本文为以后学习SLAVE做一个记录不保证正确性。因为算法没看懂。
一、问题由来
今天朋友问我这样一个问题@K.I.S.S，在官方文档中有这样一段描述：
When using GTIDs, the slave tells the master which transactions it has already received, executed, or both. To compute this set, it reads
the global value of gtid_executed and the value of the Retrieved_gtid_set column from SHOW SLAVE STATUS. The GTID of the last transmitted
transaction is included in Retrieved_gtid_set only when the full transaction is received. The slave computes the following set:
UNION(@@global.gtid_executed, Retrieved_gtid_set)
Prior to MySQL 5.7.5, the GTID of the last transmitted transaction was included in Retrieved_gtid_set even if the transaction was only
partially transmitted, and the last received GTID was subtracted from this set. (Bug #17943188) Thus, the slave computed the following set: UNION(@@global.gtid_executed, Retrieved_gtid_set - last_received_GTID)
问为什么要减去last_received_GTID。
二、查看Bug #17943188的commit
对于这个问题我先查看了一下这个bug到底修复了什么问题如下：
BUG#17943188 SHOW SLAVE STATUS/RETRIEVED_GTID_SET MAY HAVE PARTIAL TRX OR MISS COMPLETE TRX
Problem:
=======
The SHOW SLAVE STATUS command contains the column RETRIEVED_GTID_SET.
This is supposed to contain the set of GTIDs that exist in the relay
log. However, the field is updated when the slave receiver thread
(I/O thread) receives a Gtid_log_event, which happens at the beginning
of the transaction.
If the I/O thread gets disconnected in the middle of a transaction,
RETRIEVED_GTID_SET can contain a GTID for a transaction that is only
partially received in the relay log. This transaction will
subsequently be rolled back, so it is wrong to pretend that the
transaction
