Postgres 11

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create function f() returns text language sql as $$
  SELECT string_agg (substr('abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ceil (random() * 62)::integer, 1), '')
  FROM generate_series(1, 20)
$$;

CREATE FUNCTION

create table foo(id serial primary key, url_prefix text not null default f());

CREATE TABLE

insert into foo default values;

INSERT 0 1

select * from foo;

id
url_prefix

1
I4Dnf6aI259UW3KHPBrd

SELECT 1