
-	nullable_relids = bms_intersect(nullable_relids, expr_relids);
-
 	/*
 	 * Scan through the existing EquivalenceClasses for a match
 	 */
@@ -716,6 +710,12 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 	if (newec->ec_has_volatile && sortref == 0) /* should not happen */
 		elog(ERROR, "volatile EquivalenceClass has no sortref");
 
+	/*
+	 * Get the precise set of nullable relids appearing in the expression.
+	 */
+	expr_relids = pull_varnos((Node *) expr);
+	nullable_relids = bms_intersect(nullable_relids, expr_relids);
+
 	newem = add_eq_member(newec, copyObject(expr), expr_relids,
 						  nullable_relids, false, opcintype);
 
@@ -1171,9 +1171,9 @@ generate_join_implied_equalities(PlannerInfo *root,
 	}
 
 	/*
-	 * Get all eclasses in common between inner_rel's relids and outer_relids
+	 * Get all eclasses that mention both inner and outer sides of the join
 	 */
-	matching_ecs = get_common_eclass_indexes(root, inner_rel->relids,
+	matching_ecs = get_common_eclass_indexes(root, nominal_inner_relids,
 											 outer_relids);
 
 	i = -1;