How to set postgres pg_class reltablespace to the actual value, not 0?

Question

I need pg_class.reltablespace to be the actual oid of the default tablespace instead of 0. For the existing and future new tables. I know this is per design but I need the actual oid for an automation.

Ex: This table is on the default namespace

# select relname,reltablespace from pg_class where relname = '<my table>'   ;
      relname       | reltablespace 
--------------------+---------------
 <my table>         |             0

In my case it need to be "1663":

# SELECT oid, spcname FROM pg_tablespace where spcname = 'pg_default';
 oid  |  spcname   
------+------------
 1663 | pg_default

Is there a config to change this behavior? Or some modification to the query that gets the tablespace oid

Answer 1

If the relation's tablespace is 0, that means that the table resides in the default tablespace of the database:

SELECT t.oid::regclass,
       CASE WHEN t.reltablespace <> 0
            THEN t.reltablespace
            ELSE d.dattablespace
       END
FROM pg_class AS t
   CROSS JOIN pg_database AS d
WHERE d.datname = current_database()
  AND t.relname = 'mytable';