diff --git a/tests/multi_source/main.go b/tests/multi_source/main.go
index 8d0e25a8468..e6208efa94c 100644
--- a/tests/multi_source/main.go
+++ b/tests/multi_source/main.go
@@ -282,6 +282,14 @@ func addDropColumnDDL(ctx context.Context, db *sql.DB) {
 	}
 }
 
+// addDropColumnDDL2 tests the following scenario:
+// 1. Create a table, executing DML of this table in background
+// 2. alter table add column v1 varchar(20) default "xxx" not null
+// 3. Some rows could have no data for column v1, however when we decode them,
+//    we could use the new table schema (which has the column of v1, caused by
+//    online DDL). Since the column data doesn't exist, the tidb library will
+//    fill in a default value, which is a string type. That's why we can get
+//    either []byte or a string of a column.Value
 func addDropColumnDDL2(ctx context.Context, db *sql.DB) {
 	testName := getFunctionName(addDropColumnDDL2)
 	mustCreateTable(db, testName)