...
| Copying to tmp table | 0.000263 |
| Copying to tmp table | 0.052237 |
| Sorting result | 0.000035 |
...

select `pics`.*, `users`.`username`, `users`.`displayname` from `pics` left join `users` on `users`.`id` = `pics`.`user_id` order by RAND() asc limit 4;

PRIMARY id
UNIQUE  created_at
INDEX   user_id
INDEX   location

PRIMARY id
UNIQUE  username