Avoid pushing quals down into sub-queries that have grouping sets.

tglsfdc · tglsfdc · commit 6fa403e61241 · 2020-08-22T14:46:40.000-04:00
The trouble with doing this is that an apparently-constant subquery output column isn't really constant if it is a grouping column that appears in only some of the grouping sets. A qual using such a column would be subject to incorrect const-folding after push-down, as seen in bug #16585 from Paul Sivash. To fix, just disable qual pushdown altogether if the sub-query has nonempty groupingSets. While we could imagine far less restrictive solutions, there is not much point in working harder right now, because subquery_planner() won't move HAVING clauses to WHERE within such a subquery. If the qual stays in HAVING it's not going to be a lot more useful than if we'd kept it at the outer level. Having said that, this restriction could be removed if we used a parsetree representation that distinguished such outputs from actual constants, which is something I hope to do in future. Hence, make the patch a minimal addition rather than integrating it more tightly (e.g. by renumbering the existing items in subquery_is_pushdown_safe's comment). Back-patch to 9.5 where grouping sets were introduced. Discussion: https://postgr.es/m/16585-9d8c340d23ade8c1@postgresql.org
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
@@ -2521,6 +2521,17 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
  * volatile qual could succeed for some SRF output rows and fail for others,
  * a behavior that cannot occur if it's evaluated before SRF expansion.
  *
+ * 6. If the subquery has nonempty grouping sets, we cannot push down any
+ * quals.  The concern here is that a qual referencing a "constant" grouping
+ * column could get constant-folded, which would be improper because the value
+ * is potentially nullable by grouping-set expansion.  This restriction could
+ * be removed if we had a parsetree representation that shows that such
+ * grouping columns are not really constant.  (There are other ideas that
+ * could be used to relax this restriction, but that's the approach most
+ * likely to get taken in the future.  Note that there's not much to be gained
+ * so long as subquery_planner can't move HAVING clauses to WHERE within such
+ * a subquery.)
+ *
  * In addition, we make several checks on the subquery's output columns to see
  * if it is safe to reference them in pushed-down quals.  If output column k
  * is found to be unsafe to reference, we set safetyInfo->unsafeColumns[k]
@@ -2565,6 +2576,10 @@ subquery_is_pushdown_safe(Query *subquery, Query *topquery,
 	if (subquery->limitOffset != NULL || subquery->limitCount != NULL)
 		return false;
 
+	/* Check point 6 */
+	if (subquery->groupClause && subquery->groupingSets)
+		return false;
+
 	/* Check points 3, 4, and 5 */
 	if (subquery->distinctClause ||
 		subquery->hasWindowFuncs ||
diff --git a/src/test/regress/expected/groupingsets.out b/src/test/regress/expected/groupingsets.out
@@ -434,6 +434,38 @@ select x, not x as not_x, q2 from
    |       |  4567890123456789
 (5 rows)
 
+-- check qual push-down rules for a subquery with grouping sets
+explain (verbose, costs off)
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+                 QUERY PLAN                 
+--------------------------------------------
+ Subquery Scan on ss
+   Output: ss.x, ss.q1, ss.sum
+   Filter: ((ss.x = 1) AND (ss.q1 = 123))
+   ->  GroupAggregate
+         Output: (1), i1.q1, sum(i1.q2)
+         Group Key: 1
+         Sort Key: i1.q1
+           Group Key: i1.q1
+         ->  Seq Scan on public.int8_tbl i1
+               Output: 1, i1.q1, i1.q2
+(10 rows)
+
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+ x | q1 | sum 
+---+----+-----
+(0 rows)
+
 -- simple rescan tests
 select a, b, sum(v.x)
   from (values (1),(2)) v(x), gstest_data(v.x)
diff --git a/src/test/regress/sql/groupingsets.sql b/src/test/regress/sql/groupingsets.sql
@@ -172,6 +172,22 @@ select x, not x as not_x, q2 from
   group by grouping sets(x, q2)
   order by x, q2;
 
+-- check qual push-down rules for a subquery with grouping sets
+explain (verbose, costs off)
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+
+select * from (
+  select 1 as x, q1, sum(q2)
+  from int8_tbl i1
+  group by grouping sets(1, 2)
+) ss
+where x = 1 and q1 = 123;
+
 -- simple rescan tests
 
 select a, b, sum(v.x)
