ORA-01555 快照过旧--详细解释

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  　　o The transaction slot in the rollback segment's transaction table (stored in the rollback segment's header) is overwritten, and Oracle cannot rollback the transaction header sufficiently to derive the original rollback segment transaction slot.
  　　
  　　Both of these situations are discussed below with the series of steps that cause the ORA-01555. In the steps, reference is made to 'QENV'. 'QENV' is short for 'Query Environment', which can be thought of as the environment that existed when a query is first started and to which Oracle is trying to attain a read consistent image. Associated with this environment is the SCN (System Change Number) at that time and hence, QENV 50 is the query environment with SCN 50.
  　　
  　　CASE 1 - ROLLBACK OVERWRITTEN
  　　
  　　This breaks down into two cases: another session overwriting the rollback that the current session requires or the case where the current session　overwrites the rollback information that it requires. The latter is discussed in this article because this is usually the harder one to understand.
  　　
  　　Steps:
  　　
  　　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 does some other work that generates rollback information.
  　　
  　　5. Session 1 commits the changes made in steps '3' and '4'.
  　　(Now other transactions are free to overwrite this rollback information)
  　　
  　　6. Session 1 revisits the same block B1 (perhaps for a different row).
  　　
  　　Now, Oracle can see from the block's header that it has been changed and it is later than the required QENV (which was 50). Therefore we need to get an image of the block as of this QENV.
  　　
  　　If an old enough version of the block can be found in the buffer cache then we will use this, otherwise we need to rollback the current block to generate another version of the block as at the required QENV.
  
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