How to create a scan sequence in the Postgres database?

Question

I have the following table in the database Postgres:

CREATE TABLE customer
(
  userid character varying(30) NOT NULL,
  firstname character varying(30) NOT NULL,
  lastname character varying(30) NOT NULL,
  balance double precision NOT NULL,
  PRIMARY KEY (userid)
)  

It would be possible to create a sequence of the auto increment type but one of the type VARCHAR?

'Cause I got an object of business rule that needs a guy String.

I know the procedure for creating a sequence with the type serial:

CREATE TABLE customer
(
  userid serial NOT NULL,
  ...
  ...
  ...
  PRIMARY KEY (userid)
)    

But like I said before, I need a guy VARCHAR, because the values that will be entered as the primary key will have this form:

id001  
id002  
id003  

Answer 1

With varchar you will have to create a tigger and increase its field.

Example:

CREATE FUNCTION update_transaction_id() RETURNS trigger AS $BODY$
BEGIN
  NEW.userid  :=  'id' || nextval('seq_name');
  RETURN NEW;
END; 
$BODY$ LANGUAGE 'plpgsql';

Whereas seq_name is the name of your Generator. (If it does not exist you must create it)

Create Generator: CREATE SEQUENCE seq_name START 1;

And the Trigger on the table to run Function:

CREATE TRIGGER inc_id
  AFTER INSERT
  ON customer
  FOR EACH ROW
  EXECUTE PROCEDURE update_transaction_id();