Postgresql: why can't you drop a role with privileges that does not own any ressource? - postgresql

Question

I'm confused by the nomenclature around roles and privileges with postgresql.
Given a stock postgresql install, if I do this:
postgres=# create database foo;
CREATE DATABASE
Now, there is a database.
postgres=# create role bar;
CREATE ROLE
Now, there is a user. I can delete it:
postgres=# drop role bar;
DROP ROLE
And it's gone, and I can recreate it again.
postgres=# create role bar;
CREATE ROLE
postgres=# grant all privileges on database foo to bar;
GRANT
So this role does not own anything. (Since there is nothing to own.)
The database existed before the role. The way I understand it, it's an independent entity.
And you've just "GRANT"-ed the role "bar" to do some things with this database.
Plain english. Pretty logical.
Then you changed your mind, and remove the role:
postgres=# drop role bar;
ERROR: role "bar" cannot be dropped because some objects depend on it
DETAIL: privileges for database foo
"What the what ? What do you mean 'some objects depend on it' ? THERE IS NO OBJECT YET !!!", you ask.
As I understand it, the answer lies here:
postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
--------------+----------+----------+-------------+-------------+---------------------------
foo | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =Tc/postgres +
| | | | | postgres=CTc/postgres +
| | | | | bar=CTc/postgres
So the "ressource" that depends on you is the "privilege" itself.
So you can remove the privilege, and after that, drop the role:
postgres=# revoke all on database foo from bar;
REVOKE
postgres=# \l
List of databases
Name | Owner | Encoding | Collate | Ctype | Access privileges
--------------+----------+----------+-------------+-------------+---------------------------
foo | postgres | UTF8 | en_US.UTF-8 | en_US.UTF-8 | =Tc/postgres +
| | | | | postgres=CTc/postgres
postgres=# drop user bar;
DROP ROLE
So my question is: why ? In this situation, something is preventing postgresql from doing the (obvious / expected / arguably less confusing) thing, and "cascading" the dropping of "privileges", too.
What could it be ?
It might simply be a confusing error message over a n00b issue, but I suspect something more fundamental...