? ? ? ? ? ? ? ?何為方程式索引，其實這個名字是我自己取的，好像不是很形象，不過先這樣叫吧。先看一段操作：

mysql> explain select * from user where id+1=2; +----+-------------+-------+------+---------------+------+---------+------+----- -+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows| Extra | +----+-------------+-------+------+---------------+------+---------+------+----- -+-------------+ | 1 | SIMPLE | user | ALL | NULL | NULL | NULL | NULL | 4| Using where | +----+-------------+-------+------+---------------+------+---------+------+----- -+-------------+ 1 row in set (0.00 sec)mysql> show index from user; +-------+------------+----------+--------------+-------------+-----------+------ -------+----------+--------+------+------------+---------+---------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardi nality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | +-------+------------+----------+--------------+-------------+-----------+------ -------+----------+--------+------+------------+---------+---------------+ | user | 1 | id | 1 | id | A |NULL | NULL | NULL | YES | BTREE | | | +-------+------------+----------+--------------+-------------+-----------+------ -------+----------+--------+------+------------+---------+---------------+ 1 row in set (0.00 sec) id是表的索引，但是這個索引包含在id+1=2中，此時mysql無法識別ID這個索引。解析查找的類型為ALL。

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