查询性能优化

Question

我有这样的MySQL查询：

SELECT tbl.userid, tbl.name, tbl.courseid, tbl.fullname, IFNULL(tbl.instance, '-') instance, IFNULL(tbl.activityname, '-') activityname, 
    IFNULL(tbl.module, '-') module, IFNULL(tbl.attempt, '-') attempt, IFNULL(tbl.score, '-') score, 
    CASE tbl.module WHEN 'quiz' THEN (
     CASE WHEN (tbl.attempt IS NULL AND tbl.score IS NULL) THEN 'NOT YET' 
     WHEN (tbl.attempt IS NOT NULL AND tbl.score IS NULL) THEN 'ATTEMPT' 
     WHEN (tbl.attempt IS NOT NULL AND tbl.score IS NOT NULL) THEN 'FINISH' 
     END) 
    WHEN 'scorm' THEN (
     CASE WHEN tbl.attempt IS NULL THEN 'NOT YET' 
     ELSE (SELECT `status` FROM mdl_scorm_logs WHERE scormid = tbl.instance AND userid = tbl.userid) 
     END) 
    ELSE '-' 
    END `status` 
FROM (
    SELECT tbl1.userid, tbl1.name, tbl1.courseid, tbl1.fullname, tbl2.instance, 
     CASE tbl2.module WHEN 'quiz' THEN (SELECT `name` FROM mdl_quiz WHERE id = tbl2.instance AND course = tbl1.courseid) 
     WHEN 'scorm' THEN (SELECT `name` FROM mdl_scorm WHERE id = tbl2.instance AND course = tbl1.courseid) 
     END activityname 
     , tbl2.module, 
     CASE tbl2.module WHEN 'quiz' THEN (SELECT MAX(attempt) FROM mdl_quiz_attempts WHERE quiz = tbl2.instance AND userid = tbl1.userid) 
     WHEN 'scorm' THEN (SELECT MAX(attempt) FROM mdl_scorm_scoes_track WHERE scormid = tbl2.instance AND userid = tbl1.userid) 
     END attempt, 
     (SELECT CONCAT(TRUNCATE(ROUND(gg.finalgrade/gi.grademax * 100, 2), 2), ' %') 
     FROM mdl_grade_grades gg 
     JOIN mdl_grade_items gi ON gg.itemid = gi.id 
     WHERE gg.userid = tbl1.userid AND gi.courseid = tbl1.courseid AND gi.itemname = activityname AND gi.itemtype = 'mod' 
      AND gi.itemmodule = tbl2.module AND gi.iteminstance = tbl2.instance) score 
    FROM (
     SELECT u.id userid, CONCAT(u.firstname, ' ', u.lastname) `name`, c.id courseid, c.fullname 
     FROM mdl_user u 
     JOIN mdl_role_assignments ra ON u.id = ra.userid 
     JOIN mdl_context co ON ra.contextid = co.id 
     JOIN mdl_role r ON ra.roleid = r.id 
     JOIN mdl_course c ON co.instanceid = c.id 
     JOIN mdl_enrol e ON c.id = e.courseid 
     JOIN mdl_user_enrolments ue ON e.id = ue.enrolid AND u.id = ue.userid 
     WHERE r.archetype = 'student') tbl1 
    LEFT JOIN (
     SELECT cm.course, m.name module, cm.instance FROM mdl_course_modules cm 
     JOIN mdl_modules m ON cm.module = m.id 
     WHERE m.name IN ('quiz', 'scorm') 
     ) tbl2 
    ON tbl1.courseid = tbl2.course) tbl 
WHERE tbl.userid = '4' AND tbl.courseid = '8' 
GROUP BY tbl.courseid, tbl.instance 
ORDER BY tbl.name, tbl.fullname, tbl.activityname 

结果后，我使用EXPLAIN：

id select_type table type possible_keys key key_len ref rows Extra 
1 PRIMARY <derived3> ALL     122 Using where; Using temporary; Using filesort 
3 DERIVED <derived9> ALL     42 
3 DERIVED <derived10> ALL     23 
10 DERIVED m range PRIMARY,mdl_modu_nam_ix mdl_modu_nam_ix 62  2 Using where; Using index 
10 DERIVED cm ref mdl_courmodu_mod_ix mdl_courmodu_mod_ix 8 moodle.m.id 4 
9 DERIVED r ref PRIMARY,mdl_role_arc_ix mdl_role_arc_ix 92  1 Using where; Using index 
9 DERIVED ra ref mdl_roleassi_rol_ix,mdl_roleassi_con_ix,mdl_roleassi_use_ix mdl_roleassi_rol_ix 8 moodle.r.id 5 
9 DERIVED u eq_ref PRIMARY PRIMARY 8 moodle.ra.userid 1 
9 DERIVED co eq_ref PRIMARY,mdl_cont_ins_ix PRIMARY 8 moodle.ra.contextid 1 
9 DERIVED e ref PRIMARY,mdl_enro_cou_ix mdl_enro_cou_ix 8 moodle.co.instanceid 1 Using index 
9 DERIVED c eq_ref PRIMARY PRIMARY 8 moodle.co.instanceid 1 
9 DERIVED ue eq_ref mdl_userenro_enruse_uix,mdl_userenro_enr_ix,mdl_userenro_use_ix mdl_userenro_enruse_uix 16 moodle.e.id,moodle.ra.userid 1 Using index 
8 DEPENDENT SUBQUERY gi ref PRIMARY,mdl_graditem_itenee_ix,mdl_graditem_cou_ix mdl_graditem_cou_ix 9 tbl1.courseid 1 Using where 
8 DEPENDENT SUBQUERY gg eq_ref mdl_gradgrad_useite_uix,mdl_gradgrad_ite_ix,mdl_gradgrad_use_ix mdl_gradgrad_useite_uix 16 tbl1.userid,moodle.gi.id 1 Using where 
7 DEPENDENT SUBQUERY mdl_scorm_scoes_track ref mdl_scorscoetrac_usescosco_uix,mdl_scorscoetrac_use_ix,mdl_scorscoetrac_sco_ix mdl_scorscoetrac_usescosco_uix 16 tbl1.userid,tbl2.instance 6 Using where; Using index 
6 DEPENDENT SUBQUERY mdl_quiz_attempts ref mdl_quizatte_use_ix,mdl_quizatte_qui_ix mdl_quizatte_qui_ix 8 tbl2.instance 2 Using where 
5 DEPENDENT SUBQUERY mdl_scorm eq_ref PRIMARY,mdl_scor_cou_ix PRIMARY 8 tbl2.instance 1 Using where 
4 DEPENDENT SUBQUERY mdl_quiz eq_ref PRIMARY,mdl_quiz_cou_ix PRIMARY 8 tbl2.instance 1 Using where 
2 DEPENDENT SUBQUERY mdl_scorm_logs eq_ref PRIMARY PRIMARY 16 func,func 1 Using where 

任何想法来优化呢？ 谢谢。

Answer 1

是的。将相关子查询转换为外连接。您应该可以通过查询本网站和谷歌的条款来获得良好的开端。

同样的“派生表”，我想象。

语法

SELECT ... FROM（SELECT FROM ...）可能有问题。

我可能不了解上下文，但反规范化在我看来似乎是一个非常不成熟的选择。

Answer 2

OMG。非规范化是处理这种情况的唯一方法。好的，你会优化这个查询。下一次你将做什么，你将不得不在单个查询中加入15个表格？

您的查询看起来像报告。您应该设计数据库以进行报告。看看下面的链接，它可能对你有用。

http://en.wikipedia.org/wiki/Denormalization

http://en.wikipedia.org/wiki/Star_schema