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The flag `optional=True` on `Sequence` will produce a sequence that is only used on databases which have no "autoincrementing" capability. For example, Postgres supports primary key generation using the SERIAL keyword, whereas Oracle has no such capability. Therefore, a `Sequence` placed on a primary key column with `optional=True` will only be used with an Oracle backend but not Postgres.
+A sequence can also be executed standalone, using an `Engine` or `Connection`, returning its next value in a database-independent fashion: