Fix pushdown of constants in aggregate queries

src/backend/distributed/planner/extended_op_node_utils.c

@@ -79,6 +79,9 @@ BuildExtendedOpNodeProperties(MultiExtendedOp *extendedOpNode,
 								 hasNonPartitionColumnDistinctAgg,
 								 extendedOpNode->onlyPushableWindowFunctions);
 
+	extendedOpNodeProperties.hasGroupBy = extendedOpNode->groupClauseList != NIL;
+	extendedOpNodeProperties.hasAggregate = TargetListHasAggregates(targetList);
+
 	extendedOpNodeProperties.groupedByDisjointPartitionColumn =
 		groupedByDisjointPartitionColumn;
 	extendedOpNodeProperties.repartitionSubquery = repartitionSubquery;

src/backend/distributed/planner/multi_logical_optimizer.c

@@ -228,7 +228,6 @@ static Expr * AddTypeConversion(Node *originalAggregate, Node *newExpression);
 static MultiExtendedOp * WorkerExtendedOpNode(MultiExtendedOp *originalOpNode,
 											  ExtendedOpNodeProperties *
 											  extendedOpNodeProperties);
-static bool TargetListHasAggregates(List *targetEntryList);
 static void ProcessTargetListForWorkerQuery(List *targetEntryList,
 											ExtendedOpNodeProperties *
 											extendedOpNodeProperties,
@@ -2830,7 +2829,7 @@ BuildOrderByLimitReference(bool hasDistinctOn, bool groupedByDisjointPartitionCo
  * target list contain aggregates that are not inside the window functions.
  * This function should not be called if window functions are being pulled up.
  */
-static bool
+bool
 TargetListHasAggregates(List *targetEntryList)
 {
 	TargetEntry *targetEntry = NULL;
@@ -3839,9 +3838,26 @@ CanPushDownExpression(Node *expression,
 	bool hasAggregate = contain_aggs_of_level(expression, 0);
 	bool hasWindowFunction = contain_window_function(expression);
 	if (!hasAggregate && !hasWindowFunction)
+	{
+		/*
+		 * If the query has the form SELECT expression, agg(..) FROM table;
+		 * then expression should be evaluated on the coordinator.
+		 *
+		 * Other than the efficiency part of this, we could also crash if
+		 * we pushed down the expression to the workers. When pushing down
+		 * expressions to workers we create a Var reference to the worker
+		 * tuples. If the result from worker is empty, but we need to have
+		 * at least a row in coordinator result, postgres will crash when
+		 * trying to evaluate the Var.
+		 *
+		 * For details, see https://github.com/citusdata/citus/pull/3961
+		 */
+		if (!extendedOpNodeProperties->hasAggregate ||
+			extendedOpNodeProperties->hasGroupBy)
 		{
 			return true;
 		}
+	}
 
 	/* aggregates inside pushed down window functions can be pushed down */
 	bool hasPushableWindowFunction =

src/include/distributed/extended_op_node_utils.h

@@ -31,6 +31,12 @@ typedef struct ExtendedOpNodeProperties
 	bool onlyPushableWindowFunctions;
 	bool pullUpIntermediateRows;
 	bool pushDownGroupingAndHaving;
+
+	/* indicates whether the MultiExtendedOp has a GROUP BY */
+	bool hasGroupBy;
+
+	/* indicates whether the MultiExtendedOp has an aggregate on the target list */
+	bool hasAggregate;
 } ExtendedOpNodeProperties;
 
 

src/include/distributed/multi_logical_optimizer.h

@@ -176,5 +176,6 @@ extern void FindReferencedTableColumn(Expr *columnExpression, List *parentQueryL
 									  Query *query, Oid *relationId, Var **column);
 extern char * WorkerColumnName(AttrNumber resno);
 extern bool IsGroupBySubsetOfDistinct(List *groupClauses, List *distinctClauses);
+extern bool TargetListHasAggregates(List *targetEntryList);
 
 #endif   /* MULTI_LOGICAL_OPTIMIZER_H */