How to generate a random, unique, alphanumeric ID of length N in Postgres 9.6+?

Figured this out, here’s a function that does it:

CREATE OR REPLACE FUNCTION generate_uid(size INT) RETURNS TEXT AS $$
DECLARE
  characters TEXT := 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
  bytes BYTEA := gen_random_bytes(size);
  l INT := length(characters);
  i INT := 0;
  output TEXT := '';
BEGIN
  WHILE i < size LOOP
    output := output || substr(characters, get_byte(bytes, i) % l + 1, 1);
    i := i + 1;
  END LOOP;
  RETURN output;
END;
$$ LANGUAGE plpgsql VOLATILE;

And then to run it simply do:

generate_uid(10)
-- '3Rls4DjWxJ'

Warning

When doing this you need to be sure that the length of the IDs you are creating is sufficient to avoid collisions over time as the number of objects you’ve created grows, which can be counter-intuitive because of the Birthday Paradox. So you will likely want a length greater (or much greater) than 10 for any reasonably commonly created object, I just used 10 as a simple example.

Usage

With the function defined, you can use it in a table definition, like so:

CREATE TABLE users (
  id TEXT PRIMARY KEY DEFAULT generate_uid(10),
  name TEXT NOT NULL,
  ...
);

And then when inserting data, like so:

INSERT INTO users (name) VALUES ('ian');
INSERT INTO users (name) VALUES ('victor');
SELECT * FROM users;

It will automatically generate the id values:

    id     |  name  | ...
-----------+--------+-----
owmCAx552Q | ian    |
ZIofD6l3X9 | victor |

Usage with a Prefix

Or maybe you want to add a prefix for convenience when looking at a single ID in the logs or in your debugger (similar to how Stripe does it), like so:

CREATE TABLE users (
  id TEXT PRIMARY KEY DEFAULT ('user_' || generate_uid(10)),
  name TEXT NOT NULL,
  ...
);

INSERT INTO users (name) VALUES ('ian');
INSERT INTO users (name) VALUES ('victor');
SELECT * FROM users;

      id       |  name  | ...
---------------+--------+-----
user_wABNZRD5Zk | ian    |
user_ISzGcTVj8f | victor |