Q: Why do we get ORA-01555 error?.
Ans: Suppose there are two users(User1 and User2). User1 is executing long update statement on oracle data without commit and at the same time User2 is also selecting same data which is getting updated by User1.
Both Queries are still running and no one gets output yet.
After some time, Query execution of User1 gets completed and he commits the data. Now what does happen with User2?.
He gets ORA-01555 error. The reason behind this is ,"User2 doesn't get old image of the data", because User1 has changed the data.
Solution:
Increasing UNDO tablespace size is not the solution for this.
Run Undo advisory
Increase UNDO Retension Period
Enable Undo Retention Guarantee
Run this Select Query during non-business hours where heavy transactions are not happening.
Tune the Query
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