mysql - 缓慢的 UNION 查询 - MySQL

Question

我有一个由两个快速查询组成的 UNION 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' )

第一个查询在 0.0047 秒内运行 第二次运行在 0.0043 秒内

但是，在 Union 中，他们运行 0.27 秒……这是为什么？在 UNION 之后没有 Order By，为什么 MySQL 不简单地采用两个快速查询并将它们连接起来？

Answer 1

UNION 会导致创建一个临时表，即使对于 UNION ALL 也是如此。

当执行 UNION DISTINCT(与 UNION 相同)时，将创建带有索引的临时表，以便删除重复项。使用 UNION ALL，创建了临时表，但没有索引。

这解释了使用 UNION ALL 时性能略有提高，也解释了使用 UNION 而不是两个单独的查询时性能下降的原因。

有关这方面的更多信息，请参阅 MySQL performance blog 上的以下条目:

UNION vs UNION ALL Performance

How MySQL Uses Internal Temporary Tables MySQL 文档中的页面指出在以下情况下会创建一个临时表:

... any column larger than 512 bytes in the SELECT list, if UNION or UNION ALL is used