use custom sql function as default value on non-null field

[ryparker]

I'd like to create a custom SQL function that generates an id in a format such as pc_1Pa04ELJnaZjX5rdwAEd0YMV. Id like to use this as the default value of a id field (type text). Similar to how gen_uuid() can be used for UUID type fields.

I realize that the Hasura docs have some suggested method for this however it requires enabling null values on the field. However I do not want to enable null values on the id field. Is there a way to make custom functions work as defaults just like the gen_uuid() function?

Hasura (OSS) v2

Here's an example of the sql function im trying to use: https://github.com/elitan/postgres-nanoid/blob/main/nanoid.sql

I then create another function that uses nanoid and takes 0 args

CREATE OR REPLACE FUNCTION userid()
    RETURNS text
    LANGUAGE plpgsql
    VOLATILE
    PARALLEL SAFE
    AS $$
BEGIN
    RETURN nanoid('user_', 24);
END
$$;

Id then expect userid() to appear in this list of default functions:

The current guidance found in the docs is not ideal. I don't want to set id to null for data integrity reasons.

[ryparker]

converted to issue #10489