查询使用子查询慢速运行

此查询对子查询的处理速度很慢。我还没有权限创建临时表，但无论如何要进一步优化这个查询，或者我在某处使用了破坏的逻辑吗？查询使用子查询慢速运行

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';

来源

2017-01-09 alfourn211

尝试在您在哪里条件下使用的所有字段上创建索引。子查询中的'lastlogin'，'id'，'firstaccess'，'lastaccess' – jaym

您应该添加'AND u2.id = u.id'。这可能不是一个巨大的速度提升，但结果会更准确。 – CptMisery

相关的子查询大部分时间都相当低效，您可能会获得更好的表现。大多数情况下，可以用INNER JOIN重写相关子查询。

我认为你的相关子查询可以重写如下，但你需要检查结果是否相同，因为我不知道你的表结构和数据。

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';

来源

2017-01-09 17:03:51

查询使用子查询慢速运行

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