Create PostgreSQL ROLE (user) if it doesn't exist

Question

How do I write an SQL script to create a ROLE in PostgreSQL 9.1, but without raising an error if it already exists?

The current script simply has:

CR         


        

Answer 1

The accepted answer suffers from a race condition if two such scripts are executed concurrently on the same Postgres cluster (DB server), as is common in continuous-integration environments.

It's generally safer to try to create the role and gracefully deal with problems when creating it:

DO $$
BEGIN
  CREATE ROLE my_role WITH NOLOGIN;
  EXCEPTION WHEN DUPLICATE_OBJECT THEN
  RAISE NOTICE 'not creating role my_role -- it already exists';
END
$$;