1：
primary key (`id`), unique key `uid` (`uid`,`task_id`)
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查询：
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mysql> explain select * from user_task where uid = 232; +----+-------------+-----------+------+---------------+------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+-------------+-----------+------+---------------+------+---------+-------+------+-------+ | 1 | simple | user_task | ref | uid | uid | 4 | const | 206 | | +----+-------------+-----------+------+---------------+------+---------+-------+------+-------+
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mysql> explain select * from user_task where task_id = 1454; +----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+ | 1 | simple | user_task | all | null | null | null | null | 55917 | using where | +----+-------------+-----------+------+---------------+------+---------+------+-------+-------------+ 1 row in set (0.00 sec)
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mysql> explain select * from user_task where task_id = 1454 and uid = 232; +----+-------------+-----------+-------+---------------+------+---------+-------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra | +----+-------------+-----------+-------+---------------+------+---------+-------------+------+-------+ | 1 | simple | user_task | const | uid | uid | 8 | const,const | 1 | | +----+-------------+-----------+-------+---------------+------+---------+-------------+------+-------+ 1 row in set (0.00 sec)
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如果（a， b, c）
查询a ； a b ； a b c 会用到索引
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所以建立索引的时候 如果要查询 b c ?那么可以 key(b, c)
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key 和index 不同的是 key 可以有约束 同时会有索引
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