请帮忙分析下该表的EXPLAIN？

cxm.cm

mysql> explain select tb_smlvideo_Category.SourceId,tb_smlvideo_Category.Category from tb_smlvideo_Category,(select tb_smlvideo_Category.Sourc
eId from tb_smlvideo_Category where tb_smlvideo_Category.Category='') as tb_smlvideo_CategorySourceId where tb_smlvideo_Category.SourceId=tb_smlvideo_CategorySourceId.SourceId;
+----+-------------+----------------------+--------+---------------+----------+---------+------+-------+--------------------------+
| id | select_type | table                | type   | possible_keys | key      | key_len | ref  | rows  | Extra                    |
+----+-------------+----------------------+--------+---------------+----------+---------+------+-------+--------------------------+
|  1 | PRIMARY     | <derived2>           | system | NULL          | NULL     |    NULL | NULL |     0 | const row not found      |
|  1 | PRIMARY     | tb_smlvideo_Category | index  | NULL          | Category |      54 | NULL | 19506 | Using where; Using index |
|  2 | DERIVED     | tb_smlvideo_Category | ref    | Category      | Category |      50 |      |     1 | Using where; Using index |

该表如果优化的话，还需要从哪几方面来做？

yejr

看起来基本是极限了