postgresql - How can I generate a unique string per record in a table in Postgres?

Question

Say I have a table like posts, which has typical columns like id, body, created_at. I'd like to generate a unique string with the creation of each post, for use in something like a url shortener. So maybe a 10-character alphanumeric string. It needs to be unique within the table, just like a primary key.

Ideally there would be a way for Postgres to handle both of these concerns:

1. generate the string
2. ensure its uniqueness

And they must go hand-in-hand, because my goal is to not have to worry about any uniqueness-enforcing code in my application.

Answer 1

I don't claim the following is efficient, but it is how we have done this sort of thing in the past.

CREATE FUNCTION make_uid() RETURNS text AS $$
DECLARE
    new_uid text;
    done bool;
BEGIN
    done := false;
    WHILE NOT done LOOP
        new_uid := md5(''||now()::text||random()::text);
        done := NOT exists(SELECT 1 FROM my_table WHERE uid=new_uid);
    END LOOP;
    RETURN new_uid;
END;
$$ LANGUAGE PLPGSQL VOLATILE;

make_uid() can be used as the default for a column in my_table. Something like:

ALTER TABLE my_table ADD COLUMN uid text NOT NULL DEFAULT make_uid();

md5(''||now()::text||random()::text) can be adjusted to taste. You could consider encode(...,'base64') except some of the characters used in base-64 are not URL friendly.