我有以下问题。 MySQL SHOW ENGINE INNODB状态告诉我，以下两个查询导致死锁：
UPDATE xf_user SET conversations_unread = '1' WHERE (user_id = 209495) UPDATE xf_user SET last_activity = '1431370106' WHERE (user_id = 417599) 这怎么可能？两者都是UPDATE查询，并且在WHERE中都具有单个主键相等条件。桌子上没有约束。
SHOW CREATE TABLE xf_user：
xf_user CREATE TABLE `xf_user` (
`user_id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`username` varchar(50) NOT NULL,
`email` varchar(120) NOT NULL,
`gender` enum('','male','female') NOT NULL DEFAULT '' COMMENT 'Leave empty for ''unspecified''',
`custom_title` varchar(50) NOT NULL DEFAULT '',
`language_id` int(10) unsigned NOT NULL,
`style_id` int(10) unsigned NOT NULL COMMENT '0 = use system default',
`timezone` varchar(50) NOT NULL COMMENT 'Example: ''Europe/London''',
`visible` tinyint(3) unsigned NOT NULL DEFAULT '1' COMMENT 'Show browsing activity to others',
`user_group_id` int(10) unsigned NOT NULL,
`secondary_group_ids` varbinary(255) NOT NULL,
`display_style_group_id` int(10) unsigned NOT NULL DEFAULT '0' COMMENT 'User group ID that provides user styling',
`permission_combination_id` int(10) unsigned NOT NULL,
`message_count` int(10) unsigned NOT NULL DEFAULT '0',
`conversations_unread` smallint(5) unsigned NOT NULL DEFAULT '0',
`register_date` int(10) unsigned NOT NULL DEFAULT '0',
`last_activity` int(10) unsigned NOT NULL DEFAULT '0',
`trophy_points` int(10) unsigned NOT NULL DEFAULT '0',
`alerts_unread` smallint(5) unsigned NOT NULL DEFAULT '0',
`avatar_date` int(10) unsigned NOT NULL DEFAULT '0',
`avatar_width` smallint(5) unsigned NOT NULL DEFAULT '0',
`avatar_height` smallint(5) unsigned NOT NULL DEFAULT '0',
`gravatar` varchar(120) NOT NULL DEFAULT '' COMMENT 'If specified, this is an email address corresponding to the user''s ''Gravatar''',
`user_state` enum('valid','email_confirm','email_confirm_edit','moderated','email_bounce') NOT NULL DEFAULT 'valid',
`is_moderator` tinyint(3) unsigned NOT NULL DEFAULT '0',
`is_admin` tinyint(3) unsigned NOT NULL DEFAULT '0',
`is_banned` tinyint(3) unsigned NOT NULL DEFAULT '0',
`like_count` int(10) unsigned NOT NULL DEFAULT '0',
`warning_points` int(10) unsigned NOT NULL DEFAULT '0',
`members_page` int(10) unsigned NOT NULL DEFAULT '0',
`stock_last_login` int(10) unsigned NOT NULL DEFAULT '0',
`stock_current_login` int(10) unsigned NOT NULL DEFAULT '0',
`stock_profile_validated` tinyint(1) unsigned NOT NULL DEFAULT '0',
`is_staff` tinyint(3) unsigned NOT NULL DEFAULT '0',
`is_consultant` tinyint(3) unsigned NOT NULL DEFAULT '0',
`fh_modified` int(11) NOT NULL DEFAULT '0',
`activity_visible` tinyint(3) unsigned NOT NULL DEFAULT '1',
PRIMARY KEY (`user_id`),
UNIQUE KEY `username` (`username`),
KEY `email` (`email`),
KEY `user_state` (`user_state`),
KEY `last_activity` (`last_activity`),
KEY `members_page` (`members_page`),
KEY `message_count` (`message_count`),
KEY `trophy_points` (`trophy_points`),
KEY `like_count` (`like_count`),
KEY `register_date` (`register_date`),
KEY `staff_username` (`is_staff`,`username`),
KEY `is_moderator` (`is_moderator`)
) ENGINE=InnoDB AUTO_INCREMENT=1605433 DEFAULT CHARSET=utf8
SHOW ENGINE INNODB状态摘录：
------------------------
LATEST DETECTED DEADLOCK
------------------------
2015-05-11 22:10:14 7f4240cd0700
*** (1) TRANSACTION:
TRANSACTION 3310570290, ACTIVE 1 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 21 lock struct(s), heap size 2936, 24 row lock(s), undo log entries 31
MySQL thread id 17046111, OS thread handle 0x7f42415f0700, query id 663191550 192.168.100.19 xxxxxx updating
UPDATE `xf_user` SET `conversations_unread` = '1' WHERE (user_id = 209495)
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 708 page no 290 n bits 176 index `PRIMARY` of table `xxxxxx`.`xf_user` trx id 3310570290 lock_mode X locks rec but not gap waiting
*** (2) TRANSACTION:
TRANSACTION 3310570063, ACTIVE 1 sec starting index read
mysql tables in use 1, locked 1
297 lock struct(s), heap size 30248, 986 row lock(s), undo log entries 985
MySQL thread id 17046068, OS thread handle 0x7f4240cd0700, query id 663193617 192.168.100.19 xxxxxx updating
UPDATE `xf_user` SET `last_activity` = '1431370106' WHERE (user_id = 417599)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 708 page no 290 n bits 176 index `PRIMARY` of table `xxxxxx`.`xf_user` trx id 3310570063 lock_mode X locks rec but not gap
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 708 page no 1037 n bits 176 index `PRIMARY` of table `xxxxxx`.`xf_user` trx id 3310570063 lock_mode X locks rec but not gap waiting
*** WE ROLL BACK TRANSACTION (1)
MySQL版本为mysql Ver 15.1 Distrib 10.0.15-MariaDB, for Linux (x86_64) using readline 5.1
两个查询被解析为：
1 SIMPLE xf_user range PRIMARY PRIMARY 4 NULL 1 Using where
我不能在这里发布一个完整的交易，因为这不是我的代码，导致死锁，而是安装了一千个不同插件的XenForo论坛引擎。我稍后会尝试做一些倾销。
事务隔离级别为READ COMMITTED。