Postgres 12

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CREATE TABLE test_table (
  id     int PRIMARY KEY  --!
, tvalue bool
);

CREATE OR REPLACE FUNCTION test_function(_user_id int)
  RETURNS test_table LANGUAGE sql AS
$func$
   WITH ins AS (
      INSERT INTO test_table(id, tvalue)
      VALUES (_user_id, TRUE) 
      ON CONFLICT DO NOTHING
      RETURNING *
      )
   TABLE ins
   UNION ALL
   SELECT * FROM test_table WHERE id = _user_id
   LIMIT 1
$func$;

INSERT INTO test_table(id, tvalue) VALUES (2, FALSE);  -- pre-existing row
1 rows affected

SELECT * FROM test_function(1);


id
tvalue


1
t


SELECT * FROM test_function(1);


id
tvalue


1
t


SELECT * FROM test_function(2);


id
tvalue


2
f


SELECT * FROM test_function(3);


id
tvalue


3
t


TABLE test_table ORDER BY id;


id
tvalue


1
t

2
f

3
t