What is "snapshot too old" error and how to avoid it?

First thing to be understood here is how rollback segments work. These work in a circular fashion by looping around to reuse the extents which have been released by committed transactions. Thus in case of long running queries if data is changed while the query is running, rollback segments created for that change are needed to make a read consistent image for the lon running query. When these extents are reused while these were still need by the query, this error occurs, because oracle can no longer provide a read consistent image of the data.

To avoid this error you need to postpone the reuse of extents. Thus increasing the size of rollback segments and the value of OPTIMAL parameter should do the trick. Comitting less often would be other solution. As until transaction is comitted rollback segment can not be reused thus reducing the chances of a snapshot too old error.