[postgresql] Select datatype of the field in postgres

The information schema views and pg_typeof() return incomplete type information. Of these answers, psql gives the most precise type information. (The OP might not need such precise information, but should know the limitations.)

create domain test_domain as varchar(15);

create table test (
  test_id test_domain, 
  test_vc varchar(15), 
  test_n numeric(15, 3), 
  big_n bigint,
  ip_addr inet
);

Using psql and \d public.test correctly shows the use of the data type test_domain, the length of varchar(n) columns, and the precision and scale of numeric(p, s) columns.

sandbox=# \d public.test
             Table "public.test"
 Column  |         Type          | Modifiers
---------+-----------------------+-----------
 test_id | test_domain           |
 test_vc | character varying(15) |
 test_n  | numeric(15,3)         |
 big_n   | bigint                |
 ip_addr | inet                  |

This query against an information_schema view does not show the use of test_domain at all. It also doesn't report the details of varchar(n) and numeric(p, s) columns.

select column_name, data_type 
from information_schema.columns 
where table_catalog = 'sandbox'
  and table_schema = 'public'
  and table_name = 'test';

 column_name |     data_type
-------------+-------------------
 test_id     | character varying
 test_vc     | character varying
 test_n      | numeric
 big_n       | bigint
 ip_addr     | inet

You might be able to get all that information by joining other information_schema views, or by querying the system tables directly. psql -E might help with that.

The function pg_typeof() correctly shows the use of test_domain, but doesn't report the details of varchar(n) and numeric(p, s) columns.

select pg_typeof(test_id) as test_id, 
       pg_typeof(test_vc) as test_vc,
       pg_typeof(test_n) as test_n,
       pg_typeof(big_n) as big_n,
       pg_typeof(ip_addr) as ip_addr
from test;

   test_id   |      test_vc      | test_n  | big_n  | ip_addr
-------------+-------------------+---------+--------+---------
 test_domain | character varying | numeric | bigint | inet