南京移动网站建设哪里强,网站开发和网站维护有区别吗,免费的php网站模板,我的百度账号登录Open Group于1997年10月发布#xff0c;UUID遵从此协议。UUID被设计成一个在空间和时间上的唯一值。两次调用的UUID将产生两个不同的值#xff0c;即使这些调用是在两个不连接的#xff0c;彼此独立的计算机。由一串数字表示aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeeeformat:mys…Open Group于1997年10月发布UUID遵从此协议。UUID被设计成一个在空间和时间上的唯一值。两次调用的UUID将产生两个不同的值即使这些调用是在两个不连接的彼此独立的计算机。由一串数字表示aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeeeformat:mysql SELECT UUID(); - 6ccd780c-baba-1026-9564-0040f4311e29虽然UUID()值是唯一的但它们不一定是不可猜测的或不可预测的。如果需要不可预测性UUID值应该以其他方式生成。UUID不基于statement replication.Thefirst three numbersare generated from a timestamp.Thefourth numberpreserves temporal uniqueness in case the timestamp value loses monotonicity (for example, due to daylight saving time).由于夏令时导致的Thefifth numberis an IEEE 802 node number that provides spatial uniqueness. A random number is substituted if the latter is not available (for example, because the host computer has no Ethernet card, or we do not know how to find the hardware address of an interface on your operating system). In this case, spatial uniqueness cannot be guaranteed. Nevertheless, a collision should haveverylow probability.第五个数字是它提供空间唯一一个IEEE802节点号。The MAC address of an interface is taken into account only on FreeBSD and Linux. On other operating systems, MySQL uses a randomly generated 48-bit number.uuid我在百度百科上也看到了... ...第1 2 3段是与时间有关的。time_low、time_mid、time_high_and_version转成16进制后分别对应第1 2 3段。这个时间是从1582-10-15 00:00:00.00到当前时间的100ns值。(实际上系统只能取到精确us再乘以10)。所以你短时间连续执行的话比较可能只有第一个值在改实际上1 2 3都可能会改变。第4段是你启动这个MySQL后第一次执行select uuid()时的随机数每次重启会改变。第5段是mac值转过来的同一个机器多实例的一般相同。如果mac值获取不到则是一个随机值。所以这个值可以认为是每次执行都不相同。并且不同实例之间也只有极微小概率重复。Returns a“short”universal identifier as a 64-bit unsigned integer (rather than a string-form 128-bit identifier as returned by theUUID()function).The value ofUUID_SHORT()is guaranteed to be unique if the following conditions hold:TheUUID_SHORT()return value is constructed this way:(server_id 255) (server_startup_time_in_seconds incremented_variable;mysql SELECT UUID_SHORT(); - 92395783831158784Note thatUUID_SHORT()does not work with statement-based replication.Theserver_idof the current host is unique among your set of master and slave serversserver_idis between 0 and 255You do not set back your system time for your server betweenmysqldrestarts与uuid返回固定长度字符串不同uuid_short的返回值是一个unsigned long long类型。MySQL启动后第一次执行的值是通过server_id 56 server_start_time 24来初始化。server_start_time单位是秒。之后每次执行都加1。由于每次加1都会加全局mutex锁因此多线程安全可以当作sequence来用只是初始值有点大。You do not invokeUUID_SHORT()on average more than 16 million times per second betweenmysqldrestartsIn anINSERT ... ON DUPLICATE KEY UPDATEstatement, you can use the VALUES(col_name) function in theUPDATEclause to refer to column values from theINSERTportion of the statement. In other words, VALUES(col_name) in theUPDATEclause refers to the value ofcol_namethat would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. TheVALUES()function is meaningful only in the ON DUPLICATE KEY UPDATE clause ofINSERTstatements and returns NULL otherwise. SeeSection 13.2.5.3, “INSERT ... ON DUPLICATE KEY UPDATE Syntax”.mysql INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6) - ON DUPLICATE KEY UPDATE cVALUES(a)VALUES(b);当开启gitd时binlog中会多出gtid event扫描binlog就会知道GTID对应的filename和position主从日志落后很多就会扫描很多binlog日志。Previous_gtids表示这之前的binlog运行到的GTID的位置。GTID比当前值到就去扫这个binlog比这个小就去扫描之前的binlog日志。说到切换有的时候主库恢复后可能从库还没有应用完虽然我们事先准备好了切换脚本但是到这时候我们还是很犹豫的经历过后会深有感触。金融行业不允许切半同步
