如何知道你建立的MySQL索引是高效，合理的

不管是开发人员，仍是数据库维护人员，给表建立索引是很常见的事情。普通到什么程度，初级，中级开发人员的印象中，只要写的sql语句效率不高，第一反应就是建索引，而无论这个字段是否适合建立索引。反正只要给字段建立好索引，应用就能跑的嗖嗖的，索引就是银弹，就是灵丹妙药。

案例
在某个项目现场的数据库中，开发人员给一张业务表的全部字段都建立上了索引。因而就问，为何建立这么索引。开发说，我不懂sql怎么优化，碰到慢了，就建立索引。

因此在接手一个新库时，了解哪些索引不合理，低效是颇有必要的一件事情。

高效索引特色
索引必须建立在索引选择性较高的列上，选择性的计算方式为

select count(distinct(col_name))/count(*) from tb_name

若是结果小于0.2，则不建议在此列上建立索引，不然大几率会拖慢SQL执行

查询低效索引
能够根据高效索引特色，即选择性小于0.2的列上建立索引是低效的，能够经过如下sql语句查询到低效索引

mysql> SELECT
    ->     t.TABLE_SCHEMA,t.TABLE_NAME,INDEX_NAME,CARDINALITY,
    ->     TABLE_ROWS, CARDINALITY/TABLE_ROWS AS SELECTIVITY
    -> FROM
    ->     information_schema.TABLES t,
    -> (
    ->   SELECT table_schema,table_name,index_name,cardinality
    ->   FROM information_schema.STATISTICS
    ->   WHERE (table_schema,table_name,index_name,seq_in_index) IN (
    ->   SELECT table_schema,table_name,index_name,MAX(seq_in_index)
    ->   FROM information_schema.STATISTICS
    ->   GROUP BY table_schema , table_name , index_name )
    -> ) s
    -> WHERE
    ->     t.table_schema = s.table_schema
    ->         AND t.table_name = s.table_name AND t.table_rows != 0
    ->         AND t.table_schema NOT IN ( 'mysql','performance_schema','information_schema','sys')
    ->         and index_name!='PRIMARY'
    ->         and CARDINALITY/TABLE_ROWS <=0.2
    -> ORDER BY SELECTIVITY;
+--------------+------------+------------+-------------+------------+-------------+
| TABLE_SCHEMA | TABLE_NAME | index_name | cardinality | TABLE_ROWS | SELECTIVITY |
+--------------+------------+------------+-------------+------------+-------------+
| sbtest       | sbtest1    | k_1        |       13167 |      79937 |      0.1647 |
+--------------+------------+------------+-------------+------------+-------------+
1 row in set (0.09 sec)

从上面结果就找到一个低效索引（k_1），经过下面的命令，能够更详细的获取索引信息

mysql> show index from sbtest.sbtest1;
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table   | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| sbtest1 |          0 | PRIMARY  |            1 | id          | A         |       79937 |     NULL | NULL   |      | BTREE      |         |               |
| sbtest1 |          1 | k_1      |            1 | k           | A         |       13167 |     NULL | NULL   |      | BTREE      |         |               |
+---------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)