MYSQL primary key use btree 是什么含义了解一下

CREATE TABLE `sth_definition` ( `id` int(11) NOT NULL AUTO_INCREMENT, `analyseId` bigint(20) DEFAULT NULL, `levelCondition` varchar(2048) DEFAULT NULL, `filed1` double DEFAULT NULL, `dfdf` double DEFAULT NULL, `dfsdf` double DEFAULT NULL, `dfdf` varchar(255) DEFAULT NULL, `sf` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`) USING BTREE, KEY `id` (`id`) USING BTREE ) ENGINE=InnoDB AUTO_INCREMENT=7974 DEFAULT CHARSET=utf8

primary key use btree 是什么含义了解一下