--single-transaction 的含义是，在开始 dump 前，设置隔离级别为 RR ，并且 start transaction. 通过将导出操作封装在一个事务 (Repeatable Read) 内来使得导出的数据是一个一致性快照。 测试数据库版本：
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.26-log |
+------------+
1 row in set (0.00 sec) 测试 1 ：
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sesseion A |
session B |
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mysql> set tx_isolation='repeatable-read'; Query OK, 0 rows affected (0.00 sec) |
mysql> set tx_isolation='repeatable-read'; Query OK, 0 rows affected (0.00 sec) |
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mysql> begin; Query OK, 0 rows affected (0.01 sec) |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | +----+------+ 3 rows in set (0.00 sec) mysql> insert into test01 select 4,4; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | +----+------+ 4 rows in set (0.00 sec) |
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mysql> insert into test01 select 5,5; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | +----+------+ 4 rows in set (0.00 sec) |
可以看到， RR 隔离级别下的一致性读，不是以事务 begin 的时间点建立 snapshot 的。单纯 begin 后，到第一次读取前之间其他会话的事务还是可以读取的，但是在第一次读取数据完成后，读取到的事务就不再变化了。 测试 2 ：
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session A |
session B |
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mysql> set tx_isolation='repeatable-read'; |
mysql> set tx_isolation='repeatable-read'; |
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mysql> select * from t1; Empty set (0.00 sec) |
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mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | | 5 | 5 | +----+------+ 5 rows in set (0.00 sec) |
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mysql> insert into test01 select 6,6; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | | 5 | 5 | +----+------+ 5 rows in set (0.00 sec) |
RR 隔离级别下的一致性读，发生在事务里面第一次 select 的时候。我试了下用 delete 语句替换 session A 的 select 语句，结果发现还是会读取到 session B 的事务。
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session A |
session B |
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mysql> set tx_isolation='repeatable-read'; |
mysql> set tx_isolation='repeatable-read'; mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | +----+------+ 3 rows in set (0.00 sec) |
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mysql> begin; Query OK, 0 rows affected (0.00 sec) mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | +----+------+ 3 rows in set (0.00 sec) |
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mysql> insert into test01 select 4,4; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 4 | +----+------+ 4 rows in set (0.00 sec) |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | +----+------+ 3 rows in set (0.00 sec) |
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mysql> update test01 set c2=5 where c1=4; Query OK, 1 row affected (0.00 sec) Rows matched: 1 Changed: 1 Warnings: 0 mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | | 4 | 5 | +----+------+ 4 rows in set (0.00 sec) |
Session A 第一次 select 建立一致性读后， session B 插入数据， session A 的 select 仍然读不到，但是 update 因为是当前读，所以更新到 session B 插入的数据。 所以 begin 和 start transaction 是事务开始的标志，但不是事务开始的起点。如果要将 start transaction 作为事务开始的时间点，那么必须使用： START TRANSACTION WITH consistent snapshot ###mysqldump 中的快照就是用这个实现的
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mysql> set tx_isolation='repeatable-read'; |
mysql> set tx_isolation='repeatable-read'; |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | +----+------+ 3 rows in set (0.00 sec) |
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mysql> start transaction with consistent snapshot; Query OK, 0 rows affected (0.00 sec) |
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mysql> insert into test01 select 4,4; Query OK, 1 row affected (0.00 sec) Records: 1 Duplicates: 0 Warnings: 0 |
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mysql> select * from test01; +----+------+ | c1 | c2 | +----+------+ | 1 | 1 | | 2 | 2 | | 3 | 3 | +----+------+ 3 rows in set (0.00 sec) |
发起备份 mysqldump -uroot -poracle --single-transaction --master-data=1 -R -E --triggers -B ming --ignore_table=ming.test02 > /tmp/ming_st.sql 查看备份过程的 general log ：
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[root@oradb-2062 binlog]# more /u01/mysql/3306/data/oradb-2062.log /u01/mysql_57/bin/mysqld, Version: 5.7.26-log (MySQL Community Server (GPL)). started with: Tcp port: 3306 Unix socket: /u01/mysql/3306/data/mysqld.sock Time Id Command Argument 2019-08-01T08:30:50.718358Z 12 Query show variables like 'log_output' 2019-08-01T08:31:33.211254Z 14 Connect root@localhost on using Socket 2019-08-01T08:31:33.211413Z 14 Query /*!40100 SET @@SQL_MODE='' */ 2019-08-01T08:31:33.211474Z 14 Query /*!40103 SET TIME_ZONE='+00:00' */ 2019-08-01T08:31:33.211565Z 14 Query FLUSH /*!40101 LOCAL */ TABLES 2019-08-01T08:31:33.212009Z 14 Query FLUSH TABLES WITH READ LOCK 2019-08-01T08:31:33.212047Z 14 Query SET SESSION TRANSACTION ISOLATION LEVEL REPEATABLE READ 2019-08-01T08:31:33.212070Z 14 Query START TRANSACTION /*!40100 WITH CONSISTENT SNAPSHOT */ 2019-08-01T08:31:33.212115Z 14 Query SHOW VARIABLES LIKE 'gtid\_mode' 2019-08-01T08:31:33.216296Z 14 Query SHOW MASTER STATUS 2019-08-01T08:31:33.216472Z 14 Query UNLOCK TABLES 2019-08-01T08:31:33.219582Z 14 Query SELECT LOGFILE_GROUP_NAME, FILE_NAME, TOTAL_EXTENTS, INITIAL_SIZE, ENGINE, EXTRA FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'UNDO LOG ' AND FILE_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IS NOT NULL AND LOGFILE_GROUP_NAME IN (SELECT DISTINCT LOGFILE_GROUP_NAME FROM INFORMATION_SCHEMA.FILES WHERE FILE_TYPE = 'DATAFILE' AN D TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHEMA.PARTITIONS WHERE TABLE_SCHEMA IN ('ming'))) GROUP BY LOGFILE_GROUP_NAME, FILE_NAME, ENGINE, TOTAL_EXTENTS, I NITIAL_SIZE ORDER BY LOGFILE_GROUP_NAME 2019-08-01T08:31:33.223184Z 14 Query SELECT DISTINCT TABLESPACE_NAME, FILE_NAME, LOGFILE_GROUP_NAME, EXTENT_SIZE, INITIAL_SIZE, ENGINE FROM INFORMATION_SCHEMA.FILES WHERE FILE _TYPE = 'DATAFILE' AND TABLESPACE_NAME IN (SELECT DISTINCT TABLESPACE_NAME FROM INFORMATION_SCHEMA.PARTITIONS WHERE TABLE_SCHEMA IN ('ming')) ORDER BY TABLESPACE_NAME, LOGFILE_GROUP_NAME 2019-08-01T08:31:33.223727Z 14 Query SHOW VARIABLES LIKE 'ndbinfo\_version' 2019-08-01T08:31:33.225502Z 14 Init DB ming 2019-08-01T08:31:33.225545Z 14 Query SHOW CREATE DATABASE IF NOT EXISTS `ming` 2019-08-01T08:31:33.225683Z 14 Query SAVEPOINT sp 2019-08-01T08:31:33.225750Z 14 Query show tables 2019-08-01T08:31:33.225957Z 14 Query show table status like 'mytest01' 2019-08-01T08:31:33.226083Z 14 Query SET SQL_QUOTE_SHOW_CREATE=1 2019-08-01T08:31:33.226114Z 14 Query SET SESSION character_set_results = 'binary' 2019-08-01T08:31:33.226145Z 14 Query show create table `mytest01` 2019-08-01T08:31:33.226190Z 14 Query SET SESSION character_set_results = 'utf8' 2019-08-01T08:31:33.226226Z 14 Query show fields from `mytest01` 2019-08-01T08:31:33.226468Z 14 Query show fields from `mytest01` 2019-08-01T08:31:33.226687Z 14 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `mytest01` 2019-08-01T08:31:33.226810Z 14 Query SET SESSION character_set_results = 'binary' 2019-08-01T08:31:33.226844Z 14 Query use `ming` 2019-08-01T08:31:33.226877Z 14 Query select @@collation_database 2019-08-01T08:31:33.226920Z 14 Query SHOW TRIGGERS LIKE 'mytest01' 2019-08-01T08:31:33.227098Z 14 Query SET SESSION character_set_results = 'utf8' 2019-08-01T08:31:33.227130Z 14 Query ROLLBACK TO SAVEPOINT sp 2019-08-01T08:31:33.227162Z 14 Query show table status like 'test01' 2019-08-01T08:31:33.227262Z 14 Query SET SQL_QUOTE_SHOW_CREATE=1 2019-08-01T08:31:33.227289Z 14 Query SET SESSION character_set_results = 'binary' 2019-08-01T08:31:33.227316Z 14 Query show create table `test01` 2019-08-01T08:31:33.227356Z 14 Query SET SESSION character_set_results = 'utf8' 2019-08-01T08:31:33.227389Z 14 Query show fields from `test01` 2019-08-01T08:31:33.227730Z 14 Query show fields from `test01` 2019-08-01T08:31:33.227911Z 14 Query SELECT /*!40001 SQL_NO_CACHE */ * FROM `test01` 2019-08-01T08:31:33.228005Z 14 Query SET SESSION character_set_results = 'binary' 2019-08-01T08:31:33.228053Z 14 Query use `ming` 2019-08-01T08:31:33.228084Z 14 Query select @@collation_database 2019-08-01T08:31:33.228143Z 14 Query SHOW TRIGGERS LIKE 'test01' 2019-08-01T08:31:33.228336Z 14 Query SET SESSION character_set_results = 'utf8' 2019-08-01T08:31:33.228369Z 14 Query ROLLBACK TO SAVEPOINT sp 2019-08-01T08:31:33.228399Z 14 Query show table status like 'test03' 2019-08-01T08:31:33.228501Z 14 Query SET SQL_QUOTE_SHOW_CREATE=1 2019-08-01T08:31:33.228686Z 14 Query SET SESSION character_set_results = 'binary' 2019-08-01T08:31:33.228726Z 14 Query show create table `test03` 。。。。。。 |
可以看到， mysqldump 的大致实现过程是：连接 -> 初始化信息 -> 刷新表（锁表） -> 开启事务（一致性快照） -> 记录偏移量 -> 解锁表 参考：https://yq.aliyun.com/articles/552972?spm=a2c4e.11153940.0.0.18a12415csEBjM
