Why do I sometimes get ORA-1555 error on dequeue?
You are probably using the NEXT_MESSAGE navigation option for dequeue. This uses the snapshot created during the first dequeue call. After that the other dequeue calls generate more undo which fills up the rollback segment and hence generates 1555. The workaround is to use the FIRST_MESSAGE option to dequeue the message. This will reexecute the cursor and get a new snapshot. This might not perform as well, so we suggest you dequeue them in batches: FIRST_MESSAGE for one, and NEXT_MESSAGE for the next, say, 1000 messages, and then FIRST_MESSAGE again, and so on.
