ERROR: permission denied for relation tablename on Postgres while trying a SELECT as a readonly user

Question

GRANT SELECT ON ALL TABLES IN SCHEMA public TO readonly;

The readonly user can connect, see the tables but when it tries to do a simple select it g

Answer 1

Try to add

GRANT USAGE ON SCHEMA public to readonly;

You probably were not aware that one needs to have the requisite permissions to a schema, in order to use objects in the schema.