Postgres 12

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

select version();

version

PostgreSQL 12.12 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-10), 64-bit

SELECT 1

create or replace function generate_random_id
                    ( lower_value_in bigint default 1  
                    , upper_value_in bigint default 10000000000)
                                                     
   returns bigint
  language sql
  volatile strict 
as $$
    select floor(random()*(upper_value_in-lower_value_in+1)+1)::bigint ;
$$; 

CREATE FUNCTION

create table atable
             ( id bigint primary key
             , ext_id text generated always as  (id::text) stored
             , colx text 
             ) ; 

CREATE TABLE

create or replace function insert_atable(col_x_in atable.colx%type)
   returns void 
  language plpgsql 
as $$
declare
    l_invalid_id boolean := true;
begin 
	while l_invalid_id
	loop
	   begin
		   insert into atable( id, colx)
		     values ( generate_random_id(),col_x_in); 
		   l_invalid_id := false;
	   exception 
	      when unique_violation then null;  	   
	   end;
	end loop;
end;
$$; 

CREATE FUNCTION

with col_t1_list (col1)  as 
     ( values ('lmn'), ('opq'),('rst') )
select insert_atable(col1)
  from col_t1_list; 

insert_atable

SELECT 3

select * from atable;

id
ext_id
colx

64841501
64841501
lmn

5553314550
5553314550
opq

6843167550
6843167550
rst

SELECT 3

id	ext_id	colx
64841501	64841501	lmn
5553314550	5553314550	opq
6843167550	6843167550	rst