Mark expressions nullable by grouping sets

When generating window_pathkeys, distinct_pathkeys, or sort_pathkeys,
we failed to realize that the grouping/ordering expressions might be
nullable by grouping sets.  As a result, we may incorrectly deem that
the PathKeys are redundant by EquivalenceClass processing and thus
remove them from the pathkeys list.  That would lead to wrong results
in some cases.

To fix this issue, we mark the grouping expressions nullable by
grouping sets if that is the case.  If the grouping expression is a
Var or PlaceHolderVar or constructed from those, we can just add the
RT index of the RTE_GROUP RTE to the existing nullingrels field(s);
otherwise we have to add a PlaceHolderVar to carry on the nullingrel
bit.

However, we have to manually remove this nullingrel bit from
expressions in various cases where these expressions are logically
below the grouping step, such as when we generate groupClause pathkeys
for grouping sets, or when we generate PathTarget for initial input to
grouping nodes.

Furthermore, in set_upper_references, the targetlist and quals of an
Agg node should have nullingrels that include the effects of the
grouping step, ie they will have nullingrels equal to the input
Vars/PHVs' nullingrels plus the nullingrel bit that references the
grouping RTE.  In order to perform exact nullingrels matches, we also
need to manually remove this nullingrel bit.

Bump catversion because this changes the querytree produced by the
parser.

Thanks to Tom Lane for the idea to invent a new kind of RTE.

Per reports from Geoff Winkless, Tobias Wendorff, Richard Guo from
various threads.

Author: Richard Guo
Reviewed-by: Ashutosh Bapat, Sutou Kouhei
Discussion: https://postgr.es/m/CAMbWs4_dp7e7oTwaiZeBX8+P1rXw4ThkZxh1QG81rhu9Z47VsQ@mail.gmail.com

2 files changed, 26 insertions, 0 deletions

diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c
index d871396e20c..47644b26c6a 100644
--- a/src/backend/optimizer/path/equivclass.c
+++ b/src/backend/optimizer/path/equivclass.c
@@ -726,6 +726,10 @@ get_eclass_for_sort_expr(PlannerInfo *root,
 		{
 			RelOptInfo *rel = root->simple_rel_array[i];
 
+			/* ignore the RTE_GROUP RTE */
+			if (i == root->group_rtindex)
+				continue;
+
 			if (rel == NULL)	/* must be an outer join */
 			{
 				Assert(bms_is_member(i, root->outer_join_rels));
@@ -1087,6 +1091,10 @@ generate_base_implied_equalities(PlannerInfo *root)
 		{
 			RelOptInfo *rel = root->simple_rel_array[i];
 
+			/* ignore the RTE_GROUP RTE */
+			if (i == root->group_rtindex)
+				continue;
+
 			if (rel == NULL)	/* must be an outer join */
 			{
 				Assert(bms_is_member(i, root->outer_join_rels));
@@ -3354,6 +3362,10 @@ get_eclass_indexes_for_relids(PlannerInfo *root, Relids relids)
 	{
 		RelOptInfo *rel = root->simple_rel_array[i];
 
+		/* ignore the RTE_GROUP RTE */
+		if (i == root->group_rtindex)
+			continue;
+
 		if (rel == NULL)		/* must be an outer join */
 		{
 			Assert(bms_is_member(i, root->outer_join_rels));
diff --git a/src/backend/optimizer/path/pathkeys.c b/src/backend/optimizer/path/pathkeys.c
index e25798972f6..035bbaa3856 100644
--- a/src/backend/optimizer/path/pathkeys.c
+++ b/src/backend/optimizer/path/pathkeys.c
@@ -25,6 +25,7 @@
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
 #include "partitioning/partbounds.h"
+#include "rewrite/rewriteManip.h"
 #include "utils/lsyscache.h"
 
 /* Consider reordering of GROUP BY keys? */
@@ -1341,6 +1342,7 @@ make_pathkeys_for_sortclauses(PlannerInfo *root,
 													&sortclauses,
 													tlist,
 													false,
+													false,
 													&sortable,
 													false);
 	/* It's caller error if not all clauses were sortable */
@@ -1359,6 +1361,9 @@ make_pathkeys_for_sortclauses(PlannerInfo *root,
  * give rise to redundant pathkeys are removed from the sortclauses list
  * (which therefore must be pass-by-reference in this version).
  *
+ * If remove_group_rtindex is true, then we need to remove the RT index of the
+ * grouping step from the sort expressions before we make PathKeys for them.
+ *
  * *sortable is set to true if all the sort clauses are in fact sortable.
  * If any are not, they are ignored except for setting *sortable false.
  * (In that case, the output pathkey list isn't really useful.  However,
@@ -1375,6 +1380,7 @@ make_pathkeys_for_sortclauses_extended(PlannerInfo *root,
 									   List **sortclauses,
 									   List *tlist,
 									   bool remove_redundant,
+									   bool remove_group_rtindex,
 									   bool *sortable,
 									   bool set_ec_sortref)
 {
@@ -1394,6 +1400,14 @@ make_pathkeys_for_sortclauses_extended(PlannerInfo *root,
 			*sortable = false;
 			continue;
 		}
+		if (remove_group_rtindex)
+		{
+			Assert(root->group_rtindex > 0);
+			sortkey = (Expr *)
+				remove_nulling_relids((Node *) sortkey,
+									  bms_make_singleton(root->group_rtindex),
+									  NULL);
+		}
 		pathkey = make_pathkey_from_sortop(root,
 										   sortkey,
 										   sortcl->sortop,