This is assuming other sessions change the same block your session is trying to read.
=== It's been a while since I worked with Oracle as a developer. But my understanding is that even a read-only transaction, like the one you described above, requires a point in time consistent image of the database. This would imply that if your transaction runs for a long time and meanwhile other DML sessions change lot of blocks, resulting in undo tablespace getting totally turned over, then Oracle can no longer gurantee PIT consistent view of the database to your session and barf out with snapshot-too-old error. I have no way of confirming this and I am writing this based on my limited experience with oracle. So I may be wrong.
