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

1 files changed, 10 insertions, 3 deletions

diff --git a/src/backend/parser/parse_agg.c b/src/backend/parser/parse_agg.c
index bd095d05c0b..102accd0716 100644
--- a/src/backend/parser/parse_agg.c
+++ b/src/backend/parser/parse_agg.c
@@ -1333,9 +1333,6 @@ substitute_grouped_columns_mutator(Node *node,
 
 	if (node == NULL)
 		return NULL;
-	if (IsA(node, Const) ||
-		IsA(node, Param))
-		return node;			/* constants are always acceptable */
 
 	if (IsA(node, Aggref))
 	{
@@ -1410,6 +1407,16 @@ substitute_grouped_columns_mutator(Node *node,
 	}
 
 	/*
+	 * Constants are always acceptable.  We have to do this after we checked
+	 * the subexpression as a whole for a match, because it is possible that
+	 * we have GROUP BY items that are constants, and the constants would
+	 * become not so constant after the grouping step.
+	 */
+	if (IsA(node, Const) ||
+		IsA(node, Param))
+		return node;
+
+	/*
 	 * If we have an ungrouped Var of the original query level, we have a
 	 * failure.  Vars below the original query level are not a problem, and
 	 * neither are Vars from above it.  (If such Vars are ungrouped as far as