ORA-01555: Snapshot too old

Try to run long running queries on off peak hours, when there is less DML transactions on database.

Database should be in Automatic Undo Management mode.

Use a large optimal size of rollback segment (undo) size.

Use a large optimal value for undo_retention.

Should avoid to fetch (select / cursors) between commits.

Should Commit less often at the time of long running query, to reduce Undo Segment reuse.

Hope it helps!