address comments.

plan/planbuilder.go

@@ -1166,11 +1166,12 @@ func (b *planBuilder) buildSelectPlanOfInsert(insert *ast.InsertStmt, insertPlan
 	}
 
 	// Check to guarantee that there's no generated column.
-	// This check is done after the above one is to make compatible with mysql.
-	// i.e. table t have two column a and b where b is generated column.
-	// `insert into t (b) select * from t` will raise a error which tells you that the column count is not matched.
-	// `insert into t select * from t` will raise a error which tells you that there's generated column in column list.
-	// If we do this check before the above one, the first example will raise the error same with the second example.
+	// This check should be done after the above one to make its behavior compatible with MySQL.
+	// For example, table t has two columns, namely a and b, and b is a generated column.
+	// "insert into t (b) select * from t" will raise an error that the column count is not matched.
+	// "insert into t select * from t" will raise an error that there's a generated column in the column list.
+	// If we do this check before the above one, "insert into t (b) select * from t" will raise an error
+	// that there's a generated column in the column list.
 	for _, col := range affectedValuesCols {
 		if col.IsGenerated() {
 			b.err = ErrBadGeneratedColumn.GenByArgs(col.Name.O, insertPlan.Table.Meta().Name.O)