Recursive JSON object traversal by PostgreSQL

lisp.json

{"car":"H","cdr":{"car":"e","cdr":{"car":"l","cdr":{"car":"l","cdr":{"car":"o","cdr":{"car":" ","cdr":{"car":"w","cdr":{"car":"o","cdr":{"car":"r","cdr":{"car":"l","cdr":{"car":"d","cdr":{"car":"!","cdr":{}}}}}}}}}}}}}

lisp.sql

drop table lisp;
create table lisp (
  obj jsonb
);
copy lisp
from '/lisp.json'
with csv quote e'\x01' delimiter e'\x02'
;

with recursive eval as (
  select obj->>'car' as car
       , obj->'cdr' as cdr
       , 1 as depth
  from lisp
  union all
  select cdr->>'car' as car
       , cdr->'cdr' as cdr
       , depth + 1 as depth
  from eval
  where (cdr->'cdr') is not null
)
select string_agg(car, '' order by depth) as msg
from eval
;