update与select关联执行效率问题

UPDATE fl_user_space u
SET u.`course_count` = (SELECT COUNT(*) FROM fl_course c WHERE c.uid = u.uid)
WHERE 1 = 1 
AND u.school_id = #${schoolId}#;

　　这种执行效率执行时间在秒级以上，几秒到几十秒，数据量大时甚至更长

UPDATE fl_user_space u,
(SELECT c.uid ,COUNT(course_id) AS course_count FROM fl_course c WHERE c.school_id = #${schoolId}# GROUP BY c.`uid`) b
SET u.`course_count` = b.course_count
WHERE u.`uid` = b.uid 
AND u.school_id = #${schoolId}#;

　　这种执行效率执行时间在毫秒级，1~2百毫秒左右