Unknown returned type in PostgreSQL query

It is ugly, but you can try:

SELECT a, b::text
FROM unnest(ARRAY[(1,'hello'), (3,'world')])
AS t(a integer, b unknown);

This way the type defined in AS matches the output of unnest(), which you can cast to your needs in the SELECT list.

You can try this in a small SQLFiddle.

You can do this without generating a warning by creating a type and casting the records to it:

create type t as (a integer, b varchar(255));

select * from unnest(array[(1,'hello'), (3,'world')]::t[]);
┌───┬───────┐
│ a │   b   │
├───┼───────┤
│ 1 │ hello │
│ 3 │ world │
└───┴───────┘

_{tested on 9.4 and 9.3 (db<>fiddle here)}