MySQL查询耗时分析及存储空间分析

查询耗时分析

查看执行语句：

MySQL [dbname]> show processlist;
+-----+-----------------+----------------------+--------+---------+-------+-----------------------------+------------------------------------------------------------------------------------------------------+
| Id  | User            | Host                 | db     | Command | Time  | State                       | Info                                                                                                 |
+-----+-----------------+----------------------+--------+---------+-------+-----------------------------+------------------------------------------------------------------------------------------------------+
|   1 | event_scheduler | localhost            | NULL   | Daemon  | 27675 | Waiting for next activation | NULL                                                                                                 |
|   2 | root            | 114.55.72.122:41679   | dbname| Sleep   |    25 |                             | NULL                                                                                                 |
|   3 | root            | 114.55.72.122:41682   | dbname| Sleep   |    51 |                             | NULL                                                                                                 |
|   4 | root            | 114.55.72.122:41680   | dbname| Sleep   |    25 |                             | NULL                                                                                                 |
| 218 | root            | 114.55.72.121:55010 | dbname| Killed  | 27625 | updating                    | delete from msg_log_t where recv_time < DATE_SUB(NOW(),INTERVAL 2 DAY)           

使用kill关键字取消后台耗时操作：

MySQL [dbname]> kill 218;  //其中218对应上表中查询ID
Query OK, 0 rows affected (0.00 sec)

使用explain关键字分析慢查询：

MySQL [dbname]> explain select recv_time from msg_log_t order by recv_time desc limit 10;
+----+-------------+------------------+-------+---------------+--------+---------+------+----------+-----------------------------+
| id | select_type | table            | type  | possible_keys | key    | key_len | ref  | rows     | Extra                       |
+----+-------------+------------------+-------+---------------+--------+---------+------+----------+-----------------------------+
|  1 | SIMPLE      | car_gb_msg_log_t | index | NULL          | update | 24      | NULL | 35173644 | Using index; Using filesort |
+----+-------------+------------------+-------+---------------+--------+---------+------+----------+-----------------------------+
1 row in set (0.00 sec)

如果出现Extra列出现Using filesort，即要使用外部排序，则会造成查询缓慢。

存储空间分析

查看表及索引占用存储空间：

MySQL [(none)]> use dbname;
Database changed
MySQL [dbname]> select table_name,concat(truncate(data_length/1024/1024,2),'MB') as data_size,
    -> concat(truncate(index_length/1024/1024,2),'MB') as index_size
    -> from information_schema.tables where table_schema='dbname'
    -> order by data_length desc;
+-------------------------+------------+------------+
| table_name              | data_size  | index_size |
+-------------------------+------------+------------+
| msg_log_t               | 10271.75MB | 3272.12MB  |
MySQL [dbname]> show index from msg_log_tfrom btkjdb;
+------------------+------------+----------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table            | Non_unique | Key_name | Seq_in_index | Column_name   | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------------+------------+----------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| msg_log_t|          0 | PRIMARY  |            1 | id            | A         |    41523525 |     NULL | NULL   |      | BTREE      |         |               |
| msg_log_t|          1 | car_id   |            1 | car_id        | A         |          21 |     NULL | NULL   | YES  | BTREE      |         |               |
| msg_log_t|          1 | car_id   |            2 | msg_type      | A         |          21 |     NULL | NULL   | YES  | BTREE      |         |               |
| msg_log_t|          1 | update   |            1 | car_id        | A         |          21 |     NULL | NULL   | YES  | BTREE      |         |               |
| msg_log_t|          1 | update   |            2 | length        | A         |          21 |     NULL | NULL   | YES  | BTREE      |         |               |
| msg_log_t|          1 | update   |            3 | recv_time     | A         |    41523525 |     NULL | NULL   | YES  | BTREE      |         |               |
| msg_log_t|          1 | update   |            4 | upload_status | A         |    41523525 |     NULL | NULL   | YES  | BTREE      |         |               |
+------------------+------------+----------+--------------+---------------+-----------+-------------+----------+--------+------+------------+---------+---------------+

索引优化

联合索引，如INDEX（C1,C2,C3,C4），通常是先按C1列排序，C1相同，再按C2列排序，以次类推。此时，order by后的字段，只对C1列优化。如果要order by C2，最好新建INDEX（C2），否则很可能会出现Using filesort现象。具体使用哪个索引字段，最好使用explain分析下。