MySQL 5.7之后怎么查看索引使用状况?

MySQL 5.7之后怎么查看索引使用状况?

0.在sys库中查看没用的索引

root@localhost [sys]>select * from schema_unused_indexes;
+-------------------+-------------+------------+
| object_schema     | object_name | index_name |
+-------------------+-------------+------------+
| sysbench_testdata | sbtest1     | k_1        |
| sysbench_testdata | sbtest10    | k_10       |
| sysbench_testdata | sbtest3     | k_3        |
| sysbench_testdata | sbtest4     | k_4        |
| sysbench_testdata | sbtest5     | k_5        |
| sysbench_testdata | sbtest6     | k_6        |
| sysbench_testdata | sbtest7     | k_7        |
| sysbench_testdata | sbtest8     | k_8        |
| sysbench_testdata | sbtest9     | k_9        |
+-------------------+-------------+------------+
9 rows in set (0.00 sec)

1. 经过show status like '%Handler_read%'方法查看:总体的

root@localhost [sysbench_testdata]>show status like '%Handler_read%';
+-----------------------+---------+
| Variable_name         | Value   |
+-----------------------+---------+
| Handler_read_first    | 7       |
| Handler_read_key      | 29      |
| Handler_read_last     | 0       |
| Handler_read_next     | 8446377 |
| Handler_read_prev     | 0       |
| Handler_read_rnd      | 20      |
| Handler_read_rnd_next | 8344612 |
+-----------------------+---------+
7 rows in set (0.00 sec)

• Handler_read_key这个值表明了一个行将索引值读的次数，很低的值代表增长索引获得的性能改善不高，由于索引并不常常使用。

• Handler_read_rnd_next 的值高则查询低效，而且应该创建索引补救。这个值是指在数据文件中读下一行的请求数。若是正进行大量的表扫描，Handler_read_rnd_next的值较高，则一般说明表索引不正确或查询没有利用索引

2.查看具体某一个sql的索引使用状况 :

root@localhost [sysbench_testdata]>explain select k from sbtest2 where k=432 limit 2;
+----+-------------+---------+------------+------+---------------+------+---------+-------+--------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref   | rows   | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+-------+--------+----------+-------------+
|  1 | SIMPLE      | sbtest2 | NULL       | ref  | k_2           | k_2  | 4       | const | 110944 |   100.00 | Using index |
+----+-------------+---------+------------+------+---------------+------+---------+-------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

字段说明:
Type：告诉咱们对表所使用的访问方式，主要包含以下集中类型；
◇ all：全表扫描
◇ const：读常量，且最多只会有一条记录匹配，因为是常量，因此实际上只须要读一次；
◇ eq_ref：最多只会有一条匹配结果，通常是经过主键或者惟一键索引来访问；
◇ fulltext：
◇ index：全索引扫描；
◇ index_merge：查询中同时使用两个（或更多）索引，而后对索引结果进行merge 以后再读取表数据；
◇ index_subquery：子查询中的返回结果字段组合是一个索引（或索引组合），但不是一个主键或者惟一索引；
◇ rang：索引范围扫描；
◇ ref：Join 语句中被驱动表索引引用查询；
◇ ref_or_null：与ref 的惟一区别就是在使用索引引用查询以外再增长一个空值的查询；
◇ system：系统表，表中只有一行数据；
◇ unique_subquery：子查询中的返回结果字段组合是主键或者惟一约束；

possible_keys：可能能够利用的索引的名字。这里的索引名字是建立索引时指定的索引昵称；若是索引没有昵称，则默认显示的是索引中第一个列的名字（在本例中，它是“firstname”）。默认索引名字的含义每每不是很明显。 　

key：它显示了MySQL实际使用的索引的名字。若是它为空（或NULL），则MySQL不使用索引。 　

key_len：索引中被使用部分的长度，以字节计

ref：列出是经过常量（const），仍是某个表的某个字段（若是是join）来过滤（经过key）
的；

rows：MySQL所认为的它在找到正确的结果以前必须扫描的记录数。显然，这里最理想的数字就是1。

3.经过performance_schema能够查询到.查看sbtest2表索引状况的查询语句:

root@localhost [sysbench_testdata]>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='sbtest2';

• 具体查看过程:

root@localhost [sysbench_testdata]>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='sbtest2';
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    1697669
Current database: sysbench_testdata

+-------------+-------------------+-------------+------------+------------+------------+-------------+
| object_type | object_schema     | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| TABLE       | sysbench_testdata | sbtest2     | PRIMARY    |          0 |          0 |           0 |
| TABLE       | sysbench_testdata | sbtest2     | k_2        |   76287298 |   76287298 |    76287298 |
| TABLE       | sysbench_testdata | sbtest2     | NULL       |    8344631 |    8344631 |     8344631 |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
3 rows in set (0.00 sec)

root@localhost [sysbench_testdata]>select k from sbtest2 where k=432 limit 2;                                                                                                       
+-----+
| k   |
+-----+
| 432 |
| 432 |
+-----+
2 rows in set (0.00 sec)

root@localhost [sysbench_testdata]>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='sbtest2';
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| object_type | object_schema     | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| TABLE       | sysbench_testdata | sbtest2     | PRIMARY    |          0 |          0 |           0 |
| TABLE       | sysbench_testdata | sbtest2     | k_2        |   76287300 |   76287300 |    76287300 |
| TABLE       | sysbench_testdata | sbtest2     | NULL       |    8344631 |    8344631 |     8344631 |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
3 rows in set (0.01 sec)

root@localhost [sysbench_testdata]>select k from sbtest2 where id=432 limit 2;                                                                                                               
+-------+
| k     |
+-------+
| 49866 |
+-------+
1 row in set (0.00 sec)

root@localhost [sysbench_testdata]>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='sbtest2';
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| object_type | object_schema     | object_name | index_name | count_star | count_read | COUNT_FETCH |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
| TABLE       | sysbench_testdata | sbtest2     | PRIMARY    |          1 |          1 |           1 |
| TABLE       | sysbench_testdata | sbtest2     | k_2        |   76287300 |   76287300 |    76287300 |
| TABLE       | sysbench_testdata | sbtest2     | NULL       |    8344631 |    8344631 |     8344631 |
+-------------+-------------------+-------------+------------+------------+------------+-------------+
3 rows in set (0.00 sec)

root@localhost [sysbench_testdata]>