MySQL優化查詢

Question

SELECT 
    (UserCourseProgress.TimeEnded - UserCourseProgress.TimeStarted) as SecondsSpent, 
    UserCourseProgress.UserId 
FROM 
    UserCourseProgress 
    INNER JOIN UserRoles 
    ON UserCourseProgress.UserId = UserRoles.UserId 
     AND (UserRoles.SuperId = '123' OR UserRoles.UserId = '123') 

我想優化這個查詢，因爲它需要很長時間（主要是將數據複製到Tmp表）。 該結構很簡單，一個名爲UserCourseProgress的表和另一個名爲UserRoles的表，他們都有UserId列的共同點，並且我選擇屬於超級用戶的所有userRoles。有任何想法嗎？

Answer 1

使用UNION

SELECT 
    (UserCourseProgress.TimeEnded - UserCourseProgress.TimeStarted) as SecondsSpent, 
    UserCourseProgress.UserId FROM UserCourseProgress 
INNER JOIN UserRoles ON UserCourseProgress.UserId = UserRoles.UserId 
    AND UserRoles.SuperId = '123' 

UNION 

SELECT 
    (UserCourseProgress.TimeEnded - UserCourseProgress.TimeStarted) as SecondsSpent, 
    UserCourseProgress.UserId FROM UserCourseProgress 
INNER JOIN UserRoles ON UserCourseProgress.UserId = UserRoles.UserId 
    AND UserRoles.UserId = '123'; 

添加索引

ALTER TABLE UserCourseProgress ADD INDEX (UserId); 
ALTER TABLE UserRoles ADD INDEX (SuperId, UserId); 
ALTER TABLE UserRoles ADD INDEX (UserId); 

Answer 2

使用EXPLAIN獲取查詢執行計劃。然後，看看更耗費的步驟，並在必要時使用索引。

EXPLAIN