Question

我有一个UNION查询，包含两个快速查询。

( SELECT DISTINCT ( SELECT strStatus FROM User_User_XR uuxr WHERE 
( uuxr.intUserId1 = '1' AND uuxr.intUserId2 = u.intUserId ) ) AS strFriendStatus1,
 uuxro.strStatus AS strFriendStatus2, uuxr.intUserId2 AS intUserId, u.strUserName ,
 u.strGender, IF( u.dtmBirth != '0000-00-00', FLOOR(DATEDIFF(CURDATE(), 
u.dtmBirth) / 365.25) , '?') AS intAge, u.strCountry AS strCountryCode,
 c.strCountry AS strCountry, u.strAvatar, u.fltPoints, 
IF( o.intUserId IS NULL, 'offline', 'online' ) AS strOnline, 
IF ( u.strAvatar != '', CONCAT( 'avatars/60/', u.strAvatar ), 
CONCAT( 'images/avatar_', u.strGender, '_small.png' ) ) as strAvatar,     
IF ( u.strAvatar != '', CONCAT( 'avatars/150/', u.strAvatar ),     
CONCAT( 'images/avatar_', u.strGender, '.png' )) as strLargeAvatar,
 u.dtmLastLogin, u.dtmRegistered FROM User_User_XR uuxr, 
User u LEFT JOIN User_User_XR uuxro ON uuxro.intUserId2 = '1' 
AND uuxro.intUserId1 = u.intUserId
 LEFT JOIN Online o ON o.intUserId = u.intUserId 
LEFT JOIN Country c ON c.strCountryCode = u.strCountry 
WHERE u.intUserId = uuxr.intUserId2 AND ( uuxr.strStatus = 'confirmed' ) 
AND uuxr.intUserId1='1' ) 

UNION 

( SELECT DISTINCT ( SELECT strStatus FROM User_User_XR uuxr 
WHERE ( uuxr.intUserId1 = '1' AND uuxr.intUserId2 = u.intUserId ) ) AS strFriendStatus1,
 uuxro.strStatus AS strFriendStatus2, uuxr.intUserId1 AS intUserId, u.strUserName , 
u.strGender, IF( u.dtmBirth != '0000-00-00', FLOOR(DATEDIFF(CURDATE(),
 u.dtmBirth) / 365.25) , '?') AS intAge,
 u.strCountry AS strCountryCode, c.strCountry AS strCountry, u.strAvatar, u.fltPoints, 
IF( o.intUserId IS NULL, 'offline', 'online' ) AS strOnline, 
IF ( u.strAvatar != '', CONCAT( 'avatars/60/', u.strAvatar ), 
CONCAT( 'images/avatar_', u.strGender, '_small.png' ) ) as strAvatar,
 IF ( u.strAvatar != '', CONCAT( 'avatars/150/', u.strAvatar ), 
CONCAT( 'images/avatar_', u.strGender, '.png' )) as strLargeAvatar, 
u.dtmLastLogin, u.dtmRegistered FROM User_User_XR uuxr, User u 
LEFT JOIN User_User_XR uuxro ON uuxro.intUserId2 = '1' 
AND uuxro.intUserId1 = u.intUserId 
LEFT JOIN Online o ON o.intUserId = u.intUserId 
LEFT JOIN Country c ON c.strCountryCode = u.strCountry 
WHERE u.intUserId = uuxr.intUserId1 AND ( uuxr.strStatus = 'confirmed' )
 AND uuxr.intUserId2='1' )

第一个查询以0.0047s运行第二次以0.0043s

运行

然而，在联盟，他们运行0.27s ...为什么这？在UNION之后没有Order By，为什么MySQL不会简单地采用两个快速查询并将它们连接起来？

Answer 1

UNION会导致创建临时表，即使是UNION ALL也是如此。

当执行UNION DISTINCT（与UNION相同）时，将使用索引创建临时表，以便可以删除重复项。使用UNION ALL，将创建临时表，但不包含索引。

这解释了使用UNION ALL时性能略有提升，并且还说明了使用UNION而不是两个单独查询时的性能下降。

有关此问题的详细信息，请参阅MySQL performance blog上的以下条目：

UNION vs UNION ALL Performance

MySQL文档中的How MySQL Uses Internal Temporary Tables页面指出在以下情况下创建临时表：

...任何大于512的列 SELECT列表中的字节，如果是UNION或使用UNION ALL

Answer 2

尝试使用UNION ALL。

UNION本身将删除任何重复的记录，这意味着幕后的排序。

慢UNION查询 - MySQL

2 个答案: