The `summarise_report_metrics` action in `HostsController` calls:

@hosts.map(&:hostgroup_id).compact.uniq

This loads all Host ActiveRecord objects already fetched for the page
into Ruby memory and extracts `hostgroup_id` in application code.

Using `pluck` instead:

@hosts.pluck(:hostgroup_id).compact.uniq

generates a `SELECT hostgroup_id FROM hosts WHERE ...` query that
returns only the needed column, avoiding unnecessary object allocation.
This is measurable on the hosts index page when displaying many hosts.