Add explanatory comment

sqlx.go

@@ -1035,6 +1035,9 @@ func fieldsByTraversal(v reflect.Value, traversals [][]int, values []interface{}
 		} else if len(traversal) == 1 {
 			values[i] = reflectx.FieldByIndexes(v, traversal).Addr().Interface()
 		} else {
+			// reflectx.FieldByIndexes initializes pointer fields, including pointers to nested structs.
+			// Use optDest to delay it until the first non-NULL value is scanned into a field of a nested struct.
+			// That way we can support LEFT JOINs with optional nested structs.
 			traversal := traversal
 			values[i] = optDest(func() interface{} {
 				return reflectx.FieldByIndexes(v, traversal).Addr().Interface()