ORA-01555 causes and suggestions to avoid it

Here are the causesof this error

• The undo segments are too small
•  The application fetches across commits (a design flaw)
• Delayed block cleanout

Here are some suggestions to avoid  ORA-01555: snapshot too old erro

• Adjust the undo_retention parameter (for 9i and higher releases of Oracle
•  Increase the size of the UNDO tablespace 
• Increase the size of the undo segments [if using manual UNDO management]