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

tglsfdc · tglsfdc · commit 74dae41e5ac3 · 2020-10-05T13:15:39.000-04:00
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 f3b3b8d), 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
diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
@@ -600,12 +600,6 @@ 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
 	 */
@@ -682,6 +676,12 @@ 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);
 
