Question

Here’s the query (the largest table has about 40,000 rows)

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

If I run this, it executes very quickly (.05 seconds roughly). It returns 13 rows.

When I add an ORDER BY clause at the end of the query (ordering by any column) the query takes about 10 seconds.

I’m using this database in production now, and everything is working fine. All my other queries are speedy.

Any ideas of what it could be? I ran the query in MySQL’s Query Browser, and from the command line. Both places it was dead slow with the ORDER BY.

EDIT: Tolgahan ALBAYRAK solution works, but can anyone explain why it works?

Answer 1

maybe this helps:

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