通过key_len分析联合索引的使用

The key_len column indicates the length of the key that MySQL decided to use. The length is NULL if the key column says NULL. Note that the value of key_len enables you to determine how many parts of a multiple-part key MySQL actually uses.

数据库版本及数据表结构如下

mysql> select version();
+------------+
| version()  |
+------------+
| 5.6.10-log |
+------------+
1 row in set

mysql> show create table usercasinoresult_11;
+---------------------+-------------------------------------------------+
| Table               | Create Table                                    |
+---------------------+-------------------------------------------------+
| usercasinoresult_11 | CREATE TABLE `usercasinoresult_11` (
  `CasinoID` varchar(64) NOT NULL,
  `UserID` bigint(20) NOT NULL DEFAULT '0',
  `NickName` varchar(45) NOT NULL,
  `LastUpdateTime` datetime NOT NULL,
  `GameSpecified` varchar(500) DEFAULT NULL,
  PRIMARY KEY (`CasinoID`,`UserID`),
  KEY `IX_UserID_LastUpdateTime` (`UserID`,`LastUpdateTime`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 |
+---------------------+-------------------------------------------------+
1 row in set

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