MySQL Order By 减慢连接速度

Posted 2023-02-24

【中文标题】MySQL Order By 减慢连接速度

【英文标题】：MySQL Order By Slows down Joins

【发布时间】：2012-08-17 16:24:08

【问题描述】：

is 查询有问题

SELECT *
FROM files f
LEFT JOIN xfiles x ON x.hash = f.bhash
LEFT JOIN cate c ON c.id = f.cate
LEFT JOIN users u ON u.id = f.user
LEFT JOIN userslev ul ON u.id_lev = ul.id
LEFT JOIN (

SELECT DISTINCT x.hash, s.iis
FROM anno x
JOIN seb s ON s.iis = x.icc
WHERE x.le =0
AND (
x.eve =0
OR x.eve =2
)
GROUP BY  `x`.`hash`
) AS tabel2 ON tabel2.hash = f.bhash
WHERE f.see + IFNULL( x.see, 0 ) + f.lee + IFNULL( x.lee, 0 ) >0
ORDER BY f.added DESC

显示第 0 - 29 行（总共 22,083 行，查询耗时 9.6523 秒）

如您所见，需要 9 秒！

但没有'order by'

SELECT *
FROM files f
LEFT JOIN xfiles x ON x.hash = f.bhash
LEFT JOIN cate c ON c.id = f.cate
LEFT JOIN users u ON u.id = f.user
LEFT JOIN userslev ul ON u.id_lev = ul.id
LEFT JOIN (

SELECT DISTINCT x.hash, s.iis
FROM anno x
JOIN seb s ON s.iis = x.icc
WHERE x.le =0
AND (
x.eve =0
OR x.eve =2
)
GROUP BY  `x`.`hash`
) AS tabel2 ON tabel2.hash = f.bhash
WHERE f.see + IFNULL( x.see, 0 ) + f.lee + IFNULL( x.lee, 0 ) >0

显示第 0 - 29 行（总共 22,083 行，查询耗时 0.0241 秒）

只有 0.0241 秒 :)

............

有没有办法让“订购”与美好时光？

............

索引：

Indexes: (files)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  ihash   27815   A       
 Edit    Drop   filename    BTREE   No  No  filename    27815   A       
 Edit    Drop   cate    BTREE   No  No  cate    45  A       
 Edit    Drop   user    BTREE   No  No  user    1264    A       
 Edit    Drop   bhash   BTREE   No  No  bhash (20)  27815   A       
 Edit    Drop   added   BTREE   No  No  added   27815   A       
 Edit    Drop   filename_2  FULLTEXT    No  No  filename    1           



Indexes: (xfiles)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  id  8196    A       
 Edit    Drop   hash    BTREE   Yes No  hash    8196    A       
 Edit    Drop   see BTREE   No  No  see 37  A       
 Edit    Drop   leec    BTREE   No  No  lee25   A   


Indexes: (cate)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  id  51  A   


Indexes: (users)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  id  39776   A       
 Edit    Drop   username    BTREE   Yes No  username    39776   A       
 Edit    Drop   id_lev  BTREE   No  No  id_lev  19  A       


Indexes: (userslev)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   base    BTREE   Yes No  id  22  A       


Indexes: (anno)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  id  23636   A       
 Edit    Drop   icc BTREE   No  No  icc 2626    A       
 Edit    Drop   eve BTREE   No  No  eve 8   A       
 Edit    Drop   hash    BTREE   No  No  hash    5909    A       
 Edit    Drop   le  BTREE   No  No  le  875 A   


Indexes: (seb)

Action  Keyname Type    Unique  Packed  Column  Cardinality Collation   Null    Comment
 Edit    Drop   PRIMARY BTREE   Yes No  id  58  A       
 Edit    Drop   iis BTREE   Yes No  iis 58  A   YES 

解释：

id          select_type     table       type            possible_keys       key         key_len         ref         rows    Extra
1               PRIMARY     f           ALL             NULL                NULL        NULL            NULL        27819   Using temporary; Using filesort
1               PRIMARY     x           eq_ref          hash                hash        20              f.bhash     1       Using where
1               PRIMARY     c           eq_ref          PRIMARY             PRIMARY     4               f.cate      1   
1               PRIMARY     u           eq_ref          PRIMARY             PRIMARY     4               f.user      1   
1               PRIMARY     ul          eq_ref          base                base        4               u.id_lev    1   
1               PRIMARY     <derived2>  ALL             NULL                NULL        NULL            NULL        1176    
2               DERIVED     s           index           iis                 iis         5               NULL        58      Using index; Using temporary; Using filesort
2               DERIVED     x           ref             icc,eve,le          icc         4               s.iis       9       Using where

【问题讨论】：

您尝试索引f.added 列吗？

@raina77ow 已编入索引，但我忘记放了

也许索引按 ASC 排序，而您需要对结果进行 DESC 排序

尝试用WHERE (f.see>0 OR x.see>0 OR f.lee>0 OR x.lee>0)替换WHERE f.see + IFNULL( x.see, 0 ) + f.lee + IFNULL( x.lee, 0 ) >0

@Giuseppe 尝试了 ASC，没有变化

【参考方案1】：

这里有很多关于优化订单的信息： http://dev.mysql.com/doc/refman/5.0/en/order-by-optimization.html

您能否发布这两个解释，以便我们了解查询计划的不同之处？