使用ORDER BY时查询速度慢

Question

这是查询(最大的表有大约40,000行)

SELECT
  Course.CourseID,
  Course.Description,
  UserCourse.UserID,
  UserCourse.TimeAllowed,
  UserCourse.CreatedOn,
  UserCourse.PassedOn,
  UserCourse.IssuedOn,
  C.LessonCnt
FROM
  UserCourse
INNER JOIN
  Course
USING(CourseID)
INNER JOIN
(
  SELECT CourseID, COUNT(*) AS LessonCnt FROM CourseSection GROUP BY CourseID
) C
USING(CourseID)
WHERE 
  UserCourse.UserID = 8810

如果我运行它,它会很快执行(大约.05秒).它返回13行.

当我ORDER BY在查询末尾添加一个子句(按任何列排序)时,查询大约需要10秒.

我现在在生产中使用这个数据库,一切正常.我所有的其他疑问都很快.

有什么想法可能是什么？我在MySQL的查询浏览器中运行查询,并从命令行运行.这两个地方都很慢了ORDER BY.

编辑: Tolgahan ALBAYRAK解决方案的工作原理,但谁能解释为什么它的工作原理？

Answer 1

也许这有助于:

SELECT * FROM (    
     SELECT
      Course.CourseID,
      Course.Description,
      UserCourse.UserID,
      UserCourse.TimeAllowed,
      UserCourse.CreatedOn,
      UserCourse.PassedOn,
      UserCourse.IssuedOn,
      C.LessonCnt
    FROM
      UserCourse
    INNER JOIN
      Course
    USING(CourseID)
    INNER JOIN
    (
      SELECT CourseID, COUNT(*) AS LessonCnt FROM CourseSection GROUP BY CourseID
    ) C
    USING(CourseID)
    WHERE 
      UserCourse.UserID = 8810
) ORDER BY CourseID

Answer 2

您订购的列是否已编入索引？

索引大大加快了排序和过滤速度.