Optimize sifting down in binaryheap.

Presently, each iteration of the loop in sift_down() will perform
3 comparisons if both children are larger than the parent node (2
for comparing each child to the parent node, and a third to compare
the children to each other).  By first comparing the children to
each other and then comparing the larger child to the parent node,
we can accomplish the same thing with just 2 comparisons (while
also not affecting the number of comparisons in any other case).

Author: ChangAo Chen
Reviewed-by: Robert Haas
Discussion: https://postgr.es/m/tencent_0142D8DA90940B9930BCC08348BBD6D0BB07%40qq.com

Author: nathan-bossart

src/common/binaryheap.c

@@ -323,34 +323,23 @@ sift_down(binaryheap *heap, int node_off)
 	{
 		int			left_off = left_offset(node_off);
 		int			right_off = right_offset(node_off);
-		int			swap_off = 0;
+		int			swap_off = left_off;
 
-		/* Is the left child larger than the parent? */
-		if (left_off < heap->bh_size &&
-			heap->bh_compare(node_val,
-							 heap->bh_nodes[left_off],
-							 heap->bh_arg) < 0)
-			swap_off = left_off;
-
-		/* Is the right child larger than the parent? */
+		/* Is the right child larger than the left child? */
 		if (right_off < heap->bh_size &&
-			heap->bh_compare(node_val,
+			heap->bh_compare(heap->bh_nodes[left_off],
 							 heap->bh_nodes[right_off],
 							 heap->bh_arg) < 0)
-		{
-			/* swap with the larger child */
-			if (!swap_off ||
-				heap->bh_compare(heap->bh_nodes[left_off],
-								 heap->bh_nodes[right_off],
-								 heap->bh_arg) < 0)
-				swap_off = right_off;
-		}
+			swap_off = right_off;
 
 		/*
-		 * If we didn't find anything to swap, the heap condition is
+		 * If no children or parent is >= the larger child, heap condition is
 		 * satisfied, and we're done.
 		 */
-		if (!swap_off)
+		if (left_off >= heap->bh_size ||
+			heap->bh_compare(node_val,
+							 heap->bh_nodes[swap_off],
+							 heap->bh_arg) >= 0)
 			break;
 
 		/*