CTE upsert for postgresql

pg_cte_upsert.txt

db=> create table what(id serial primary key, v text);
CREATE TABLE
db=> with upsert as (update what set v='updated value' where id=42 returning *) insert into what(id, v) select 42, 'inserted value' where not exists (select * from upsert);
INSERT 0 1
db=> select * from what;
id |       v
----+----------------
42 | inserted value
(1 row)

db=> with upsert as (update what set v='updated value' where id=42 returning *) insert into what(id, v) select 42, 'inserted value' where not exists (select * from upsert);
INSERT 0 0
db=> select * from what;
id |       v
----+---------------
42 | updated value
(1 row)