The dreaded long transaction error.

We recently had to call informix in order to "fix" our database after a long transaction error. The logical logs had filled as a result of a "simple" unload from a large table which did group by's, order by's and also had amin(unique..) thrown in there for good measure! The logs couldn't be backed up as the job was still regarded as runnning by the database (even though it wasn't). A few questions : 1. Why would the above example even use the logical logs? 2. Is there any way to use software to automatically (and selectively!) abort a job which is threatening to fill the logs? 3. Any good rule of thumb for how big the logs should be? (we dont use transaction logging). 4. Does anyone know what Informix did to free the logs?