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mysql索引分析对于开发人员是非常有必要的，充分理解mysql的查询索引机制，才能写出高效稳定的应用程序。这里记录下，平时的mysql索引使用经验小结，方便以后生疏了再来回顾。
测试版本5.7.16
!=是否走索引
网上的部分文章说，都有说逆向查询条件不走索引，但是本人在测试中发现这并不是绝对的。
原文地址 https://mp.weixin.qq.com/s/EJQHJv1WJu-aglWkIUX6aA
举例
user表有两个字段，id自增主键，name姓名(普通索引)。
user_copy表有四个字段，id自增主键，name姓名(普通索引), age, sex。
执行 explain select * from user where name != ‘xuliang’;
mysql> explain select * from user where name != 'xuliang';
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | user | NULL | index | idx_name | idx_name | 767 | NULL | 1 | 100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
执行 explain select * from user_copy where name != ‘xuliang’;
mysql> explain select * from user_copy where name != 'xuliang';
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user_copy | NULL | ALL | idx_name | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
为什么!=查询时一个查询结果显示type=index,一个显示type=ALL呢？
因为索引的关键信息存储在叶子结点上，叶子结点存储着索引列的全部关键值 和 对应行记录的指针。在第一个例子中，因为select * 只包含两列，主键id和name，在叶子结点中可以直接拿到，所以走索引全扫描。而在第二个例子中，因为select *还包含了age,sex字段，所以还要根据rowid去寻找对应的行记录，个人猜测，mysql可能考虑到 != 会匹配到更多的行记录，还不如走全表扫描。
为了进一步验证，我们执行
explain select id,name from user_copy where name != ‘xuliang’;
mysql> explain select id,name from user_copy where name != 'xuliang';
+----+-------------+-----------+------------+-------+---------------+----------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+----------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | user_copy | NULL | index | idx_name | idx_name | 767 | NULL | 1 | 100.00 | Using where; Using index |
+----+-------------+-----------+------------+-------+---------------+----------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
select count(1)会走索引
explain select count(1) from t_sp_item
执行结果
