user creation dumping using trigger and fucntions - postgresql

Question

As PostgreSQL doesn't dump object creation date so I want to manually dump user creation date by using trigger and functions. I have created trigger and functions but it's not working.
CREATE TABLE user_audits (
usesysid INT GENERATED ALWAYS AS IDENTITY,
usename varchar NOT NULL,
created_on TIMESTAMP(6) NOT NULL
);
============================
CREATE OR REPLACE FUNCTION user_creation()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
BEGIN
IF NEW.usename <> OLD.usename THEN
INSERT INTO user_audits(usesysid,usename,created_on)
VALUES(usesysid,usename,now());
END IF;
RETURN NEW;
END;
$$
=================================
CREATE TRIGGER user_creation
BEFORE UPDATE
ON user
FOR EACH ROW
EXECUTE PROCEDURE user_creation();
This is important for audit purpose, for now I am using log file to check creation date but it will rotate after sometime.
Please suggest the better way to dump user creation date in table so that I can retrieve the information anytime.
Thanks

Answer 1

I created a similar excercise with the following tables:
The user_tbl table having only a identity column usersysid and the username
CREATE TABLE user_tbl (
usersysid INT GENERATED ALWAYS AS IDENTITY,
username varchar NOT NULL
);
The user_audits table, slightly modified version of yours: where i added an id identity field. I removed the identity from the usersysid field (since it'll be populated with the one coming from user_tbl)
CREATE TABLE user_audits (
id INT GENERATED ALWAYS AS IDENTITY,
usersysid INT,
username varchar NOT NULL,
created_on TIMESTAMP(6) NOT NULL
);
Now the function, I check if the OLD.username is null, this means that is an insert, if NEW.username <> OLD.username then is an update.
CREATE OR REPLACE FUNCTION user_creation()
RETURNS TRIGGER
LANGUAGE PLPGSQL
AS
$$
BEGIN
IF OLD.username is null OR NEW.username <> OLD.username THEN
INSERT INTO user_audits(usersysid,username,created_on)
VALUES(NEW.usersysid,NEW.username,now());
END IF;
RETURN NEW;
END;
$$
;
And finally the trigger, which is fired both on INSERT or UPDATE
CREATE TRIGGER user_creation
BEFORE INSERT OR UPDATE
ON user_tbl
FOR EACH ROW
EXECUTE PROCEDURE user_creation();
Now if I create two new rows and update one with the following
insert into user_tbl (username) values('Carlo');
insert into user_tbl (username) values('Gianni');
update user_tbl set username='Giorgio' where usersysid=1;
I end up with the user_tbl containing the 2 expected rows
defaultdb=> select * from user_tbl;
usersysid | username
-----------+----------
2 | Gianni
1 | Giorgio
(2 rows)
and the user_audits tables containing 3 rows (2 for the insert + 1 for the update)
defaultdb=> select * from user_audits;
id | usersysid | username | created_on
----+-----------+----------+----------------------------
1 | 1 | Carlo | 2021-06-04 13:57:44.810889
2 | 2 | Gianni | 2021-06-04 13:58:14.680878
3 | 1 | Giorgio | 2021-06-04 13:58:44.702364
(3 rows)