Description of problem:
The 'critical updates available' report takes over 15 minutes to run with around 400 systems in a group.

This causes an ISE due to the query taking too long to return.

This is due to a correlated subquery in the "relevant_to_server_group" query.

Version-Release number of selected component (if applicable):
2.6, 2.7

How reproducible:
a. Click on 'Systems'.
b. Click on 'System Groups'.
c. Click on a Group Name that has over 300 entries
d. Click on 'critical updates available'

Actual results:
15+ minute query time

Expected results:
Query response in under 15 seconds

Additional info:
Reproducible on Oracle and PostgreSQL