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
-
eeeeeeeeeeee
format:
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.
The
first three numbers
are generated from a timestamp.
The
fourth number
preserves temporal uniqueness in case the timestamp value loses monotonicity (for example, due to daylight saving time).
由于夏令时导致的
The
fifth number
is 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 have
very
low 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 the
UUID()
function).
The value of
UUID_SHORT()
is guaranteed to be unique if the following conditions hold:
The
UUID_SHORT()
return value is constructed this way:
(server_id & 255) <
+ (server_startup_time_in_seconds <
+ incremented_variable++;
mysql> SELECT UUID_SHORT(); -> 92395783831158784
Note that
UUID_SHORT()
does not work with statement-based replication.
The
server_id
of the current host is unique among your set of master and slave servers
server_id
is between 0 and 255
You do not set back your system time for your server between
mysqld
restarts
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 invoke
UUID_SHORT()
on average more than 16 million times per second between
mysqld
restarts
In an
INSERT ... ON DUPLICATE KEY UPDATE
statement, you can use the VALUES(
col_name
) function in the
UPDATE
clause to refer to column values from the
INSERT
portion of the statement. In other words, VALUES(
col_name
) in the
UPDATE
clause refers to the value of
col_name
that would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The
VALUES()
function is meaningful only in the ON DUPLICATE KEY UPDATE clause of
INSERT
statements and returns NULL otherwise. See
Section 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 c=VALUES(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，
