I did a simple routine to trap in a TXT log file all the active transactions. I realized that I need to be SYSDBA in order to run a query against MON$TRANSACTIONS. So, every time a user enters or exits from the applitaction I save a log of all the transactions. I found that I had some read/write transactions running for 2 hours. And that is a HUGE time I think! 

I read somewhere that the "read only" transactions are absolutelly harmless to the GC and sweeping. Is that so?