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# Historical Session Limitations

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The `HistoryPolicy` provides a very flexible means of accommodating a wide variety of historical schemas. However, be aware of the following restrictions:

- You cannot use the `HistoryPolicy`, if your design combines both current and historical data in a single schema.

- You cannot use historical sessions, nor historical queries, with EJB entity beans.

- TopLink assumes that the current version of an object corresponds to the row in the historical table whose row end field is `NULL`.

- You cannot directly map the start and end fields of a history table because they do not exist in the regular schema.

- You cannot query on ranges of historical objects, only as of a specific point in time.