oracle可以从rollback segments中获得旧信息(旧数据管理者)
这样支持了multi-version read consistency
当查询开始后所要获得的数据就已经决定了.
please note that a user who make change to his or her data can view the change before committing(with the same session).
每个数据块的数据头存储transaction slots for transactions to identify themselves within a block as they are modifying data.(ITL)
ITL包含三个结构:Transaction ID, the undo block address(UBA)指向前镜象在undo的地址. SCN if the transaction has been committed.
select * from v$rollstat;
wraps 指跨extent就会有
采取高峰期使用的数量
select * from v$transaction;
user_ublk一个transaction使用的data blocks数.
显示活动事务信息
SELECT a.username, b.name, c.used_ublk
FROM v$session a, v$rollname b, v$transaction c
WHERE a.saddr=c.ses_addr AND b.usn=c.xidusn
AND a.username=’SCOTT’;
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