Support parallel aggregation.

Parallel workers can now partially aggregate the data and pass the
transition values back to the leader, which can combine the partial
results to produce the final answer.

David Rowley, based on earlier work by Haribabu Kommi.  Reviewed by
Álvaro Herrera, Tomas Vondra, Amit Kapila, James Sewell, and me.

src/backend/nodes/copyfuncs.c

@@ -1233,6 +1233,7 @@ _copyAggref(const Aggref *from)
 
 	COPY_SCALAR_FIELD(aggfnoid);
 	COPY_SCALAR_FIELD(aggtype);
+	COPY_SCALAR_FIELD(aggoutputtype);
 	COPY_SCALAR_FIELD(aggcollid);
 	COPY_SCALAR_FIELD(inputcollid);
 	COPY_NODE_FIELD(aggdirectargs);

src/backend/nodes/equalfuncs.c

@@ -192,6 +192,7 @@ _equalAggref(const Aggref *a, const Aggref *b)
 {
 	COMPARE_SCALAR_FIELD(aggfnoid);
 	COMPARE_SCALAR_FIELD(aggtype);
+	COMPARE_SCALAR_FIELD(aggoutputtype);
 	COMPARE_SCALAR_FIELD(aggcollid);
 	COMPARE_SCALAR_FIELD(inputcollid);
 	COMPARE_NODE_FIELD(aggdirectargs);

src/backend/nodes/nodeFuncs.c

@@ -57,7 +57,7 @@ exprType(const Node *expr)
 			type = ((const Param *) expr)->paramtype;
 			break;
 		case T_Aggref:
-			type = ((const Aggref *) expr)->aggtype;
+			type = ((const Aggref *) expr)->aggoutputtype;
 			break;
 		case T_GroupingFunc:
 			type = INT4OID;

src/backend/nodes/outfuncs.c

@@ -1033,6 +1033,7 @@ _outAggref(StringInfo str, const Aggref *node)
 
 	WRITE_OID_FIELD(aggfnoid);
 	WRITE_OID_FIELD(aggtype);
+	WRITE_OID_FIELD(aggoutputtype);
 	WRITE_OID_FIELD(aggcollid);
 	WRITE_OID_FIELD(inputcollid);
 	WRITE_NODE_FIELD(aggdirectargs);

src/backend/nodes/readfuncs.c

@@ -552,6 +552,7 @@ _readAggref(void)
 
 	READ_OID_FIELD(aggfnoid);
 	READ_OID_FIELD(aggtype);
+	READ_OID_FIELD(aggoutputtype);
 	READ_OID_FIELD(aggcollid);
 	READ_OID_FIELD(inputcollid);
 	READ_NODE_FIELD(aggdirectargs);