mySql LEFT JOIN 优化 求帮我指出问题
每个LEFT JOIN 单个查询很快，但是整体运行查询6.7s（主要是最后一个LEFT JOIN 加上就会很慢）
下面是执行计划
SELECT a.user_id,a.user_name,a.name ,REPLACE(GROUP_CONCAT(DISTINCT b.class_name),',','、') AS v1
,REPLACE(GROUP_CONCAT(DISTINCT c.class_name),',','、') AS v2
,REPLACE(GROUP_CONCAT(DISTINCT d.class_subject_name),',','、') AS v3
FROM yf_user a
LEFT JOIN (
SELECT a.school_id,a.teacher_id1,a.class_id,a.class_name
FROM yf_class a
INNER JOIN yf_grade b
ON a.school_id=b.school_id AND a.grade_id=b.grade_id
WHERE a.school_id=1 AND b.grade_code<>'199' AND a.class_type<>'199' AND b.status=1 )b
ON a.school_id=b.school_id AND a.user_id=b.teacher_id1
LEFT JOIN (
SELECT a.school_id,a.teacher_id2,a.class_id,a.class_name
FROM yf_class a
INNER JOIN yf_grade b
ON a.school_id=b.school_id AND a.grade_id=b.grade_id
WHERE a.school_id=1 AND b.grade_code<>'199' AND a.class_type<>'199' AND b.status=1 )c
ON a.school_id=c.school_id AND a.user_id=c.teacher_id2
LEFT JOIN (
SELECT a.school_id,a.user_id,CONCAT(c.class_name,d.subject_name) AS class_subject_name
FROM yf_tea a
INNER JOIN yf_grade b
ON a.school_id=b.school_id AND a.grade_id=b.grade_id
INNER JOIN yf_class c
ON a.school_id=c.school_id AND a.class_id=c.class_id
INNER JOIN yf_subject d
ON a.school_id=d.school_id AND a.subject_id=d.subject_id
WHERE a.school_id=1 AND b.grade_code<>'199' AND c.class_type<>'199' AND b.status=1 ) d
ON a.school_id=d.school_id AND a.user_id=d.user_id
WHERE a.school_id=1 AND a.user_type=1
GROUP BY a.user_id HAVING (v1 IS NOT NULL OR v2 IS NOT NULL OR v3 IS NOT NULL)
ORDER BY a.sort ASC,a.user_name ASC,a.name ASC;
