Fix insertion of empty map into JSON column by using _dummy subcolumn

lib/column/json.go

@@ -501,6 +501,13 @@ func appendStructOrMap(jCol *JSONObject, data any) error {
 				Err:        fmt.Errorf("map keys must be string for column %s", jCol.Name()),
 			}
 		}
+		if jCol.columns == nil && vData.Len() == 0 {
+			// if map is empty, we need to create an empty Tuple to make sure subcolumns protocol is happy
+			// _dummy is a ClickHouse internal name for empty Tuple subcolumn
+			// it has the same effect as `INSERT INTO single_json_type_table VALUES ('{}');`
+			jCol.upsertValue("_dummy", "Int8")
+			return jCol.insertEmptyColumn("_dummy")
+		}
 		return iterateMap(vData, jCol, 0)
 	}
 	return &UnsupportedColumnTypeError{

tests/issues/1113_test.go

@@ -0,0 +1,39 @@
+package issues
+
+import (
+	"context"
+	"testing"
+
+	"github.com/ClickHouse/clickhouse-go/v2"
+	clickhouse_tests "github.com/ClickHouse/clickhouse-go/v2/tests"
+	"github.com/stretchr/testify/require"
+)
+
+func Test1113(t *testing.T) {
+	var (
+		conn, err = clickhouse_tests.GetConnection("issues", clickhouse.Settings{
+			"max_execution_time":             60,
+			"allow_experimental_object_type": true,
+		}, nil, &clickhouse.Compression{
+			Method: clickhouse.CompressionLZ4,
+		})
+	)
+	ctx := context.Background()
+	require.NoError(t, err)
+	const ddl = "CREATE TABLE test_1113 (col_1 JSON, col_2 JSON) Engine MergeTree() ORDER BY tuple()"
+	require.NoError(t, conn.Exec(ctx, ddl))
+	defer func() {
+		conn.Exec(ctx, "DROP TABLE IF EXISTS test_1113")
+	}()
+
+	batch, err := conn.PrepareBatch(context.Background(), "INSERT INTO test_1113")
+	require.NoError(t, err)
+
+	v1 := map[string]struct {
+		Str string
+	}{"a": {Str: "value"}}
+	v2 := map[string]any{}
+
+	require.NoError(t, batch.Append(v1, v2))
+	require.NoError(t, batch.Send())
+}