PostgreSQL: Auto-increment based on multi-column unique constraint

Question

One of my tables has the following definition:

CREATE TABLE incidents
(
  id serial NOT NULL,
  report integer NOT NULL,
  year integer NOT NULL,
  month int         


        

Answer 1

I think I found better solution. It doesn't depends on grp Type (it can be enum, integer and string) and can be used in a lot of cases.

myFunc() - function for a trigger. You can name it as you want. number - autoincrement column which grows up for each exists value of grp. grp - your column you want to count in number. myTrigger - trigger for your table. myTable - table where you want to make trigger. unique_grp_number_key - unique constraint key. We need make it for unique pair of values: grp and number.

ALTER TABLE "myTable"
    ADD CONSTRAINT "unique_grp_number_key" UNIQUE(grp, number);

CREATE OR REPLACE FUNCTION myFunc() RETURNS trigger AS $body_start$
BEGIN
    SELECT COALESCE(MAX(number) + 1, 1)
        INTO NEW.number
        FROM "myTable"
        WHERE grp = NEW.grp;
    RETURN NEW;
END;
$body_start$ LANGUAGE plpgsql;

CREATE TRIGGER myTrigger BEFORE INSERT ON "myTable"
    FOR EACH ROW
    WHEN (NEW.number IS NULL) 
    EXECUTE PROCEDURE myFunc();

How does it work? When you insert something in myTable, trigger invokes and checks if number field is empty. If it is empty, myFunc() select MAX value of number where grp equals to new grp value which you want to insert. It returns max value + 1 like auto_increment and replaces null number field to new autoincrement value.

This solution is more unique than Denis de Bernardy cause it doesn't depend on grp Type, but thanks to him, his code helps me write my solution. Maybe it's too late to write answer, but i can't found unique solution for this problem in stackoverflow, so it can help someone. Enjoy and thanks for help!