ORA-01555 快照过旧--详细解释

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  　　It is under this condition that Oracle may not be able to get the required rollback information because Session 1's changes have generated rollback information that has overwritten it and returns the ORA-1555 error.
  　　
  　　CASE 2 - ROLLBACK TRANSACTION SLOT OVERWRITTEN
  　　
  　　1. Session 1 starts query at time T1 and QENV 50
  　　
  　　2. Session 1 selects block B1 during this query
  　　
  　　3. Session 1 updates the block at SCN 51
  　　
  　　4. Session 1 commits the changes
  　　(Now other transactions are free to overwrite this rollback information)
  　　
  　　5. A session (Session 1, another session or a number of other sessions) then use the same rollback segment for a series of committed transactions.
  　　
  　　These transactions each consume a slot in the rollback segment transaction table sUCh that it eventually wraps around (the slots are written to in a circular fashion) and overwrites all the slots. Note that Oracle is free to reuse these slots since all transactions are committed.
  　　
  　　6. Session 1's query then visits a block that has been changed since the initial QENV was established. Oracle therefore needs to derive(获得) an image of the block as at that point in time.
  　　
  　　Next Oracle attempts to lookup the rollback segment header's transaction slot pointed to by the top of the data block. It then realises that this has been overwritten and attempts to rollback the changes made to the rollback segment header to get the original transaction slot entry.
  　　
  　　If it cannot rollback the rollback segment transaction table sufficiently it will return ORA-1555 since Oracle can no longer derive the required version of the data block.
  　　
  　　It is also possible to encounter a variant of the transaction slot being overwritten when using block cleanout. This is briefly described below :
  
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