EnsAlertRequest Summary Queries timing out /

Off the back of the Interface Monitoring post I had created a class that queries the Ens.AlertRequest global and returns the entries between 6pm the night before and 6am in the morning.   

I tested this build in our T&D environments and the build worked very well.

However in our production environment the query is being truncated, by what I believe to be a timeout and I get a partial query output.

In the System>SQL pages my 12 hour query times out.

I compared the Global size by running a SELECT MAX(ID) query and got a return of 60,244,962 records.  This may go some way to explain why the query is taking longer and possibly timing out.

In our T&D environments (which we don't purge), we have only 1 million and 2.5 million records.

For messages we in production we have a 90 day purge limit.

I wonder if the Ens.AlertRequest global isn't being purged and if so why it isn't as this global is considered to be a message in my mind.

Any advice please.

Regards

Stuart