Support Parallel Append plan nodes.

When we create an Append node, we can spread out the workers over the
subplans instead of piling on to each subplan one at a time, which
should typically be a bit more efficient, both because the startup
cost of any plan executed entirely by one worker is paid only once and
also because of reduced contention.  We can also construct Append
plans using a mix of partial and non-partial subplans, which may allow
for parallelism in places that otherwise couldn't support it.
Unfortunately, this patch doesn't handle the important case of
parallelizing UNION ALL by running each branch in a separate worker;
the executor infrastructure is added here, but more planner work is
needed.

Amit Khandekar, Robert Haas, Amul Sul, reviewed and tested by
Ashutosh Bapat, Amit Langote, Rafia Sabih, Amit Kapila, and
Rajkumar Raghuwanshi.

Discussion: http://postgr.es/m/CAJ3gD9dy0K_E8r727heqXoBmWZ83HwLFwdcaSSmBQ1+S+vRuUQ@mail.gmail.com

src/backend/nodes/copyfuncs.c

@@ -242,6 +242,7 @@ _copyAppend(const Append *from)
 	 */
 	COPY_NODE_FIELD(partitioned_rels);
 	COPY_NODE_FIELD(appendplans);
+	COPY_SCALAR_FIELD(first_partial_plan);
 
 	return newnode;
 }

src/backend/nodes/list.c

@@ -1249,6 +1249,44 @@ list_copy_tail(const List *oldlist, int nskip)
 	return newlist;
 }
 
+/*
+ * Sort a list using qsort. A sorted list is built but the cells of the
+ * original list are re-used.  The comparator function receives arguments of
+ * type ListCell **
+ */
+List *
+list_qsort(const List *list, list_qsort_comparator cmp)
+{
+	ListCell   *cell;
+	int			i;
+	int			len = list_length(list);
+	ListCell  **list_arr;
+	List	   *new_list;
+
+	if (len == 0)
+		return NIL;
+
+	i = 0;
+	list_arr = palloc(sizeof(ListCell *) * len);
+	foreach(cell, list)
+		list_arr[i++] = cell;
+
+	qsort(list_arr, len, sizeof(ListCell *), cmp);
+
+	new_list = (List *) palloc(sizeof(List));
+	new_list->type = list->type;
+	new_list->length = len;
+	new_list->head = list_arr[0];
+	new_list->tail = list_arr[len - 1];
+
+	for (i = 0; i < len - 1; i++)
+		list_arr[i]->next = list_arr[i + 1];
+
+	list_arr[len - 1]->next = NULL;
+	pfree(list_arr);
+	return new_list;
+}
+
 /*
  * Temporary compatibility functions
  *

src/backend/nodes/outfuncs.c

@@ -399,6 +399,7 @@ _outAppend(StringInfo str, const Append *node)
 
 	WRITE_NODE_FIELD(partitioned_rels);
 	WRITE_NODE_FIELD(appendplans);
+	WRITE_INT_FIELD(first_partial_plan);
 }
 
 static void

src/backend/nodes/readfuncs.c

@@ -1600,6 +1600,7 @@ _readAppend(void)
 
 	READ_NODE_FIELD(partitioned_rels);
 	READ_NODE_FIELD(appendplans);
+	READ_INT_FIELD(first_partial_plan);
 
 	READ_DONE();
 }