一个案例彻底弄懂如何正确使用mysql inndb联合索引

SELECT `id`, `title`
FROM `th_content`
WHERE `audit_time` < 1541984478
 AND `status` = 'ONLINE'
ORDER BY `audit_time` DESC, `id` DESC
LIMIT 5;

CREATE TABLE `th_content` (
 `id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
 `title` varchar(500) CHARACTER SET utf8 NOT NULL DEFAULT '' COMMENT '内容标题',
 `content` mediumtext CHARACTER SET utf8 NOT NULL COMMENT '正文内容',
 `audit_time` int(11) unsigned NOT NULL DEFAULT '0' COMMENT '审核时间',
 `last_edit_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT '最近编辑时间',
 `status` enum('CREATED','CHECKING','IGNORED','ONLINE','OFFLINE') CHARACTER SET utf8 NOT NULL DEFAULT 'CREATED' COMMENT '资讯状态',
 PRIMARY KEY (`id`),
 KEY `idx_at_let` (`audit_time`,`last_edit_time`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;

ALTER TABLE `th_content` ADD INDEX `idx_audit_status` (`audit_time`, `status`);

mysql> explain select `id`, `title` from `th_content` where `audit_time` < 1541984478 and `status` = 'ONLINE' order by `audit_time` desc, `id` desc limit 5;
+----+-------------+------------+-------+------------------------------------------+------------------+---------+------+--------+-------------+
| id | select_type | table  | type | possible_keys       | key    | key_len | ref | rows | Extra  |
+----+-------------+------------+-------+------------------------------------------+------------------+---------+------+--------+-------------+
| 1 | SIMPLE  | th_content | range | idx_at_ft_pt_let,idx_audit_status  | idx_audit_status | 4  | NULL | 209754 | Using where |
+----+-------------+------------+-------+------------------------------------------+------------------+---------+------+--------+-------------+

ALTER TABLE `th_content` DROP INDEX `idx_audit_status`;
ALTER TABLE `th_content` ADD INDEX `idx_status_audit` (`status`, `audit_time`);