Consistent read means that when there are many transactions updating the same data rows at the same time concurrently it will generate multiple versions in the undo tablespaces named undo log files.All the other reads on the same data will get the old version before those transactions are committed. If it has the low isolation level,it will read from the undo tablespaces before committed,but after committed,new updates will be refreshed to data files in the disk and then you can get the new row,but after rollback,undo tablespaces will discard the changes.