Re: How to eliminate extra "NOT EXISTS"-query here?

Not sure you need to use array why not simple table joins, so a table with your criteria x y z t joined to stuff to give you candidates that do match, then left join with coalesce to add the 'd' 

select

--a.id,b.test_id,

coalesce(a.id,b.test_id) as finalresult

from test a

left join (

select

test_id

from stuff a

inner join (values ('x'),('y'),('z'),('t')) b (v) using(v)

group by 1

)b on(a.id=b.test_id);