什麼會導致連接比分爲兩個查詢慢？

Question

我正在執行JOIN從多個表執行分面搜索。當避免JOIN並將查詢分成兩個不同的查詢時，我注意到一個很大的性能提升，所以我假設我的JOIN沒有被優化。

結構：

-- 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> 

我試圖提取有標籤的所有用戶，按照search.number和其他表把元數據的順序。查詢：

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 

我注意到刪除JOIN到用戶表（並做了之後）使查詢快得多。我如何優化它以在相同的查詢中工作？我試着改變FROM到標籤，而不是搜索，但沒有奏效...

這是EXPLAIN顯示：

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 

EXPLAIN查詢的建議回答：

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 

Answer 1

創建上表的索引運行以下查詢：

Table ColumnName 
------ ---------- 
tags  user_id, skillid (Both column in one index) 

試試這個：

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 

編輯

嘗試用適當的指數這兩個查詢，讓我知道，下面的查詢是爲您或不工作。

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; 

OR

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;