mysql5.7 order by id desc limit n ;Optimizer selects primary key index
CREATE TABLE `test` ( `id` bigint UNSIGNED NOT NULL AUTO_INCREMENT, `month` timestamp NOT NULL DEFAULT '2018-01-01 00:00:00', `b` varchar(255) NOT NULL DEFAULT '', `c` varchar(255) NOT NULL DEFAULT '', `d` varchar(255) NOT NULL DEFAULT '', `e` varchar(255) NOT NULL DEFAULT…