optimize sql query to get tags related to runners

The query generated by ActsAsTaggableOn `@taggable_type.all_tags` is
very inefficient (joins too much, grouping, inner select, ...).

app/finders/autocomplete/acts_as_taggable_on/tags_finder.rb

@@ -11,21 +11,31 @@ module Autocomplete
       end
       def execute
-        @tags = @taggable_type.all_tags
+        @tags = ::ActsAsTaggableOn::Tag.all
+        filter_by_taggable_type!
         search!
         limit!
         @tags
       end
+      def filter_by_taggable_type!
+        # rubocop: disable CodeReuse/ActiveRecord
+        @tags = @tags
+          .joins(:taggings)
+          .where(taggings: { taggable_type: @taggable_type.name })
+          .distinct
+        # rubocop: enable CodeReuse/ActiveRecord
+      end
+
       def search!
         search = @params[:search]
         return unless search
         if search.empty?
-          @tags = @taggable_type.none
+          @tags = ::ActsAsTaggableOn::Tag.none
           return
         end