[collectd] how to set a threshold using postgresql plugin

[Gerardo Herzig]

Hi all. Im sending collectd values from a postgres DB to our collectd central server.
In DB, there is using postgres plugin like:

<Query db_movements>
  Statement "select sum(numbackends) as backends, sum(tup_inserted) as inserts, sum(tup_updated) as updates, sum(tup_deleted) as deletes 
  from pg_stat_database;"
  <Result>
    Type gauge
    InstancePrefix total_backends
    ValuesFrom "backends"
  </Result>
</Query>
<Database template1>
        Interval 10
        Host localhost
        User postgres
        Query db_movements
</Database>

And, in the "central" collectd server i want to set a threshold for that server, so
" If that db server is over 100 connections in use, ALERT "

At that point, i got confused about how to get it work.
I have something like that:

<Host "db_server">
    <Plugin "postgresql-template1">
      <Instance "total_backends">
        FailureMax 100.0
      </Instance>
    </Plugin>
 </Host>


It throws no error, but even if i put a FailureMax of 2 or so, alert is not raising.
So, anyone can help me to know to set that alert?

Thanks

Gerardo