What happens when DB engine runs out of numbers to use for primary keys?

Question

Since DBs do not reuse numbers of deleted records it is possible to run out of numbers, especially if you pick not really a big integer type for this column.
What would

Answer 1

In Postgres, the "serial" type is equivalent to the creation of a SEQUENCE with the NO CYCLE option, and setting the default of the field to nextval. Exhausting such a sequence produces an error:

http://www.postgresql.org/docs/8.3/interactive/sql-createsequence.html