Changing primary key int type to serial

Converting an int to a serial more or less only means adding a sequence default to the value, so to make it a serial;

• Pick a starting value for the serial, greater than any existing value in the table
  SELECT MAX(id)+1 FROM mytable

• Create a sequence for the serial (tablename_columnname_seq is a good name)
  CREATE SEQUENCE test_id_seq MINVALUE 3 (assuming you want to start at 3)

• Alter the default of the column to use the sequence
  ALTER TABLE test ALTER id SET DEFAULT nextval('test_id_seq')

• Alter the sequence to be owned by the table/column;
  ALTER SEQUENCE test_id_seq OWNED BY test.id

A very simple SQLfiddle demo.

And as always, make a habit of running a full backup before running altering SQL queries from random people on the Internet ;-)

-- temp schema for testing-- ----------------------------DROP SCHEMA tmp CASCADE;CREATE SCHEMA tmp ;SET search_path=tmp;CREATE TABLE bagger        ( id  INTEGER NOT NULL PRIMARY KEY        , tralala varchar        );INSERT INTO bagger(id,tralala)SELECT gs, 'zzz_' || gs::textFROM generate_series(1,100) gs        ;DELETE FROM bagger WHERE random() <0.9;-- SELECT * FROM bagger;        -- CREATE A sequence and tie it to bagger.id        -- -------------------------------------------CREATE SEQUENCE bagger_id_seq;ALTER TABLE bagger        ALTER COLUMN id SET NOT NULL        , ALTER COLUMN id SET DEFAULT nextval('player_id_seq')        ;ALTER SEQUENCE bagger_id_seq        OWNED BY bagger.id        ;SELECT setval('bagger_id_seq', MAX(ba.id))FROM bagger ba        ;     -- Check the result     -- ------------------SELECT * FROM bagger;\d bagger\d bagger_id_seq