Insert if not exists, else return id in postgresql

Question

I have a simple table in PostgreSQL that has three columns:

• id serial primary key
• key varchar
• value varchar

I have already see

Answer 1

with vals as (
  select 'key5' as key, 'value2' as value
)
insert into Test1 (key, value)
select v.key, v.value
from vals as v
where not exists (select * from Test1 as t where t.key = v.key and t.value = v.value)
returning id

sql fiddle demo