01.索引字段数据类型隐式转换，将导致索引失效。
案例如下：
work@ (xxx) > show create table sign\G;
*************************** 1. row ***************************
Table: user_sign
Create Table: CREATE TABLE `sign` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`appid` bigint(20) unsigned NOT NULL,
`sign_task_id` bigint(20) unsigned NOT NULL,
`uid` varchar(64) NOT NULL,
`push_flag` int(10) NOT NULL DEFAULT '0' COMMENT '签到提醒标志 0=不提醒, 1=提醒',
`current_counter` bigint(20) unsigned NOT NULL DEFAULT '0',
`total_counter` bigint(20) unsigned NOT NULL DEFAULT '0',
`last_sign_time` bigint(20) NOT NULL DEFAULT '0',
`create_time` bigint(20) NOT NULL DEFAULT '0',
`update_time` bigint(20) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`,`appid`),
UNIQUE KEY `uk_appid_task_id_uid` (`sign_task_id`,`uid`,`appid`) USING BTREE
) ENGINE=InnoDB AUTO_INCREMENT=1259002273035255811 DEFAULT CHARSET=utf8mb4
1 row in set (0.00 sec)
ERROR:
No query specified
work@ (xxx) > select * from sign WHERE sign_task_id = 1 AND uid = 1 and appid = 959462;
+---------------------+--------+--------------+-----+-----------+-----------------+---------------+----------------+---------------+---------------+
| id | appid | sign_task_id | uid | push_flag | current_counter | total_counter | last_sign_time | create_time | update_time |
+---------------------+--------+--------------+-----+-----------+-----------------+---------------+----------------+---------------+---------------+
| 1194541191434104834 | 959462 | 1 | 1 | 0 | 1 | 69 | 1583115350530 | 1573635788187 | 1583115350530 |
+---------------------+--------+--------------+-----+-----------+-----------------+---------------+----------------+---------------+---------------+
1 row in set (5.30 sec)
Sat May 9 14:13:33 2020 work@ (xxx) > desc select * from sign WHERE sign_task_id = 1 AND uid = 1 and appid = 959462;
+----+-------------+-----------+------------+------+----------------------+----------------------+---------+-------+---------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+----------------------+----------------------+---------+-------+---------+----------+-----------------------+
| 1 | SIMPLE | user_sign | NULL | ref | uk_appid_task_id_uid | uk_appid_task_id_uid | 8 | const | 4762484 | 1.00 | Using index condition |
+----+-------------+-----------+------------+------+----------------------+----------------------+---------+-------+---------+----------+-----------------------+
1 row in set, 2 warnings (0.00 sec)
Sat May 9 14:18:32 2020
发现上面的sql执行花费了5秒多，该表数据量为9825755
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work@ (xxx) > select * from sign WHERE sign_task_id = 1 AND uid = '1' and appid = 959462;
+---------------------+--------+--------------+-----+-----------+-----------------+---------------+----------------+---------------+---------------+
| id | appid | sign_task_id | uid | push_flag | current_counter | total_counter | last_sign_time | create_time | update_time |
+---------------------+--------+--------------+-----+-----------+-----------------+---------------+----------------+---------------+---------------+
| 1194541191434104834 | 959462 | 1 | 1 | 0 | 1 | 69 | 1583115350530 | 1573635788187 | 1583115350530 |
+---------------------+--------+--------------+-----+-----------+-----------------+---------------+----------------+---------------+---------------+
1 row in set (0.01 sec)
Sat May 9 14:18:39 2020 work@ (xxx) > desc select * from sign WHERE sign_task_id = 1 AND uid = '1' and appid = 959462;
+----+-------------+-----------+------------+-------+----------------------+----------------------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+----------------------+----------------------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | user_sign | NULL | const | uk_appid_task_id_uid | uk_appid_task_id_uid | 274 | const,const,const | 1 | 100.00 | NULL |
+----+-------------+-----------+------------+-------+----------------------+----------------------+---------+-------------------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
Sat May 9 14:18:46 2020
发现上面的sql执行，花费了0.01秒，仅仅是将“uid = 1”改成了“uid = '1'”而已，结果差距这么大。这是因为uid字段是一个varchar类型。若使用uid=1，则将会发生数据类型的隐式转换，故而导致索引失效，从而导致是一个慢查询。
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