postgresql message_id auto increment for each user - postgresql

Question

I am building chatroom using Go,PostgreSQL.
I have this table -> messages
id | user | message_id | message
I want for each user, message_id Start from 1 and And increase automatically.
for example :
id | user | message_id | message
1 | one | 1 | Hello
2 | two | 1 | Hi!
3 | one | 2 | How are you?
4 | one | 3 | :)
Thanks in advance.

Answer 1

Without duplicate values and to safely increase message_id field we use SELECT FOR UPDATE command.
FOR UPDATE causes the rows retrieved by the SELECT statement to be locked as though for update. This prevents them from being locked, modified or deleted by other transactions until the current transaction ends. That is, other transactions that attempt UPDATE, DELETE, SELECT FOR UPDATE, SELECT FOR NO KEY UPDATE, SELECT FOR SHARE or SELECT FOR KEY SHARE of these rows will be blocked until the current transaction ends; conversely, SELECT FOR UPDATE will wait for a concurrent transaction that has run any of those commands on the same row, and will then lock and return the updated row.
Now let's write a simple function for inserting messages:
CREATE OR REPLACE FUNCTION insert_message(p_user character varying, p_message text)
RETURNS void
LANGUAGE plpgsql
AS $function$
DECLARE
v_new_id integer;
v_id integer;
begin
-- selecting user last inserting message and get last message_id, and blocking this record for update command
select mm.id, mm.message_id into v_id, v_new_id
from messages mm
where
mm."user" = p_user
order by
mm.message_id desc
limit 1
for update;
if (v_new_id is null) then
v_new_id = 0;
end if;
insert into messages
(
"user",
message_id,
message_text
) values
(
p_user,
v_new_id + 1,
p_message
);
-- after inserting message we must unblock record
if (v_new_id > 0) then
update messages
set
updated_date = now()
where id = v_id;
end if;
END;
$function$
;
As you can see from the code for increasing message_id we must filter records by user and then the ordered message_id field descending. Given that in future this table will have million records then performance will be bad. For good performancing let's write new function:
CREATE OR REPLACE FUNCTION insert_message(p_user character varying, p_message text)
RETURNS void
LANGUAGE plpgsql
AS $function$
DECLARE
v_new_id integer;
v_id integer;
begin
-- in users table add new last_id field, for storing last message_id value by user
-- and then block this record
select last_id into v_new_id from users
where "user" = p_user
for update;
insert into messages
(
"user",
message_id,
message_text
) values
(
p_user,
v_new_id + 1,
p_message
);
-- unblocking user record
update users
set
last_id = v_new_id + 1
where "user" = p_user;
END;
$function$
;
This code was simple and high-performance.