Select from PostgreSQL function that returns composite type

Question

How to include a function that returns a composite type in a SELECT?
I have composite type:

CREATE TYPE public.dm_nameid AS (
  id   public.dm_int,
  name public.dm_str
);

Also, I have a function that returns this type fn_GetLinkedProject(integer). And I need to make something like this:

SELECT 
    p.id, p.data, p.name, 
    pl.id linked_id, pl.name linked_name
FROM tb_projects p
   left join "fn_GetLinkedProject"(p.id) pl

How can I do this?

I have read this article.

I don't want following method:

SELECT
 p.id, p.data, p.name, 
    (select pl1.id from "fn_GetLinkedProject"(p.id) pl1 ) linked_id,
    (select pl2.name from "fn_GetLinkedProject"(p.id) pl2 ) linked_name
FROM tb_projects p

Answer 1

Postgres 9.3 or later

Use a LATERAL join!

SELECT p.id, p.name, p.data, f.*
FROM   tb_projects p
LEFT   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_name
FROM   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_name
FROM  (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 demo

CREATE 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