Trying to understand pg_get_expr()

Given:

select version();
version

-----------------------------------------------
PostgreSQL 17.9 (Ubuntu 17.9-1.pgdg24.04+1)

and:

CREATE TABLE default_test (
id integer,
fld_1 varchar DEFAULT 'test',
fld_2 integer DEFAULT 0
);

Then:

SELECT
adrelid::regclass,
pg_typeof(pg_get_expr(adbin, adrelid)),
pg_get_expr(adbin, adrelid)
FROM
pg_attrdef
WHERE
adrelid = 'default_test'::regclass;

adrelid | pg_typeof | pg_get_expr
--------------+-----------+---------------------------
default_test | text | 'test'::character varying
default_test | text | 0

and:

SELECT
adrelid::regclass,
pg_typeof(pg_get_expr(adbin, adrelid)),
pg_get_expr(adbin, adrelid)
FROM
pg_attrdef
WHERE
adrelid = 'default_test'::regclass
AND pg_get_expr(adbin, adrelid) = '0';

adrelid | pg_typeof | pg_get_expr
--------------+-----------+-------------
default_test | text | 0

SELECT
adrelid::regclass,
pg_typeof(pg_get_expr(adbin, adrelid)),
pg_get_expr(adbin, adrelid)
FROM
pg_attrdef
WHERE
adrelid = 'default_test'::regclass
AND pg_get_expr(adbin, adrelid) = 'test';

adrelid | pg_typeof | pg_get_expr
---------+-----------+-------------
(0 rows)

Why does the = 'test' not return anything?

--
Adrian Klaver
adrian(dot)klaver(at)aklaver(dot)com