CREATE SCHEMA test AUTHORIZATION root;
SET search_path = test, pg_catalog;
CREATE TABLE table1 (
    id integer NOT NULL,
    val integer NOT NULL
) WITHOUT OIDS;
SET check_function_bodies = false;
CREATE FUNCTION function1 (integer) RETURNS SETOF table1
    AS '
select * from test.table1 t where t.id = $1
'
    LANGUAGE sql;
INSERT INTO table1 (id, val) VALUES ( 1 ,  10 );
INSERT INTO table1 (id, val) VALUES ( 2 ,  20 );
INSERT INTO table1 (id, val) VALUES ( 3 ,  30 );
ALTER TABLE ONLY table1
    ADD CONSTRAINT table1_pkey PRIMARY KEY (id);
    
EXPLAIN ANALYZE SELECT * FROM test.function1 ( 1 )

Function Scan on function1  (cost= 0 . 00 .. 12 . 50  rows= 1000  width= 8 ) (actual time= 0 . 000 .. 0 . 000  rows= 1  loops= 1 )
Total runtime:  0 . 000  ms