Creating a new column with SERIAL as data type

Hi group,
 I have an existing table with data loaded. Now I
wanted to create a column with SERIAL data type and
want to make that coulmn values as PRIMARY key. 

I did the following:
friends=> alter table friend  add column fr_id serial;
NOTICE:  ALTER TABLE will create implicit sequence
"friend_fr_id_seq" for "serial" column "friend.fr_id"
ERROR:  adding columns with defaults is not
implemented
HINT:  Add the column, then use ALTER TABLE SET
DEFAULT.

Can any one suggest what is wrong here. 

Thanks
Kumar



		
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