The integer64 extension changes the default type used for Integer to be the same type as used for :Bignum. In general, this means that instead of Integer resulting in a 32-bit database integer type, it will result in a 64-bit database integer type. This affects the default type used for primary_key and foreign_key when using the schema modification methods.
Note that it doesn't make sense to use this extension on SQLite, since the integer type will automatically handle 64-bit integers, and it treats the integer type specially when the column is also the primary key.
To load the extension into the database: