add a comment to the usage of a anti-pattern query

app/models/ci/runner.rb

@@ -31,6 +31,12 @@ module Ci
     scope :active, -> { where(active: true) }
     scope :paused, -> { where(active: false) }
     scope :online, -> { where('contacted_at > ?', contact_time_deadline) }
+    # The following query using negation is cheaper than using `contacted_at <= ?`
+    # because there are less runners online than have been created. The
+    # resulting query is quickly finding online ones and then uses the regular
+    # indexed search and rejects the ones that are in the previous set. If we
+    # did `contacted_at <= ?` the query would effectively have to do a seq
+    # scan.
     scope :offline, -> { where.not(id: online) }
     scope :ordered, -> { order(id: :desc) }