sql语句的where条件顺序和索引的顺序

建表

CREATE TABLE `Problem_submit` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `status` int(11) NOT NULL,
  `user_id` int(11) NOT NULL,
  `Username` varchar(30) NOT NULL,
  `problem_id` int(11) NOT NULL,
  `time_used` int(11) NOT NULL,
  `memory_used` int(11) NOT NULL,
  `submit_date` datetime NOT NULL,
  `ip` char(15) NOT NULL,
  `compiler_id` int(11) NOT NULL,
  `source` longtext NOT NULL,
  `code_length` int(11) NOT NULL,
  `error_message` longtext,
  `contest_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`,`submit_date`),
  KEY `contest_id,problem_id,status 查询考试问题状态` (`contest_id`,`problem_id`,`status`) USING BTREE,
  KEY `problem_id 默认id降序` (`problem_id`,`id`) USING BTREE COMMENT 'id避免外部排序',
  KEY `status 默认id降序` (`status`,`id`) USING BTREE COMMENT 'id避免外部排序',
  KEY `compiler_id 默认id降序` (`compiler_id`,`id`) USING BTREE COMMENT 'id避免外部排序',
  KEY `user_id,status 查询用户提交状态` (`user_id`,`status`,`id`) USING BTREE,
  KEY `user_id 默认id降序` (`user_id`,`id`) USING BTREE,
  KEY `contest_id 默认id降序` (`contest_id`,`id`) USING BTREE,
  KEY `contest_id,problem_id 查询考试问题` (`contest_id`,`problem_id`,`id`) USING BTREE
) ENGINE=MyISAM AUTO_INCREMENT=1060256 DEFAULT CHARSET=utf8;

查询实验：

1. select * from Problem_submit where user_id=200 and status=1
2. select * from Problem_submit where status=1 and user_id=200
3. select * from Problem_submit where status=2 and user_id=4358 and compiler_id=1
4. select * from Problem_submit where status=2 and compiler_id=1 and user_id=4358
   都会走索引，因为Mysql会进行查询的计划优化