查詢使用子查詢慢速運行

此查詢對子查詢的處理速度很慢。我還沒有權限創建臨時表，但無論如何要進一步優化這個查詢，或者我在某處使用了破壞的邏輯嗎？查詢使用子查詢慢速運行

SET SESSION TRANSACTION ISOLATION LEVEL READ UNCOMMITTED ; 
SELECT 
     CONCAT(u.firstname , ' ' , u.lastname) as Student_name, 
     u.id as Student_ID, 
     FROM_UNIXTIME(u.firstaccess) as first_moodle_access, 
     FROM_UNIXTIME(u.lastaccess) as last_moodle_access, 
     FROM_UNIXTIME(last.timeaccess) as last_course_access, 
     grp.name as Cohorts, 
     cour.fullname as course 
FROM 
mdl_user as u 
Inner Join mdl_user_lastaccess as last on u.id = last.userid 
Inner Join mdl_course as cour on last.courseid = cour.id 
Inner Join mdl_groups as grp on cour.id = grp.courseid 
WHERE u.lastlogin = (
        Select Max(u2.lastlogin) 
        From mdl_user as u2 
        where u2.lastlogin = u.lastlogin 
        ) 
AND cour.id = '10189' AND 
u.firstaccess > '0' AND 
u.lastaccess > '0';

來源

2017-01-09 alfourn211

嘗試在您在哪裏條件下使用的所有字段上創建索引。子查詢中的'lastlogin'，'id'，'firstaccess'，'lastaccess' – jaym

您應該添加'AND u2.id = u.id'。這可能不是一個巨大的速度提升，但結果會更準確。 – CptMisery

相關的子查詢大部分時間都相當低效，您可能會獲得更好的表現。大多數情況下，可以用INNER JOIN重寫相關子查詢。

我認爲你的相關子查詢可以重寫如下，但你需要檢查結果是否相同，因爲我不知道你的表結構和數據。

SELECT 
     CONCAT(u.firstname , ' ' , u.lastname) as Student_name, 
     u.id as Student_ID, 
     FROM_UNIXTIME(u.firstaccess) as first_moodle_access, 
     FROM_UNIXTIME(u.lastaccess) as last_moodle_access, 
     FROM_UNIXTIME(last.timeaccess) as last_course_access, 
     grp.name as Cohorts, 
     cour.fullname as course 
FROM 
mdl_user as u 
Inner Join mdl_user_lastaccess as last on u.id = last.userid 
Inner Join mdl_course as cour on last.courseid = cour.id 
Inner Join mdl_groups as grp on cour.id = grp.courseid 
Inner Join(
    Select 
    Max(lastlogin) 
    FROM 
    mdl_user 
) AS u2 
ON 
    u.lastlogin = u2.lastlogin 
WHERE 
cour.id = '10189' AND 
u.firstaccess > '0' AND 
u.lastaccess > '0';

來源

2017-01-09 17:03:51

查詢使用子查詢慢速運行

回答

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