Mysqldump --single-transaction 选项解析

一、解析
查看general_log：

206 Query SETSESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ
##更改隔离级别（采取快照，保证了事务中一致性读） 206 Query START TRANSACTION/*!40100 WITH CONSISTENT SNAPSHOT */
##事务开始
206 Query UNLOCKTABLES
206 Init DB sanguo_single
206 Query SAVEPOINTsp
##设置保存点
206 Query showtables
206 Query showtable status like 'achievements'4
206 Query SETSQL_QUOTE_SHOW_CREATE=15
206 Query SETSESSION character_set_results = 'binary'5
206 Query showcreate table `achievements`
##获取表结构
206 Query SETSESSION character_set_results = 'utf8'
##设置备份参数
206 Query showfields from `achievements`
206 Query SELECT/*!40001 SQL_NO_CACHE */ * FROM `achievements`6
##获取表的数据 206 Query SETSESSION character_set_results = 'binary'
206 Query use`sanguo_single`
206 Query select@@collation_database
206 Query SHOWTRIGGERS LIKE 'achievements'
206 Query SETSESSION character_set_results = 'utf8'
206 Query ROLLBACK TOSAVEPOINT sp
##回滚保存点（撤销select对于achievements表的影响）。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。## achievements表备份结束，其他表以此类推。可以看出--single-transaction是一张一张表进行备份然后一一进行撤销select
二、实验1. --single-transaction 选项备份时执行dml语句

事务一	show processlist;	事务二
mysqldump --single-transactionsanguo_single >sanguo_single.sql	| 213 | root | localhost | sanguo_single |Query | 5 | Sending data | SELECT /*!40001SQL_NO_CACHE */ * FROM `player_pve_battles` | +-----+------+-----------+---------------+---------+-------+--------------+-------------------------------------------------------------+ 5 rows in set (0.00 sec)	mysql> delete from player_pve_battles; Query OK, 52646 rows affected (18.24 sec)

------------
TRANSACTIONS
------------
Trx idcounter 12887
Purgedone for trx's n:o < 12794 undo n:o < 0 state: running but idle
Historylist length 639
LIST OFTRANSACTIONS FOR EACH SESSION:
---TRANSACTION0, not started
MySQLthread id 178, OS thread handle 0x7f61312f1700, query id 48564 localhost rootinit
showengine innodb status
---TRANSACTION12793, not started
MySQLthread id 208, OS thread handle 0x7f608bfff700, query id 39534 localhost rootcleaning up
---TRANSACTION12886, ACTIVE 4 sec starting index read
mysqltables in use 1, locked 1
20188lock struct(s), heap size 1799720, 64197 row lock(s)
MySQL thread id 210, OS thread handle 0x7f613122e700, queryid 48563 localhost root updating ###Delete没有被锁住，然后他对player_pve_battles加了个锁。deletefrom player_pve_battles
---TRANSACTION12792, ACTIVE 625 sec
MySQLthread id 207, OS thread handle 0x7f61312b0700, query id 39535 localhost rootcleaning up
Trx read view will not see trx with id >= 12793, sees< 12793
2. --single-transaction 选项备份时执行ddl语句

事务一	事务二
mysqldump --single-transactionsanguo_single >sanguo_single.sql	mysql> drop table player_pve_battles; ^CCtrl-C -- sending "KILL QUERY210" to server ... Ctrl-C -- query aborted. ERROR 1317 (70100): Query execution wasinterrupted

结论：因为-single-transaction选项备份时涉及到的是select语句，所以dml语句是被允许的，ddl不被允许直到回滚保存点撤销select 语句