Autoincrement separately for each foreign key

Question

I have two tables:

 CREATE TABLE "user"
    (
      username character varying(35) NOT NULL,
      CONSTRAINT user_pk PRIMARY KEY (username)
    )
    CREATE TABLE item
    (
      id serial NOT NULL,
      username character varying(35),
      user_item_number integer,
      item_value character varying(35),
      CONSTRAINT item_pk PRIMARY KEY (id),
      CONSTRAINT item_fk FOREIGN KEY (username)
          REFERENCES "user" (username) MATCH SIMPLE
          ON UPDATE NO ACTION ON DELETE NO ACTION,
      CONSTRAINT unique_item_username UNIQUE (username, user_item_number)
    )

I would like to auto increment user_item_number separately for each username. Following image shows example. For each username: (user1, user2) user_item_number starts form 1 and is incremented by 1.

I suppose that I should use some trigger before insert which get max value of user_item_number for username which is inserted and increment it. But I don't know how to write that trigger.

I also don't know how to consider a concurency access (concurency insert more than one row with the same value of username). I don't want to get constraint violation error when two rows with the same username and user_item_number are inserted, I would like that trigger catch that error and one again increment user_item_number value.

Any idea?

Answer 1

It is really hard to generate and maintain such gapless sequence.

the better way to obtain same results is to use window functions to generate such sequences on the fly. Something like:

SELECT id, username, row_number() OVER (PARTITION BY username ORDER BY id) as user_item_number, item_value
from item_table;

It will give you desired results and wont cause any problems with concurrency. Also it will always maintain sequences gapless.

Answer 2

Auto inc should be unique and only one per table.

So based on what you seem to want

User(User_ID PK, ...)
UserItem(User_Item_ID PK, User_ID FK, ...)
UserItemValue(User_Item_Value_ID PK, User_Item_ID FK, ...)

is where you should be heading from a normalisation point of view