ORA-01555 快照过旧--详细解释

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  　　Session 1 starts a query at QENV 50. After this another process updates the blocks that Session 1 will require. When Session 1 encounters these blocks it determines that the blocks have changed and have not yet been cleaned out (via delayed block cleanout). Session 1 must determine whether　the rows in the block existed at QENV 50, were subsequently changed,
  　　
  　　In order to do this, Oracle must look at the relevant rollback segment transaction table slot to determine the committed SCN. If this SCN is after the QENV then Oracle must try to construct an older version of the block and if it is before then the block just needs clean out to be good enough for　the QENV.
  　　
  　　If the transaction（处理） slot has been overwritten and the transaction table cannot be rolled back to a sufficiently old enough version then Oracle cannot derive the block image and will return ORA-1555.
  　　
  　　(Note: Normally Oracle can use an algorithm for determining a block's SCN during block cleanout even when the rollback segment slot has been overwritten.
  　　
  　　But in this case Oracle cannot guarantee that the version of the block has not changed since the start of the query).
  　　
  　　解决问题的办法
  　　
  　　This section lists some of the solutions that can be used to avoid（避免）the ORA-01555 problems discussed in this article. It addresses the cases where rollback segment information is overwritten by the same session and when the rollback segment transaction table entry is overwritten.
  　　
  　　It is worth highlighting（最重要的） that if a single session experiences the ORA-01555 and it is not one of the special cases listed at the end of this article, then the session must be using an Oracle extension延伸 whereby由此 fetches across commits are tolerated. This does not follow the ANSI model and in the rare cases where ORA-01555 is returned one of the solutions below must be used.
  
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