Postgres 14

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE tableList(
    "unique_id" INT GENERATED BY DEFAULT AS IDENTITY (START WITH 4),
    "next_id" int, 
    "randomCol1" text
);
    
INSERT INTO tableList
    ("next_id", "randomCol1")
VALUES
    (5, 'string 1'),
    (6, 'string 2'),
    (0, 'string 3');

SELECT * FROM tableList;

CREATE TABLE



INSERT 0 3




unique_id
next_id
randomCol1


4
5
string 1

5
6
string 2

6
0
string 3



SELECT 3


CREATE OR REPLACE FUNCTION insert_value (
	_unique_id int, _my_string text
)  RETURNS void
AS $body$
DECLARE 
	_next_val int;
	_new_id int;
BEGIN
    SELECT
    	next_id
    FROM tableList
    WHERE unique_id = _unique_id
    INTO
        _next_val;
        
    INSERT INTO tableList("next_id", "randomCol1")
    VALUES (_next_val, _my_string)
    RETURNING unique_id
    INTO _new_id;
    
    UPDATE tableList t
    SET next_id = _new_id
    WHERE unique_id = _unique_id;
END;
$body$ LANGUAGE plpgsql

CREATE FUNCTION


SELECT insert_value(4, 'my_string');

SELECT * FROM tableList


insert_value






SELECT 1




unique_id
next_id
randomCol1


5
6
string 2

6
0
string 3

7
5
my_string

4
7
string 1



SELECT 4