Prevent panic when index matcher returns nil tree. Fixes #37

go.mod

@@ -10,3 +10,5 @@ require (
 	go.etcd.io/bbolt v1.3.3 // indirect
 	golang.org/x/tools v0.0.0-20190524210228-3d17549cdc6b
 )
+
+go 1.13

query/matcher.go

@@ -452,6 +452,11 @@ type indexResultTable struct {
 func (i *indexResultTable) Iterate(fn func([]byte, record.Record) error) error {
 	var err error
 
+	// if no results,
+	if i.tree == nil {
+		return nil
+	}
+
 	i.tree.Ascend(func(it btree.Item) bool {
 		var rec record.Record
 		recordID := []byte(it.(Item))

query/query_test.go

@@ -97,6 +97,33 @@ func TestSelect(t *testing.T) {
 		require.NoError(t, err)
 	})
 
+	t.Run("WithEmptyIndex", func(t *testing.T) {
+		db, err := genji.New(memory.NewEngine())
+		require.NoError(t, err)
+
+		var n int
+		err = db.Update(func(tx *genji.Tx) error {
+			tb, err := tx.CreateTable("test")
+			if err != nil {
+				return err
+			}
+
+			_, err = tb.CreateIndex("a", index.Options{})
+			if err != nil {
+				return err
+			}
+
+			res := Select().From(tb).Where(And(StringField("a").Eq("foo"))).Limit(1).Run(tx)
+			if res.Err() != nil {
+				return res.Err()
+			}
+
+			n, err = res.Count()
+			return err
+		})
+		require.NoError(t, err)
+		require.Equal(t, 0, n)
+	})
 }
 
 func TestDelete(t *testing.T) {