Without the cast, the return type could be long or long long,
depending on what int64 is underneath. This doesn't affect code
correctness, but it could result in format-mismatch warnings when
attempting to printf such values using PRId64.
Reported-by: Thomas Munro <thomas.munro@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/CA+hUKGJc4s+Wyb3EFOQNN9VVK+Qv40r2LK41o9PkS9ThxviTvQ@mail.gmail.com