If a table has no primary key (like the rails schema migrations table)
we should return nil instead of allow an error to be raised. Other
database adapters return nil, and the schema migrations expects it to be
nil in the schema migrations rake tasks.
If a table has no primary key (like the rails schema migrations table) we should return nil instead of allow an error to be raised. Other database adapters return nil, and the schema migrations expects it to be nil in the schema migrations rake tasks.
Here is how the jdbc postgres adapter implements the same method