Why did PostgreSQL merge users and groups into roles?

Question

From the PostgreSQL docs:

The concept of roles subsumes the concepts of \"users\" and \"groups\". In PostgreSQL versions before 8.1, users and group

Answer 1

The merge has many advantages and no disadvantages. For instance, you can now seamlessly convert a "user" to a "group" and vice versa by adding / removing the LOGIN privilege.

ALTER ROLE myrole LOGIN;
ALTER ROLE myrole NOLOGIN;

Or you can GRANT membership in any other login ("user") or non-login role ("group") to a role:

GRANT joe TO sue;

You can still:

CREATE USER james;

That's just a role with login privilege now. Or:

CREATE GROUP workers;

That's effectively the same as CREATE ROLE now.

The manual has it all.