Select datatype of the field in postgres

You can get data types from the information_schema (8.4 docs referenced here, but this is not a new feature):

=# select column_name, data_type from information_schema.columns
-# where table_name="config";
    column_name     | data_type 
--------------------+-----------
 id                 | integer
 default_printer_id | integer
 master_host_enable | boolean
(3 rows)