materialize implementation and and optimization

graph/iterator.go

@@ -153,6 +153,19 @@ func Next(it Iterator) (Value, bool) {
 	return nil, false
 }
 
+// Height is a convienence function to measure the height of an iterator tree.
+func Height(it Iterator) int {
+	subs := it.SubIterators()
+	maxDepth := 0
+	for _, sub := range subs {
+		h := Height(sub)
+		if h > maxDepth {
+			maxDepth = h
+		}
+	}
+	return maxDepth + 1
+}
+
 // FixedIterator wraps iterators that are modifiable by addition of fixed value sets.
 type FixedIterator interface {
 	Iterator
@@ -180,6 +193,7 @@ const (
 	Fixed
 	Not
 	Optional
+	Materialize
 )
 
 var (
@@ -200,6 +214,7 @@ var (
 		"fixed",
 		"not",
 		"optional",
+		"materialize",
 	}
 )