diff --git a/sql/expression/function/aggregation/count_test.go b/sql/expression/function/aggregation/count_test.go
index 10c769853b..87aa278ba6 100644
--- a/sql/expression/function/aggregation/count_test.go
+++ b/sql/expression/function/aggregation/count_test.go
@@ -137,6 +137,21 @@ func TestCountDistinctEvalString(t *testing.T) {
 	require.Equal(int64(2), evalBuffer(t, b))
 }
 
+func TestCountDistinctEvalMultiColumn(t *testing.T) {
+	require := require.New(t)
+	ctx := sql.NewEmptyContext()
+
+	c := NewCountDistinct(
+		expression.NewGetField(0, types.Text, "", true),
+		expression.NewGetField(1, types.Text, "", true),
+	)
+	b, _ := c.NewBuffer(ctx)
+
+	require.NoError(b.Update(ctx, sql.NewRow("a,", "b")))
+	require.NoError(b.Update(ctx, sql.NewRow("a", ",b")))
+	require.Equal(int64(2), evalBuffer(t, b))
+}
+
 func TestCountDistinctEvalExtendedType(t *testing.T) {
 	require := require.New(t)
 	ctx := sql.NewEmptyContext()
diff --git a/sql/expression/function/aggregation/unary_agg_buffers.go b/sql/expression/function/aggregation/unary_agg_buffers.go
index f3e123b286..4e65caf0f1 100644
--- a/sql/expression/function/aggregation/unary_agg_buffers.go
+++ b/sql/expression/function/aggregation/unary_agg_buffers.go
@@ -486,7 +486,7 @@ func (c *countDistinctBuffer) Update(ctx *sql.Context, row sql.Row) error {
 		}
 	}
 
-	var str string
+	hash := xxhash.New()
 	for _, val := range value.(sql.Row) {
 		// skip nil values
 		if val == nil {
@@ -500,16 +500,11 @@ func (c *countDistinctBuffer) Update(ctx *sql.Context, row sql.Row) error {
 		if !ok {
 			return fmt.Errorf("count distinct unable to hash value: %s", err)
 		}
-		str += vv + ","
-	}
-
-	hash := xxhash.New()
-	_, err := hash.WriteString(str)
-	if err != nil {
-		return err
+		if _, err = fmt.Fprintf(hash, "%d:%s", len(vv), vv); err != nil {
+			return err
+		}
 	}
-	h := hash.Sum64()
-	c.seen[h] = struct{}{}
+	c.seen[hash.Sum64()] = struct{}{}
 
 	return nil
 }