In case of parallel update and select, if the consistent past image is not available for the user who has already started reading the data from undo, such type of user is going to get snapshot too old error.
We can minimise the occurance of snapshot too old error by commiting more frequently or by creating a bigger undo tablespace.
To minimise, the occurance of snapshot too old error(if possible to eliminate) oracle has introduced retension guarantee in 10g version of oracle.
If we create undo tablespace with retension guarantee, oracle retains the data in the undo segments as per the parameter value "undo_retension".
Once we create undo tablespace, by default all 10 undo segments will be online.
