> The latter one should employ some resource management/limitation
> scheme, taking into account page fetches, CPU time, kW*h of power
> consumed, etc per user account. A good idea would be valuation of all
> these resources in money units and provide a daily bill to user.

Actually, the idea of quotas is important and fruitful one. Most
advanced DBMS have the possibility to limit not only elapsed time, but
also CPU time, memory and disk usage, etc.
IMO this should be done as a cohesive set of features based neither on
database nor on client configuration, but on user accounts, so any
query run by a particular database user is automatically terminated
after exceeding any kind of quota given to this specific user. This
functionality could be surfaced as a system table

TABLE RDB$QUOTAS
(
RDB$USER INTEGER REFERENCES RDB$USERS(..),
RDB$MAX_ELAPSED_TIME_MSEC INTEGER,
RDB$MAX_CPU_TIME_MSEC INTEGER,
...
)

For example, users responsible for creating backups should be given
sufficiant quotas to do so, while the "user" represeting a connection
from web-server script should have tighter limits.