Question

I would like to replace some of the sequences I use for id’s in my postgresql db with my own custom made id generator. The generator would produce a random number with a checkdigit at the end. So this:

SELECT nextval('customers')

would be replaced by something like this:

SELECT get_new_rand_id('customer')

The function would then return a numerical value such as: [1-9][0-9]{9} where the last digit is a checksum.

The concerns I have is:

1. How do I make the thing atomic
2. How do I avoid returning the same id twice (this would be caught by trying to insert it into a column with unique constraint but then its to late to I think)
3. Is this a good idea at all?

Note1: I do not want to use uuid since it is to be communicated with customers and 10 digits is far simpler to communicate than the 36 character uuid.

Note2: The function would rarely be called with SELECT get_new_rand_id() but would be assigned as default value on the id-column instead of nextval().

EDIT: Ok, good discussusion below! Here are some explanation for why:

1. So why would I over-comlicate things this way? The purpouse is to hide the primary key from the customers.

   I give each new customer a unique
   customerId (generated serial number in
   the db). Since I communicate that
   number with the customer it is a
   fairly simple task for my competitors
   to monitor my business (there are
   other numbers such as invoice nr and
   order nr that have the same
   properties). It is this monitoring I
   would like to make a little bit
   harder (note: not impossible but
   harder).

2. Why the check digit?

   Before there was any talk of hiding the serial nr I added a checkdigit to ordernr since there were klumbsy fingers at some points in the production, and my thought was that this would be a good practice to keep in the future.

After reading the discussion I can certainly see that my approach is not the best way to solve my problem, but I have no other good idea of how to solve it, so please help me out here.

1. Should I add an extra column where I put the id I expose to the customer and keep the serial as primary key?
2. How can I generate the id to expose in a sane and efficient way?
3. Is the checkdigit necessary?

Answer 1

For generating unique and random-looking identifiers from a serial, using ciphers might be a good idea. Since their output is bijective (there is a one-to-one mapping between input and output values) — you will not have any collisions, unlike hashes. Which means your identifiers don’t have to be as long as hashes.

Most cryptographic ciphers work on 64-bit or larger blocks, but the PostgreSQL wiki has an example PL/pgSQL procedure for a “non-cryptographic” cipher function that works on (32-bit) int type. Disclaimer: I have not tried using this function myself.

To use it for your primary keys, run the CREATE FUNCTION call from the wiki page, and then on your empty tables do:

ALTER TABLE foo ALTER COLUMN foo_id SET DEFAULT pseudo_encrypt(nextval('foo_foo_id_seq')::int);

And voila!

pg=> insert into foo (foo_id) values(default);
pg=> insert into foo (foo_id) values(default);
pg=> insert into foo (foo_id) values(default);
pg=> select * from foo;
  foo_id   
------------
 1241588087
 1500453386
 1755259484
(4 rows)