无论是开发人员，还是数据库维护人员，给表创建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，而不管这个字段是否适合创建索引。反正只要给字段创建好索引，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所有字段都创建上了索引。于是就问，为什么创建这么索引。开发说，我不懂sql怎么优化，碰到慢了，就创建索引。
所以在接手一个新库时，了解哪些索引不合理，低效是很有必要的一件事情。
高效索引特点 索引必须创建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
如果结果小于0.2，则不建议在此列上创建索引，否则大概率会拖慢SQL执行
查询低效索引 可以根据高效索引特点，即选择性小于0.2的列上创建索引是低效的，可以通过以下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），通过下面的命令，可以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)
