Description of problem:

On large databases, host collection widget takes too long to render. 

We created a test ruby script to simulate and trace the same steps performed by the host collection widget and look at the results:


[root@inhas62145 ~]# cat /tmp/host_collection_widget.rb 
start_time = Time.now
puts "Start time #{start_time}"

User.current = User.anonymous_api_admin

query_start = Time.now
organizations = Organization.current.present? ? [Organization.current] : User.current.allowed_organizations
query_end = Time.now

puts "Organizations query #{query_end - query_start}"
puts "-------------"

query_start = Time.now
critical, warning, ok = Katello::HostCollection.lists_by_updates_needed(organizations)
query_end = Time.now
puts "host_collection list  query #{query_end - query_start}"
puts "-------------"

query_start = Time.now
host_collections = (critical + warning + ok)[0..6]
query_end = Time.now
puts "host_coolections top 0...6 query #{query_end - query_start}"
puts "-------------"

end_time = Time.now
elapsed_time = end_time - start_time
puts "End time #{end_time}"
puts "Elapsed time #{elapsed_time}"
puts "-------------"


irb(main):001:0> load '/tmp/host_collection_widget.rb'
Start time 2016-10-12 14:49:41 -0400
Organizations query 0.003659478
-------------
host_collection list  query 50.057783585
-------------
host_coolections top 0...6 query 4.094e-06
-------------
End time 2016-10-12 14:50:31 -0400
Elapsed time 50.093940405
-------------
=> true
irb(main):002:0> exit


Version-Release number of selected component (if applicable):
6.2.2

How reproducible:
100% with large databases

Steps to Reproduce:
1. Launch dashboard with host_collection widget


Actual results:

 Slow to render widget

Expected results:

 Should return on a reasonable time