Oracle在无备份情况下通过PRM恢复被截断truncate掉的表
情况 当某张表被意外truncated掉了，需要恢复其上的所有数据时。表空间的多个数据文件均存放在ASM上，且没有任何形式的备份。 注意这边文章针对的是PRM在 数据字典模式下的Truncate恢复选项不可用时使用，数据字典模式下的Truncate恢复选项是最简单、易用的
情况
当某张表被意外truncated掉了，需要恢复其上的所有数据时。表空间的多个数据文件均存放在ASM上，且没有任何形式的备份。
注意这边文章针对的是PRM在 数据字典模式下的Truncate恢复选项不可用时使用，数据字典模式下的Truncate恢复选项是最简单、易用的一种模式，具体使用见《使用PRM恢复Oracle数据库中误truncate截断的表数据》http://www.parnassusdata.com/zh-hans/node/52
PRM 3.0的下载地址： http://parnassusdata.com/sites/default/files/ParnassusData_PRMForOracle_3002.zip
PRM 的官方网站：?http://www.parnassusdata.com/
PRM背景
PRM恢复表数据时存在多种模式， PRM需要知道哪些表上的数据块是需要被读取并取出数据的。默认的表现形式是直接从segment header数据段头里获取EXTENT MAP即盘区图，另一种方案就是由PRM自己去构建一个盘区图。 这些盘区图可以通过，PRM的SCAN DATABASE选项来获得： ? ?Recovery Wizard => Non-Dictionary Mode，如果是ASM则选择Non-Dictionary Mode(ASM)
也可以在左侧属性图处选择SCAN -DATABASE
执行SCAN Database后会生成SEG$和EXT$的数据到PRM内嵌的数据库中，之后可以选择SCAN TABLES FROM SEGMENTS 或者 SCAN TABLES FROM EXTENTS。 ?FROM Segments 意味着使用Segment Header中获得的Extent MAP信息，而FROM Extents意味着使用PRM自己扫描获得的EXTENT信息。
请注意当TRUNCATE发生后， 数据表Table的Segment Header中的Extent MAP信息就会被清空了， 但实际存放数据的数据块中的行数据还是在哪里的，除非被其他数据表/索引的增长而覆盖了。 所以当Truncate发生后选择SCAN TABLES FROM SEGMENT ?是找不回数据的，必须使用SCAN TABLES FROM EXTENTS， EXTENT的信息是PRM自己去数据文件中扫描获得的，所以只要有数据的地方PRM就会自己去找到。
除了Truncate需要使用到 SCAN TABLES FROM EXTENTS之外对于DROP TABLE的恢复也可以用到SCAN TABLES FROM?EXTENTS ?， 总之当Segment Header找不到(可能存放Segment Header的数据文件丢失了)、或者已损坏(可能Segment Header的数据块被损坏了）、或者其中的Extent Map数据无效（Truncate、DROP或逻辑损坏)时都可以使用SCAN TABLES FROM?EXTENTS 。 ?
但是如果不存在上述的问题时，建议用SCAN TABLES FROM?SEGMENTS ?，因为从Segment Header获取信息更方便也更高效一些。
在PRM中同一个程序实例 同时只能使用SCAN TABLES FROM?SEGMENTS 或者?SCAN TABLES FROM?EXTENTS ??中的一个。
使用SCAN TABLES FROM?EXTENTS ??后需要找到对应被TRUNCATE掉的表的原始DATA_OBJECT_ID，即左侧属性图中的一个对象，并将其DataBridge 数据搭桥传输到目标数据库中即可。
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原文地址：Oracle在无备份情况下通过PRM恢复被截断truncate掉的表, 感谢原作者分享。
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