Dependent SERIAL column in PostgreSQL - sql

Question

I want to get this result in my table contacts:
|contact_id | user_id | user_contact_id |
+-----------+------------------+----------------------+
| 1 | 1 | 1 |
+-----------+------------------+----------------------+
| 2 | 1 | 2 |
+-----------+------------------+----------------------+
| 3 | 1 | 3 |
+-----------+------------------+----------------------+
| 4 | 2 | 1 |
+-----------+------------------+----------------------+
| 5 | 2 | 2 |
+-----------+------------------+----------------------+
| 6 | 2 | 3 |
+-----------+------------------+----------------------+
| 7 | 3 | 1 |
+-----------+------------------+----------------------+
I'm going to insert only user_id.
INSERT INTO contacts (user_id) VALUES ($user_id);
The contact_id will auto-increment because it's a serial. I want user_contact_id to also populate automatically by the DB itself, so it is 100% stable with concurrent writes.

Answer 1

As other users suggested only sequence-s or serial type are guaranteed to be concurrent safe.
If you really need to have user_contact_id "restarted" every user_id maybe you could use following view:
create view contacts_v as
select
contact_id,
user_id,
rank() over (partition by user_id order by contact_id) as user_contact_id
from contacts;

Answer 2

INSERT INTO CONTACTS
(user_id, user_contact_id)
VALUES
($user_id, (SELECT COALESCE(MAX(user_contact_id), 0) + 1 FROM CONTACTS WHERE user_id = $user_id))

Answer 3

You should use a sequence as a default value for your user_contact_id. And this is exactly what the SERIAL column type is doing.
http://www.postgresql.org/docs/9.1/static/datatype-numeric.html
http://www.postgresql.org/docs/9.4/static/sql-createsequence.html
Sequences are safe with concurrent writes.
CREATE TABLE contacts (
contact_id SERIAL PRIMARY KEY,
user_id INTEGER,
user_contact_id SERIAL
);
INSERT INTO contacts (user_id) VALUES
(1), (1), (1), (2), (2), (2), (3);
And here are the results:
> SELECT * FROM contacts;
contact_id | user_id | user_contact_id
------------+---------+-----------------
1 | 1 | 1
2 | 1 | 2
3 | 1 | 3
4 | 2 | 4
5 | 2 | 5
6 | 2 | 6
7 | 3 | 7
> \d contacts
Table "public.contacts"
Column | Type | Modifiers
-----------------+---------+--------------------------------------------------------------------
contact_id | integer | not null default nextval('contacts_contact_id_seq'::regclass)
user_id | integer |
user_contact_id | integer | not null default nextval('contacts_user_contact_id_seq'::regclass)
Indexes:
"contacts_pkey" PRIMARY KEY, btree (contact_id)