1.一个事务，就会给一个gtid编号。来看看例子：

mysql>showmasterstatus;+---------------+----------+--------------+------------------+------------------------------------------------+|File|Position|Binlog_Do_DB|Binlog_Ignore_DB|Executed_Gtid_Set|+---------------+----------+--------------+------------------+------------------------------------------------+|binlog.000017|58105|||b526042e-89a7-11e8-bddb-000c29d7d637:1-1304086|+---------------+----------+--------------+------------------+------------------------------------------------+1rowinset(0.00sec)mysql>insertintostudent(id)values(5);QueryOK,1rowaffected(0.01sec)mysql>showmasterstatus;+---------------+----------+--------------+------------------+------------------------------------------------+|File|Position|Binlog_Do_DB|Binlog_Ignore_DB|Executed_Gtid_Set|+---------------+----------+--------------+------------------+------------------------------------------------+|binlog.000017|58432|||b526042e-89a7-11e8-bddb-000c29d7d637:1-1304087|+---------------+----------+--------------+------------------+------------------------------------------------+1rowinset(0.00sec)mysql>begin->;QueryOK,0rowsaffected(0.00sec)mysql>insertintostudent(id)values(6);QueryOK,1rowaffected(0.00sec)mysql>insertintostudent(id)values(7);QueryOK,1rowaffected(0.00sec)mysql>commit;QueryOK,0rowsaffected(0.01sec)mysql>showmasterstatus;+---------------+----------+--------------+------------------+------------------------------------------------+|File|Position|Binlog_Do_DB|Binlog_Ignore_DB|Executed_Gtid_Set|+---------------+----------+--------------+------------------+------------------------------------------------+|binlog.000017|58918|||b526042e-89a7-11e8-bddb-000c29d7d637:1-1304088|+---------------+----------+--------------+------------------+------------------------------------------------+1rowinset(0.00sec)mysql>begin;QueryOK,0rowsaffected(0.00sec)mysql>insertintostudent(id)values(8);QueryOK,1rowaffected(0.00sec)mysql>rollback;QueryOK,0rowsaffected(0.00sec)mysql>showmasterstatus;+---------------+----------+--------------+------------------+------------------------------------------------+|File|Position|Binlog_Do_DB|Binlog_Ignore_DB|Executed_Gtid_Set|+---------------+----------+--------------+------------------+------------------------------------------------+|binlog.000017|58918|||b526042e-89a7-11e8-bddb-000c29d7d637:1-1304088|+---------------+----------+--------------+------------------+------------------------------------------------+1rowinset(0.00sec)mysql>quitBye[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'/*!*/;QueryOK,0rowsaffected(0.00sec)mysql>select*fromstudentwhereid=4;+----+------+------+---------+---------+---------------------+---------------------+|id|name|age|textcol|blobcol|birday|birday2|+----+------+------+---------+---------+---------------------+---------------------+|4|3|3|3|NULL|2018-07-2510:42:29|2018-07-2510:42:33|+----+------+------+---------+---------+---------------------+---------------------+1rowinset(0.00sec)mysql>CREATETABLE`student`(->`id`intNOTNULLAUTO_INCREMENT,->`name`varchar(255)NULL,->`age`intNULL,->`textcol`textNULL,->`blobcol`blobNULL,->`birday`timestampNULLONUPDATECURRENT_TIMESTAMP,->`birday2`datetimeNULLONUPDATECURRENT_TIMESTAMP,->PRIMARYKEY(`id`)->)->;QueryOK,0rowsaffected(0.00sec)mysql>showmasterstatus;+---------------+----------+--------------+------------------+------------------------------------------------+|File|Position|Binlog_Do_DB|Binlog_Ignore_DB|Executed_Gtid_Set|+---------------+----------+--------------+------------------+------------------------------------------------+|binlog.000017|58105|||b526042e-89a7-11e8-bddb-000c29d7d637:1-1304086|+---------------+----------+--------------+------------------+------------------------------------------------+1rowinset(0.00sec)mysql>insertintostudnet(id)values(5);ERROR1837(HY000):When@@SESSION.GTID_NEXTissettoaGTID,youmustexplicitlysetittoadifferentvalueafteraCOMMITorROLLBACK.PleasecheckGTID_NEXTvariablemanualpagefordetailedexplanation.Current@@SESSION.GTID_NEXTis'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。。。。。。