Add some note on adding index to HABTM table

activerecord/lib/active_record/associations.rb

@@ -1432,6 +1432,10 @@ module ActiveRecord
       #     end
       #   end
       #
+      # It's also a good idea to add indexes to each of those columns to speed up the joins process.
+      # However, in MySQL it is advised to add a compound index for both of the columns as MySQL only
+      # uses one index per table during the lookup.
+      #
       # Adds the following methods for retrieval and query:
       #
       # [collection(force_reload = false)]