$ heroku plugins:install heroku-plugin-psql
$ heroku psql --app example-app
--> Connecting to DATABASE_URL
DATABASE=> help
You are using psql, the command-line interface to PostgreSQL.
Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help with psql commands
       \g or terminate with semicolon to execute query
       \q to quit
DATABASE=> \q
$ heroku psql --help
Like heroku pg:psql but for non-Heroku databases

USAGE
  $ heroku psql [DATABASE]

OPTIONS
  -a, --app=app        (required) app to run command against
  -r, --remote=remote  git remote of app to use
  --command=command    like --command (-c) in psql, runs single command and exits
  --file=file          like --file (-f) in psql, runs commands from a file and exits

EXAMPLE
  $ heroku psql -a sushi
       DATABASE=> 
...