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Oracle ORA_ROWSCN 伪列 说明

原创 Linux操作系统 作者:vcdog 时间:2011-08-27 05:31:07 0 删除 编辑


一. 官网对该伪列的说明



       Foreach row, ORA_ROWSCN returns the conservative upper bound system change number(SCN) of the most recent change to the row in the current session. Thispseudocolumn is useful for determining approximately when a row was lastupdated.

       Itis not absolutely precise, because Oracle tracks SCNs by transaction committedfor the block in which the row resides. You can obtain a more fine-grainedapproximation of the SCN by creating your tables with row-level dependencytracking. Refer to CREATE TABLE ... NOROWDEPENDENCIES| ROWDEPENDENCIES for more information on row-level dependency tracking.


       ROWDEPENDENCIESSpecify ROWDEPENDENCIES if you want to enable row-level dependency tracking.This setting is useful primarily to allow for parallel propagation inreplication environments. It increases the size of each row by 6 bytes.

       NOROWDEPENDENCIESSpecify NOROWDEPENDENCIES if you do not want table to use the row-leveldependency tracking feature. This is the default.


       也就是说,使用表的默认创建参数,即norowdependencies时,此时的ora_rawscn 取自data block header的SCN,那么这时候,对于同一个block里的row而言,他们的ora_rowscn 是一样的。

       而在创建table时指定为rowdependencies时,那么会为每行row 保存一个ora_rowscn. 这样对于同一个block里的row,会有多个ora_rowscn 值。通过dump block,可以发现每个row 会多出一个dscn的信息,该信息就是用来保存ora_rowscn的。  



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