How to reset Postgres' primary key sequence when it falls out of sync?

Question

I ran into the problem that my primary key sequence is not in sync with my table rows.

That is, when I insert a new row I get a duplicate key error because the sequence implied in the serial datatype returns a number that already exists.

It seems to be caused by import/restores not maintaining the sequence properly.

Answer 1

-- Login to psql and run the following

-- What is the result?
SELECT MAX(id) FROM your_table;

-- Then run...
-- This should be higher than the last result.
SELECT nextval('your_table_id_seq');

-- If it's not higher... run this set the sequence last to your highest id. 
-- (wise to run a quick pg_dump first...)

BEGIN;
-- protect against concurrent inserts while you update the counter
LOCK TABLE your_table IN EXCLUSIVE MODE;
-- Update the sequence
SELECT setval('your_table_id_seq',(SELECT GREATEST(MAX(your_id), nextval('your_table_id_seq')-1) FROM your_table))
COMMIT;

Source - Ruby Forum