我创建了一个带有多个连接的查询，用于从多个表中获取数据。对于 5000 多条记录，它的工作速度非常慢。我已经检查并为查询中使用的每个表正确设置了索引。我尝试使用多个选项优化此查询，但它不起作用，我不完全理解 EXPLAIN 或 EXPLAIN EXTENDED 帮助。
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询问
EXPLAIN EXTENDED
SELECT @rownum := @rownum + 1 AS id,
u.id AS userid,
u.username AS employeeid,
u.firstname,
u.lastname,
u.email AS email,
u.city AS state,
c.fullname AS course,
c.id AS courseid,
c.fullname AS coursename,
fi2.data branchid,
fi3.data branchname,
fi6.data areaname,
fi7.data regname,
fi8.data designation,
fi9.data department,
fi10.data zone,
fi11.data branchcategory,
fi12.data branchdistrict,
fi13.data branchstate,
fi17.data gender,
fi21.data employeecategory,
fi22.data persontype,
fi23.data assignmentstatus,
fi30.data POSITION,
fi32.data dateofjoining,
fi33.data dateofbirth,
st.scoid,
st.scormid,
st.attempt,
st.value cmistarttime,
st5.value cmilessonstatus,
st5.timemodified cmitimemodified,
st3.value cmitotaltime,
st2.value cmiscore
FROM (
SELECT @rownum := 0
) r,
mdl_user u
JOIN mdl_role_assignments ra ON ra.userid = u.id
JOIN mdl_context ctx ON ctx.id = ra.contextid
JOIN mdl_course c ON c.id = ctx.instanceid
JOIN mdl_scorm s ON s.course = c.id
JOIN mdl_scorm_scoes_track st ON st.scormid = s.id AND st.userid = u.id
JOIN mdl_scorm_scoes_track AS st2 ON (u.id = st2.userid AND st2.scormid
= s.id AND st2.scoid = st.scoid AND st2.attempt = st.attempt)
JOIN mdl_scorm_scoes_track AS st3 ON (u.id = st3.userid AND st3.scormid
= s.id AND st3.scoid = st.scoid AND st3.attempt = st.attempt)
JOIN mdl_scorm_scoes_track AS st5 ON (u.id = st5.userid AND st5.scormid
= s.id AND st5.scoid = st.scoid AND st5.attempt = st.attempt)
JOIN mdl_user_info_data AS fi2 ON u.id = fi2.userid
JOIN mdl_user_info_data AS fi3 ON u.id = fi3.userid
JOIN mdl_user_info_data AS fi6 ON u.id = fi6.userid
JOIN mdl_user_info_data AS fi7 ON u.id = fi7.userid
JOIN mdl_user_info_data AS fi8 ON u.id = fi8.userid
JOIN mdl_user_info_data AS fi9 ON u.id = fi9.userid
JOIN mdl_user_info_data AS fi10 ON u.id = fi10.userid
JOIN mdl_user_info_data AS fi11 ON u.id = fi11.userid
JOIN mdl_user_info_data AS fi12 ON u.id = fi12.userid
JOIN mdl_user_info_data AS fi13 ON u.id = fi13.userid
JOIN mdl_user_info_data AS fi17 ON u.id = fi17.userid
JOIN mdl_user_info_data AS fi21 ON u.id = fi21.userid
JOIN mdl_user_info_data AS fi22 ON u.id = fi22.userid
JOIN mdl_user_info_data AS fi23 ON u.id = fi23.userid
JOIN mdl_user_info_data AS fi30 ON u.id = fi30.userid
JOIN mdl_user_info_data AS fi32 ON u.id = fi32.userid
JOIN mdl_user_info_data AS fi33 ON u.id = fi33.userid
WHERE c.id > 0 AND
u.id > 0 AND
u.deleted = 0 AND
u.suspended = 0 AND
u.confirmed = 1 AND
u.id <= 1000 AND
c.visible = 1 AND
st.element LIKE '%x.start.time%' AND
st2.element LIKE '%cmi.core.score.raw%' AND
st3.element LIKE '%cmi.core.total_time%' AND
st5.element LIKE '%cmi.core.lesson_status%' AND
fi2.fieldid = 2 AND
fi3.fieldid = 3 AND
fi6.fieldid = 6 AND
fi7.fieldid = 7 AND
fi8.fieldid = 8 AND
fi9.fieldid = 9 AND
fi10.fieldid = 10 AND
fi11.fieldid = 11 AND
fi12.fieldid = 12 AND
fi13.fieldid = 13 AND
fi17.fieldid = 17 AND
fi21.fieldid = 21 AND
fi22.fieldid = 22 AND
fi23.fieldid = 23 AND
fi30.fieldid = 30 AND
fi32.fieldid = 32 AND
fi33.fieldid = 33 AND
ra.roleid = 5 AND
ctx.contextlevel = 50
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