All those DROP IF EXISTS and CREATE IF NOT EXISTS etc. just give a lot more text to
read and understand. If in fact a user creates the same thing twice, they should be
able to understand the error message from postgres.
The jwt_token type was not created consistently in all examples, which can
be confusing when following those. To return an object with a single key
named token, it's enough to have an OUT parameter to the function.
Resolves https://github.com/PostgREST/postgrest-docs/issues/280
