PERF: Add index on group to category_groups (#8231)

app/models/category_group.rb

@@ -23,3 +23,7 @@ end
 #  updated_at      :datetime         not null
 #  permission_type :integer          default(1)
 #
+# Indexes
+#
+#  index_category_groups_on_group_id  (group_id)
+#

db/migrate/20191022161944_add_index_on_group_to_category_groups.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddIndexOnGroupToCategoryGroups < ActiveRecord::Migration[6.0]
+  def change
+    add_index :category_groups, :group_id
+  end
+end