How to specify the primary key number space for a postgres table? I have a postgres table that contains data where the id has been set by the code that injected the data. So lets say the id is 0..49. 

How can I instruct postgres to start with 50 on the id column for the next insert?
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.