MySQL 添加索引报错：BLOB/TEXT column used in key specification without a key length

 ALTER  TABLE   `ods_stock_k_30m` MODIFY  stock_code VARCHAR(30);
 ALTER  TABLE   `ods_stock_k_30m` MODIFY  stock_date VARCHAR(30);
 ALTER  TABLE  `ods_stock_k_30m`  ADD  INDEX stock_index_name ( stock_code,stock_date);