http://blog.sina.com.cn/s/blog_5037eacb0102vm5c.html
官方MySQL中查看索引是否被使用到：
在percona版本或marida中可以通过 information_schea.index_statistics查看得到，
在官方版本中如何查看呢？
select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage;
应该可以通过上面的sql得到。 如果read,fetch的次数都为0的话，应该是没有被使用过的。
通过下面的例子，可以证实：
启动mysql:
mysql> show create table a.t3;
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| t3 | CREATE TABLE `t3` (
`id` int(11) DEFAULT NULL,
`name` varchar(10) NOT NULL DEFAULT ‘bb‘,
KEY `idx_t3` (`id`),
KEY `idx_t3_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+-------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.01 sec)
mysql> select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name=‘t3‘;
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| TABLE | a | t3 | idx_t3 | 0 | 0 | 0 |
| TABLE | a | t3 | idx_t3_name | 0 | 0 | 0 |
| TABLE | a | t3 | NULL | 0 | 0 | 0 |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
3 rows in set (0.01 sec)
mysql> explain select id from t3 where id=1;
ERROR 1046 (3D000): No database selected
mysql> explain select id from a.t3 where id=1;
+----+-------------+-------+------+---------------+--------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+--------+---------+-------+------+-------------+
| 1 | SIMPLE | t3 | ref | idx_t3 | idx_t3 | 5 | const | 1 | Using index |
+----+-------------+-------+------+---------------+--------+---------+-------+------+-------------+
1 row in set (0.00 sec)
mysql> select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name=‘t3‘;
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| TABLE | a | t3 | idx_t3 | 0 | 0 | 0 |
| TABLE | a | t3 | idx_t3_name | 0 | 0 | 0 |
| TABLE | a | t3 | NULL | 0 | 0 | 0 |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
3 rows in set (0.01 sec)
mysql> use a;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> select id from t3 where id=1;
Empty set (0.00 sec)
mysql> select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name=‘t3‘;
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| TABLE | a | t3 | idx_t3 | 1 | 1 | 1 |
| TABLE | a | t3 | idx_t3_name | 0 | 0 | 0 |
| TABLE | a | t3 | NULL | 0 | 0 | 0 |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
3 rows in set (0.00 sec)
mysql> select id from t3 where id=10; +------+
| id |
+------+
| 10 |
| 10 |
+------+
2 rows in set (0.00 sec)
mysql> select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name=‘t3‘;
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| TABLE | a | t3 | idx_t3 | 4 | 4 | 4 |
| TABLE | a | t3 | idx_t3_name | 0 | 0 | 0 |
| TABLE | a | t3 | NULL | 0 | 0 | 0 |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
3 rows in set (0.01 sec)
mysql> select name from t3 where name=‘a‘;
Empty set (0.00 sec)
mysql> select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name=‘t3‘;
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| TABLE | a | t3 | idx_t3 | 4 | 4 | 4 |
| TABLE | a | t3 | idx_t3_name | 1 | 1 | 1 |
| TABLE | a | t3 | NULL | 0 | 0 | 0 |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
3 rows in set (0.01 sec)
mysql> select name from t3 where name=‘name1‘;
+-------+
| name |
+-------+
| name1 |
+-------+
1 row in set (0.01 sec)
mysql> select object_type,object_schema,object_name,index_name,count_star,count_read,COUNT_FETCH from performance_schema.table_io_waits_summary_by_index_usage where object_name=‘t3‘;
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| object_type | object_schema | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
| TABLE | a | t3 | idx_t3 | 4 | 4 | 4 |
| TABLE | a | t3 | idx_t3_name | 3 | 3 | 3 |
| TABLE | a | t3 | NULL | 0 | 0 | 0 |
+-------------+---------------+-------------+-------------+------------+------------+-------------+
3 rows in set (0.01 sec)
索引被使用之后， 对应的值会增加。