Generating incremental numbers based on a different column - postgresql

Question

I have got a composite primary key in a table in PostgreSQL (I am using pgAdmin4)
Let's call the the two primary keys productno and version.
version represents the version of productno.
So if I create a new dataset, then it needs to be checked if a dataset with this productno already exists.
If productno doesn't exist yet, then version should be (version) 1
If productno exists once, then version should be 2
If productno exists twice, then version should be 3
... and so on
So that we get something like:
productno | version
-----|-----------
1 | 1
1 | 2
1 | 3
2 | 1
2 | 2
I found a quite similar problem: auto increment on composite primary key
But I can't use this solution because PostgreSQL syntax is obviously a bit different - so tried a lot around with functions and triggers but couldn't figure out the right way to do it.

Answer 1

You can keep the version numbers in a separate table (one for each "base PK" value). That is way more efficient than doing a max() + 1 on every insert and has the additional benefit that it's safe for concurrent transactions.
So first we need a table that keeps track of the version numbers:
create table version_counter
(
product_no integer primary key,
version_nr integer not null
);
Then we create a function that increments the version for a given product_no and returns that new version number:
create function next_version(p_product_no int)
returns integer
as
$$
insert into version_counter (product_no, version_nr)
values (p_product_no, 1)
on conflict (product_no)
do update
set version_nr = version_counter.version_nr + 1
returning version_nr;
$$
language sql
volatile;
The trick here is the the insert on conflict which increments an existing value or inserts a new row if the passed product_no does not yet exists.
For the product table:
create table product
(
product_no integer not null,
version_nr integer not null,
created_at timestamp default clock_timestamp(),
primary key (product_no, version_nr)
);
then create a trigger:
create function increment_version()
returns trigger
as
$$
begin
new.version_nr := next_version(new.product_no);
return new;
end;
$$
language plpgsql;
create trigger base_table_insert_trigger
before insert on product
for each row
execute procedure increment_version();
This is safe for concurrent transactions because the row in version_counter will be locked for that product_no until the transaction inserting the row into the product table is committed - which will commit the change to the version_counter table as well (and free the lock on that row).
If two concurrent transactions insert the same value for product_no, one of them will wait until the other finishes.
If two concurrent transactions insert different values for product_no, they can work without having to wait for the other.
If we then insert these rows:
insert into product (product_no) values (1);
insert into product (product_no) values (2);
insert into product (product_no) values (3);
insert into product (product_no) values (1);
insert into product (product_no) values (3);
insert into product (product_no) values (2);
The product table looks like this:
select *
from product
order by product_no, version_nr;
product_no | version_nr | created_at
-----------+------------+------------------------
1 | 1 | 2019-08-23 10:50:57.880
1 | 2 | 2019-08-23 10:50:57.947
2 | 1 | 2019-08-23 10:50:57.899
2 | 2 | 2019-08-23 10:50:57.989
3 | 1 | 2019-08-23 10:50:57.926
3 | 2 | 2019-08-23 10:50:57.966
Online example: https://rextester.com/CULK95702

Answer 2

You can do it like this:
-- Check if pk exists
SELECT pk INTO temp_pk FROM table a WHERE a.pk = v_pk1;
-- If exists, inserts it
IF temp_pk IS NOT NULL THEN
INSERT INTO table(pk, versionpk) VALUES (v_pk1, temp_pk);
END IF;

Answer 3

So - I got it work now
So if you want a column to update depending on another column in pg sql - have a look at this:
This is the function I use:
CREATE FUNCTION public.testfunction()
RETURNS trigger
LANGUAGE 'plpgsql'
COST 100
VOLATILE NOT LEAKPROOF
AS $BODY$
DECLARE v_productno INTEGER := NEW.productno;
BEGIN
IF NOT EXISTS (SELECT *
FROM testtable
WHERE productno = v_productno)
THEN
NEW.version := 1;
ELSE
NEW.version := (SELECT MAX(testtable.version)+1
FROM testtable
WHERE testtable.productno = v_productno);
END IF;
RETURN NEW;
END;
$BODY$;
And this is the trigger that runs the function:
CREATE TRIGGER testtrigger
BEFORE INSERT
ON public.testtable
FOR EACH ROW
EXECUTE PROCEDURE public.testfunction();
Thank you #ChechoCZ, you definetly helped me getting in the right direction.