1.一个事务，就会给一个gtid编号。来看看例子：
mysql> show master status;
+---------------+----------+--------------+------------------+------------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+---------------+----------+--------------+------------------+------------------------------------------------+
| binlog.000017 | 58105 | | | b526042e-89a7-11e8-bddb-000c29d7d637:1-1304086 |
+---------------+----------+--------------+------------------+------------------------------------------------+
1 row in set (0.00 sec)
mysql> insert into student(id) values(5);
Query OK, 1 row affected (0.01 sec)
mysql> show master status;
+---------------+----------+--------------+------------------+------------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+---------------+----------+--------------+------------------+------------------------------------------------+
| binlog.000017 | 58432 | | | b526042e-89a7-11e8-bddb-000c29d7d637:1-1304087 |
+---------------+----------+--------------+------------------+------------------------------------------------+
1 row in set (0.00 sec)
mysql> begin
-> ;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into student(id) values(6);
Query OK, 1 row affected (0.00 sec)
mysql> insert into student(id) values(7);
Query OK, 1 row affected (0.00 sec)
mysql> commit;
Query OK, 0 rows affected (0.01 sec)
mysql> show master status;
+---------------+----------+--------------+------------------+------------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+---------------+----------+--------------+------------------+------------------------------------------------+
| binlog.000017 | 58918 | | | b526042e-89a7-11e8-bddb-000c29d7d637:1-1304088 |
+---------------+----------+--------------+------------------+------------------------------------------------+
1 row in set (0.00 sec)
mysql> begin ;
Query OK, 0 rows affected (0.00 sec)
mysql> insert into student(id) values(8);
Query OK, 1 row affected (0.00 sec)
mysql> rollback;
Query OK, 0 rows affected (0.00 sec)
mysql> show master status;
+---------------+----------+--------------+------------------+------------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+---------------+----------+--------------+------------------+------------------------------------------------+
| binlog.000017 | 58918 | | | b526042e-89a7-11e8-bddb-000c29d7d637:1-1304088 |
+---------------+----------+--------------+------------------+------------------------------------------------+
1 row in set (0.00 sec)
mysql> quit
Bye
[root@yw-gz-hd-test-211 ~]# cat /data/mysql3308/auto.cnf
[auto]
server-uuid=b526042e-89a7-11e8-bddb-000c29d7d637
仔细看看上面的例子，当前gtid：b526042e-89a7-11e8-bddb-000c29d7d637:1304086，然后我插入一条数据，执行一个事务，gtid加1，变成b526042e-89a7-11e8-bddb-000c29d7d637:1304087，然后我用begin commit,标志另外一个事务，插入2条数据，gtid加1，变成b526042e-89a7-11e8-bddb-000c29d7d637:1304088，然后我rollback，gtid不加1.
到这里，我们停下来，思考一下。。。。。。。。。。。
gtid是什么东东？明白了吧，有所领悟了吧。
2.为什么说gtid能避免重复执行，避免丢数据呢？
来看下下面这个例子：
mysql> SET @@SESSION.GTID_NEXT= 'b526042e-89a7-11e8-bddb-000c29d7d637:1304076';
Query OK, 0 rows affected (0.00 sec)
mysql> select * from student where id=4;
+----+------+------+---------+---------+---------------------+---------------------+
| id | name | age | textcol | blobcol | birday | birday2 |
+----+------+------+---------+---------+---------------------+---------------------+
| 4 | 3 | 3 | 3 | NULL | 2018-07-25 10:42:29 | 2018-07-25 10:42:33 |
+----+------+------+---------+---------+---------------------+---------------------+
1 row in set (0.00 sec)
mysql> CREATE TABLE `student` (
-> `id` int NOT NULL AUTO_INCREMENT ,
-> `name` varchar(255) NULL ,
-> `age` int NULL ,
-> `textcol` text NULL ,
-> `blobcol` blob NULL ,
-> `birday` timestamp NULL ON UPDATE CURRENT_TIMESTAMP ,
-> `birday2` datetime NULL ON UPDATE CURRENT_TIMESTAMP ,
-> PRIMARY KEY (`id`)
-> )
-> ;
Query OK, 0 rows affected (0.00 sec)
mysql> show master status;
+---------------+----------+--------------+------------------+------------------------------------------------+
| File | Position | Binlog_Do_DB | Binlog_Ignore_DB | Executed_Gtid_Set |
+---------------+----------+--------------+------------------+------------------------------------------------+
| binlog.000017 | 58105 | | | b526042e-89a7-11e8-bddb-000c29d7d637:1-1304086 |
+---------------+----------+--------------+------------------+------------------------------------------------+
1 row in set (0.00 sec)
mysql> insert into studnet(id) values(5);
ERROR 1837 (HY000): When @@SESSION.GTID_NEXT is set to a GTID, you must explicitly set it to a different value after a COMMIT or ROLLBACK. Please check GTID_NEXT variable manual page for detailed explanation. Current @@SESSION.GTID_NEXT is 'b526042e-89a7-11e8-bddb-000c29d7d637:1304076'.
这个信息量有点大。一点点理解，首先我
SET @@SESSION.GTID_NEXT= 'b526042e-89a7-11e8-bddb-000c29d7d637:1304076'
但是当前gtid是b526042e-89a7-11e8-bddb-000c29d7d637:1-1304086 。什么意思，就是说1304076 事务包含在1-1304086。也就是说，这个事务已经执行了，不需要再执行了。理解了吗？
思考一下。。。。。。。。。
这个理解通了，你就会发现，为什么我发布了一个create table 命令，查询返回Query OK, 0 rows affected (0.00 sec)
就是什么作用也没有。意思就是说，这个命令create table 我已经执行过了，所以不需要执行。
再来看看错误：ERROR 1837 (HY000): When @@SESSION.GTID_NEXT is set to a GTID, you must explicitly set it to a different value after a COMMIT or ROLLBACK. Please check GTID_NEXT variable manual page for detailed explanation. Current @@SESSION.GTID_NEXT is 'b526042e-89a7-11e8-bddb-000c29d7d637:1304076'.
意思就是说，你每执行一个命令，都必须先执行 set @@SESSION.GTID_NEXT = xxxxx。
为什么会这样呢？
思考一下。。。。。。
