>> But even with read only database each transaction will get
>> unique transaction id,
>
> Actually, they don't. There's no place to store transaction ids,
> so a read-only database just uses the same number over and over.

Have you tried
SELECT CURRENT_TRANSACTION FROM RDB$DATABASE;
COMMIT;
SELECT CURRENT_TRANSACTION FROM RDB$DATABASE;
COMMIT;
SELECT CURRENT_TRANSACTION FROM RDB$DATABASE;
...

? The numbers will increase. (of course just in fb server, not in database)

Ivan