postgresql - Adding 'serial' to existing column in Postgres

Question

I have a small table (~30 rows) in my Postgres 9.0 database with an integer ID field (the primary key) which currently contains unique sequential integers starting at 1, but which was not created using the 'serial' keyword.

How can I alter this table such that from now on inserts to this table will cause this field to behave as if it had been created with 'serial' as a type?

Answer 1

Look at the following commands (especially the commented block).

DROP TABLE foo;
DROP TABLE bar;

CREATE TABLE foo (a int, b text);
CREATE TABLE bar (a serial, b text);

INSERT INTO foo (a, b) SELECT i, 'foo ' || i::text FROM generate_series(1, 5) i;
INSERT INTO bar (b) SELECT 'bar ' || i::text FROM generate_series(1, 5) i;

-- blocks of commands to turn foo into bar
CREATE SEQUENCE foo_a_seq;
ALTER TABLE foo ALTER COLUMN a SET DEFAULT nextval('foo_a_seq');
ALTER TABLE foo ALTER COLUMN a SET NOT NULL;
ALTER SEQUENCE foo_a_seq OWNED BY foo.a;    -- 8.2 or later

SELECT MAX(a) FROM foo;
SELECT setval('foo_a_seq', 5);  -- replace 5 by SELECT MAX result

INSERT INTO foo (b) VALUES('teste');
INSERT INTO bar (b) VALUES('teste');

SELECT * FROM foo;
SELECT * FROM bar;