tappables sort by time query

query.sql

SELECT tappables.id, max(
  case 
  when tappables.type = 'SurveyReceipt' 
    then tappables.created_at 
  when taps.user_id = 2816 
    then taps.created_at 
  else
    taps.skewed_created_at 
  end) as last_tap 
FROM tappables 
left join taps on taps.tappable_id = tappables.id 
where tappables.type != 'SurveyReceipt' 
group by tappables.id 
order by last_tap desc

table

                                         Table "public.taps"
      Column       |            Type             |                     Modifiers                      
-------------------+-----------------------------+----------------------------------------------------
 id                | integer                     | not null default nextval('taps_id_seq1'::regclass)
 tappable_id       | integer                     | 
 user_id           | integer                     | 
 created_at        | timestamp without time zone | 
 updated_at        | timestamp without time zone | 
 original          | boolean                     | 
 source            | text                        | 
 skewed_created_at | timestamp without time zone | 
Indexes:
    "taps_pkey1" PRIMARY KEY, btree (id)
    "taps_created_at_idx" btree (created_at)
    "taps_skewed_created_at_idx" btree (skewed_created_at)
    "taps_tappable_id_idx" btree (tappable_id)
    "taps_user_id_idx" btree (user_id)
Foreign-key constraints:
    "taps_clip_id_fk" FOREIGN KEY (tappable_id) REFERENCES tappables(id)
    "taps_user_id_fk" FOREIGN KEY (user_id) REFERENCES users(id)