Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined variable $img_type in /www/wwwroot/www.zhaohaoblog.com/wp-content/plugins/wpjam-basic/cdn/remote.php on line 86

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined variable $img_type in /www/wwwroot/www.zhaohaoblog.com/wp-content/plugins/wpjam-basic/cdn/remote.php on line 86

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 800

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 806

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 808

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 811

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 828

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 829

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

Warning: Undefined array key 0 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 75

Warning: Undefined array key 1 in /www/wwwroot/www.zhaohaoblog.com/wp-includes/media.php on line 76

文章目录

• MVCC全称Mutli Version Concurreny Control,多版本并发控制，也可称之为一致性非锁定读；它通过行的多版本控制方式来读取当前执行时间数据库中的行数据。实质上使用的是快照数据，这样就可以实现不加锁读。MVCC 主要应用于 Read Commited 和 Repeatable read 两个事务隔离级别。 1. innoDB 的逻辑存储结构 innoDB 的数据保存在表空间中，表空间又包含各种段，其中有数据段，索引段，回滚段。InnoDB中数据以B+Tree的数据结构存储的，非叶子节点也就是索引，叶子节点也就是数据行，回滚段用于存储undoLog，undoLog中记录的就是多版本数据，用于快照读和事务失败后的数据回滚,MySQL在合适的时机会清理undoLog。 2. MVVC的实现 MVCC的实现依赖于 每行的隐藏字段，DB_TRX_ID,DB_ROLL_PTR,删除标记位,还有read_view。 3. 三个隐藏字段```DB_TRX_ID``` 事务id占6 字节，表示这一行数据最后插入或修改的事务id。此外删除在内部也被当作一次更新，在行的特殊位置添加一个删除标记（记录头信息有一个字节存储是否删除的标记）。```DB_ROLL_PTR``` 回滚指针占7字节，回滚指针指向被写在 ```Rollback segment```中的undoLog记录，在该行数据被更新的时候，undoLog 会记录该行修改前内容到undoLog。```DB_ROW_ID``` 行ID占7字节，他就项自增主键一样随着插入新数据自增。如果表中不存主键 或者 唯一索引，那么数据库 就会采用 ```DB_ROW_ID ```生成聚簇索引。否则```DB_ROW_ID ```不会出现在索引中.4. undo logundo log是为回滚而用，具体内容就是copy事务前的数据库内容（行）到undo buffer，在适合的时间把undo buffer中的内容刷新到磁盘。undo buffer与redo buffer一样，也是环形缓冲，但当缓冲满的时候，undo buffer中的内容会也会被刷新到磁盘；与redo log不同的是，磁盘上不存在单独的undo log文件，所有的undo log均存放在主ibd数据文件中（表空间），即使客户端设置了每表一个数据文件也是如此。 undo log 在 ```Rollback segment```中又被细分为 insert 和 update undo log , insert 类型的undo log 仅仅用于事务回滚,当事务一旦提交，insert undo log 就会被丢弃。update的undo log 被用于 一致性的读和事务回滚，update undo log 的清理 是在 没有事务 需要对这部分数据快照进行一致性读的时候 进行清理。undo log 的创建 每次对数据进行更新操作时，都会copy 当前数据,保存到undo log 中。并修改 当前行的 回滚指针指向 undo log 中的 旧数据行。5. read_view 判断数据行可见性在innodb中，创建一个新事务的时候，innodb会将当前系统中的活跃事务列表创建一个副本（read view），副本中保存的是系统当前不应该被本事务看到的其他事务id列表。当用户在这个事务中要读取该行记录的时候，innodb会将该行当前的版本号与该read view进行比较。6. (提交读)Read Commited ，(可重复读)Repeatable read 数据可见性判断Read Commited 和 Repeatable read 采用相同的数据可见性判断逻辑。 那么怎么在相同的判断逻辑下 分别 实现 RC 和 RR 级别的？Read Commited在每次语句执行的过程中，都关闭read_view, 重新创建当前的一份新的read_view。 这样就可以根据当前的全局事务链表创建read_view的事务区间，实现read committed隔离级别。Repeatable read在repeatable read的隔离级别下，创建事务trx结构的时候，就生成了当前的global read view。 使用trx_assign_read_view函数创建，一直维持到事务结束，这样就实现了repeatable read隔离级别。

InnoDB Multi-Versioning read committed && MVCC MySQL InnoDB 索引原理