Online Status with FR & SQL

[Gunther]

I am trying to minimize the failure rate for detecting a user online status
using
FR105 & MySQL411.

Sometimes the AcctStopTime in the radacct table remains 0 even the user is
not anymore online
for whatever reason (reboot, connection lost or ...). I can therefore not
just check
if the AcctStopTime for a particular user is 0. Since there is no record
when the
NAS unit sent the last update to FR and it is not recorded in the radacct
table ...
how do I know that the AcctStopTime=0 is not a 'leftover' ...
Would it make sense to add a TIMESTAMP to the radacct table to record the
last update?
And would the use of TIMESTAMP for the radacct table produce some form of
performance degrade?
A TIMESTAMP would allow me to see if the row was updated within
Idle-Timeout.
Any hints from experience?

Thanks,
Gunther