Select from PostgreSQL function that returns composite type

Postgres 9.3 or later

Use a LATERAL join!

SELECT p.id, p.name, p.data, f.*FROM   tb_projects pLEFT   JOIN LATERAL fn_getlinkedproject(p.id) f(linked_id, lined_name) ON TRUE;

Result:

 id |  data  |  name  | linked_id | linked_name----+--------+--------+-----------+-------------  1 | data_1 | name_1 |         2 | name_2  2 | data_2 | name_2 |         3 | name_3  3 | data_3 | name_3 |         1 | name_1

See:

• What is the difference between LATERAL and a subquery in PostgreSQL?

Postgres 9.2 or older

Inferior for several reasons. Attaching column aliases is not as simple. Rather rename other conflicting names:

SELECT p.id AS p_id, p.data AS p_data, p.name AS p_name, (fn_getlinkedproject(p.id)).*FROM   tb_projects p;

Result:

 p_id | p_data | p_name | id |  name------+--------+--------+----+--------    1 | data_1 | name_1 |  2 | name_2    2 | data_2 | name_2 |  3 | name_3    3 | data_3 | name_3 |  1 | name_1

To rename the result columns, you have to:

SELECT p.id, p.data, p.name     ,(fn_getlinkedproject(p.id)).id   AS linked_id     ,(fn_getlinkedproject(p.id)).name AS linked_nameFROM   tb_projects p;

Both old solutions resolve to the same (poor) query plan of calling the function repeatedly.

To avoid that, use a subquery:

SELECT p.id, p.data, p.name    , (p.x).id AS linked_id, (p.x).name AS linked_nameFROM  (SELECT *, fn_getlinkedproject(id) AS x FROM tb_projects) p;

Note the placement of essential parentheses.
Read the manual about composite types.

Demo

CREATE TYPE dm_nameid AS (  id   int, name text); -- types simplified for demoCREATE TABLE tb_projects(  id   int, data text, name text);INSERT INTO tb_projects VALUES  (1, 'data_1', 'name_1'), (2, 'data_2', 'name_2'), (3, 'data_3', 'name_3');CREATE function fn_getlinkedproject(integer)  -- avoiding CaMeL-case for demo  RETURNS dm_nameid LANGUAGE sql AS'SELECT id, name FROM tb_projects WHERE id = ($1 % 3) + 1';

db<>fiddle here