How to specify the primary key number space for a postgres table?
If you want to happen it automatically (I mean, without the application doing that), you have to create a sequence:

CREATE SEQUENCE some_table_pk_seq START 50;


Then change the column to use this as a default:

ALTER TABLE some_table
ALTER COLUMN id SET DEFAULT nextval('some_table_pk_seq'::regclass);


And that's it.  Remember that the default will be used only if you don't pass a value to the given column in your INSERT statements.