ORA-01555 快照过旧--详细解释

• 　　
  　　CASE 1 - ROLLBACK OVERWRITTEN
  　　
  　　1.　Increase size of rollback segment which will reduce the likelihood of overwriting rollback information that is needed.
  　　
  　　2.　Reduce the number of commits (same reason as 1).
  　　
  　　3.　Run the processing against a range of data rather than the whole table.
  　　(Same reason as 1).
  　　
  　　4.　Add additional（附加） rollback segments. This will allow the updates etc. to be spread across more rollback segments thereby reducing the chances of overwriting required rollback information.
  　　
  　　5.　If fetching across commits, the code can be changed so that this is not done.
  　　
  　　6.　Ensure that the outer select does not revisit the same block at different times during the processing. This can be achieved by :
  　　
  　　- Using a full table scan rather than an index lookup
  　　- Introducing a dummy sort so that we retrieve all the data, sort it and then sequentially visit these data blocks.
  　　
  　　CASE 2 - ROLLBACK TRANSACTION SLOT OVERWRITTEN
  　　
  　　1. Use any of the methods outlined above except for '6'. This will allow transactions to spread their work across multiple rollback segments therefore reducing the likelihood or rollback segment transaction table slots being consumed.
  　　
  　　2. If it is suspected that the block cleanout variant is the cause, then force block cleanout to occur prior to the transaction that returns the ORA-1555. This can be achieved by issuing the following in SQL*Plus, SQL*DBA or Server Manager :
  　　
  　　alter session set optimizer_goal = rule;
  
  　　select count(*) from table_name;
  　　
  　　If indexes are being accessed then the problem may be an index block and clean out can be forced by ensuring that all the index is traversed. Eg, if the index is on a numeric column with a minimum value of 25 then the following query will force cleanout of the index :
  
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