How to update unique values in SQL using a PostgreSQL sequence?

Question

In SQL, how do update a table, setting a column to a different value for each row?

I want to update some rows in a PostgreSQL database, setting one column to a number from a sequence, where that column has a unique constraint. I hoped that I could just use:

update person set unique_number = (select nextval('number_sequence') );

but it seems that nextval is only called once, so the update uses the same number for every row, and I get a 'duplicate key violates unique constraint' error. What should I do instead?

Answer 1

Don't use a subselect, rather use the nextval function directly, like this:

update person set unique_number = nextval('number_sequence');

Answer 2

I consider pg's sequences a hack and signs that incremental integers aren't the best way to key rows. Although pgsql didn't get native support for UUIDs until 8.3

http://www.postgresql.org/docs/8.3/interactive/datatype-uuid.html

The benefits of UUID is that the combination are nearly infinite, unlike a random number which will hit a collision one day.