How to detect if default value is constant or expression in Postgresql - postgresql

Question

In Postgresql, how can I know default value is constant or expression?
Example:
CREATE TABLE IF NOT EXISTS default_test (
id serial primary key,
name varchar(255) NOT NULL,
father_name varchar(255) DEFAULT 'NULL', -- CONSTANT
nn2 varchar(255) DEFAULT ('NULL'), -- CONSTANT
nn3 varchar(255) DEFAULT (null), -- CONSTANT
nn4 varchar(255) DEFAULT 'the default value', -- CONSTANT
last_name varchar(255) NULL, -- CONSTANT
dt timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, -- EXPRESSION
dt2 timestamp NOT NULL DEFAULT '2011-11-11 00:00:00', -- CONSTANT
ts timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP /*ON UPDATE CURRENT_TIMESTAMP*/, -- EXPRESSION
ts2 timestamp NOT NULL DEFAULT '2011-11-11 00:00:00', -- CONSTANT
date_col date DEFAULT (CURRENT_DATE + INTERVAL '2 YEAR'), -- EXPRESSION
i INT DEFAULT -1, -- CONSTANT
c VARCHAR(10) DEFAULT '', -- CONSTANT
price DOUBLE precision DEFAULT 0.00, -- CONSTANT
-- literal defaults
i2 INT DEFAULT 0, -- CONSTANT
i3 INT DEFAULT 3, -- CONSTANT
pi_val FLOAT DEFAULT 3.14, -- CONSTANT
--
f FLOAT DEFAULT 3.14, -- CONSTANT
d DATE DEFAULT (CURRENT_DATE + INTERVAL '1 YEAR'), -- EXPRESSION
p POINT DEFAULT (Point(0,0)), -- EXPRESSION
j JSON DEFAULT '{"a":"b"}' -- CONSTANT
)
Similar part in MySQL
See https://dev.mysql.com/doc/refman/8.0/en/data-type-defaults.html
Column with default expression have DEFAULT_GENERATED in Extra column in Information Schema
https://dev.mysql.com/doc/refman/8.0/en/information-schema-columns-table.html

Answer 1

Yo can get that info in adbin field from attdef in the pg_catalog
If you have for example:
create table table1(
w_def_expr text default current_user,
w_def_cons text default 'hi',
wo_def text
);
You can get that info with this query:
select adrelid, tab.relname, attname, pg_get_expr(adbin, 0) expr,
split_part(substr(adbin,2),' ',1) expr_type
from pg_attribute col
inner join pg_class tab on col.attrelid = tab.oid
inner join pg_attrdef def on col.attrelid = def.adrelid and col.attnum = def.adnum
where tab.relname='table1';
Getting something like:
The way of getting info in adbin is not standar and can be change in the future.
I didn't find a way to do it better.
Here you can se an answer from 2017 saying that is not possible