If you have some sort of non-production version of an application running on
Heroku, you may encounter a time when you need to wipe your database and
start fresh. For a rails project, it may be tempting to do heroku run rake db:drop db:setup. Heroku doesn't want you to accidentally do anything
stupid, so it prevents you from running rake db:drop. Instead, you must
send a more explicit command
$ heroku pg:reset DATABASE_URL
Heroku will ask you to confirm that you indeed want to wipe out the database and will then proceed.
For the curious reader, running heroku config will list the values of a
number of variables including DATABASE_URL.