ORA-01555: Snapshot too old

This error occurs when there is insufficient rollback segments and the solution is to increase the rollback segments size.

This error is related to read consistency.

For example : You launched a long-running query at 2am, and it will continue for the next hour. Since undo retention is set to 1800 seconds, any DML transactions that might affect rows during the query’s runtime will be safe in the undo tablespace for the next 15min.
After 15 minutes, an error will show up if a long-running query needs the same updated rows.

Solution: Increase undo retention till it is of adequate value.

SQL> alter system set undo_retention = 3600 scope=both;
System altered.