Postgres 12

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create table post(
  id    serial primary key,
  title text not null,
  body  text not null,
  created_at timestamp not null 
);

INSERT INTO post VALUES
  (1, 'First Post', 'bla bla', now())
, (2, '2nd Post', 'bla blub', now());

-- for use below
CREATE TYPE foo_type AS (id int, title text);
CREATE TABLE foo_tbl (id int, title text);
CREATE TEMP TABLE foo_tmptbl (id int, title text);
CREATE VIEW foo_view AS SELECT id, title FROM post;
2 rows affected

-- produces anonymous record
SELECT ROW(id, title) AS foo FROM post;

foo

(1,"First Post")

(2,"2nd Post")

-- expansion fails!
SELECT (ROW(id, title)).* FROM post;
ERROR:  record type has not been registered

-- expansion works after cast:
SELECT (ROW(id, title)::foo_type).* FROM post;

id
title

1
First Post

2
2nd Post

-- expansion works for derived table!
SELECT (foo).* FROM (SELECT id, title FROM post) foo;

id
title

1
First Post

2
2nd Post

SELECT ('(1,"First Post")'::foo_type).*;

id
title

1
First Post

SELECT ('(1,"First Post")'::foo_tbl).*;

id
title

1
First Post

SELECT ('(1,"First Post")'::foo_tmptbl).*;

id
title

1
First Post

SELECT ('(1,"First Post")'::foo_view).*;

id
title

1
First Post

-- helper function
CREATE OR REPLACE FUNCTION f_expand_record(anyelement)
  RETURNS record LANGUAGE sql AS
'SELECT $1';
-- now we can provide a target definition list
SELECT * FROM f_expand_record(ROW(1,text 'First Post')) AS t(id int, title text);

id
title

1
First Post

-- notably does NOT work for row literals
SELECT * FROM f_expand_record('(1,"First Post")') AS t(id int, title text);
ERROR:  could not determine polymorphic type because input has type unknown