mysql – 什么可以导致连接比分为两个查询慢？
作者：互联网
我正在从多个表执行JOIN以执行分面搜索.当避免JOIN并将查询分成两个不同的时候我注意到了一个很大的性能提升,所以我假设我的JOIN没有优化.
结构：
-- tags
userId | tagId
1 3
1 4
2 3
2 9
-- search
userId | number | countryId | stateId ...
1 13 221 55
-- countries
countryId | countryName
221 Somewhere
-- users
userId | profileImageLink
1 | <photo link>
我正在尝试提取所有具有标记的用户,根据search.number进行排序并从其他表中提取元数据.查询：
SELECT
search.*, users.a, users.b, users.c, users.d, users.e, users.f, countries.location_country, states.location_state, cities.location_city
FROM search
RIGHT JOIN tags
ON search.user_id = tags.user_id
LEFT JOIN users
ON users.user_id=search.user_id
LEFT JOIN countries
ON countries.countryId=search.countryId
LEFT JOIN states
ON states.countryId=search.countryId AND states.stateId=search.stateId
LEFT JOIN cities
ON cities.countryId=search.countryId AND cities.stateId=search.stateId AND cities.cityId=search.cityId
WHERE
tags.skillId =52772
ORDER BY
search.number DESC LIMIT 0,200
我注意到将JOIN删除到users表(之后这样做)使查询更快.如何优化它以在同一查询中工作？我已经尝试将FROM更改为标签而不是搜索,但这不起作用……
这是EXPLAIN显示的内容：
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE tags ref skill_user,skillId skill_user 4 const 184854 Using index; Using temporary; Using filesort
1 SIMPLE search eq_ref user_id user_id 4 tags.user_id 1
1 SIMPLE countries eq_ref PRIMARY PRIMARY 2 search.countryId 1
1 SIMPLE states eq_ref PRIMARY,state PRIMARY 3 search.stateId 1
1 SIMPLE cities eq_ref PRIMARY,city PRIMARY 3 search.cityId 1
1 SIMPLE users eq_ref user_id user_id 4 search.user_id 1
EXPLAIN without the LEFT JOIN users:
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE tags ref skill_user,skillId skill_user 4 const 155870 Using index
1 SIMPLE search eq_ref user_id user_id 4 tags.user_id 1
1 SIMPLE countries eq_ref PRIMARY PRIMARY 2 search.countryId 1
1 SIMPLE states eq_ref PRIMARY,state PRIMARY 3 search.stateId 1
1 SIMPLE cities eq_ref PRIMARY,city PRIMARY 3 search.cityId 1
在回答中建议查询的解释：
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE tags index NULL userid_skill 8 NULL 22689539 Using where; Using index; Using temporary; Using filesort
1 SIMPLE search eq_ref user_id user_id 4 tags.user_id 1
1 SIMPLE users eq_ref user_id user_id 4 search.user_id 1
1 SIMPLE countries eq_ref PRIMARY PRIMARY 2 search.countryId 1
1 SIMPLE states eq_ref PRIMARY,state PRIMARY 3 search.stateId 1
1 SIMPLE cities eq_ref PRIMARY,city PRIMARY 3 search.cityId 1
解决方法:
在表上创建索引以在查询下运行：
Table ColumnName
------ ----------
tags user_id, skillid (Both column in one index)
试试这个：
SELECT s.*, u.a, u.b, u.c, u.d, u.e, u.f, c.location_country,
st.location_state, ct.location_city
FROM tags t
LEFT JOIN search s ON t.user_id = s.user_id
LEFT JOIN users u ON t.user_id = u.user_id
LEFT JOIN countries c ON s.countryId = c.countryId
LEFT JOIN states st ON s.stateId = st.stateId
LEFT JOIN cities ci ON s.cityId= ct.cityId
WHERE t.skillId =52772
ORDER BY s.number DESC
LIMIT 0,200
编辑
尝试使用适当的索引这两个查询,让我知道以下查询是否适合您.
SELECT s.*, u.a, u.b, u.c, u.d, u.e, u.f, c.location_country,
st.location_state, ct.location_city
FROM (SELECT user_id FROM tags WHERE t.skillId = 52772) AS t
LEFT JOIN search s ON t.user_id = s.user_id
LEFT JOIN users u ON t.user_id = u.user_id
LEFT JOIN countries c ON s.countryId = c.countryId
LEFT JOIN states st ON s.stateId = st.stateId
LEFT JOIN cities ci ON s.cityId= ct.cityId
ORDER BY s.number DESC
LIMIT 0,200;
要么
SELECT s.*, u.a, u.b, u.c, u.d, u.e, u.f, c.location_country,
st.location_state, ct.location_city
FROM (SELECT t.user_id, s.* FROM tags t
LEFT JOIN search s ON t.user_id = s.user_id
WHERE t.skillId = 52772
ORDER BY s.number DESC
LIMIT 0,200) AS t
LEFT JOIN users u ON t.user_id = u.user_id
LEFT JOIN countries c ON s.countryId = c.countryId
LEFT JOIN states st ON s.stateId = st.stateId
LEFT JOIN cities ci ON s.cityId= ct.cityId
ORDER BY s.number DESC
LIMIT 0,200;
标签：mysql,sql,query-optimization,faceted-search 来源： https://codeday.me/bug/20190704/1372252.html