Page History

...

1. The number or size of records is very large, but the join cardinality is low: In this scenario, some users may prefer to have an on-disk table-store with an in-memory subscription-store.
2. The number or size of records in each partition of both tables is small(ish): In this scenario, a user may prefer to have an in-memory subscription-store. To my understanding, in the worst case scenario, the The cardinality of a subscription-store keys is the cardinality of FK from the specified input table. In this case, even if the cardinality is high (on an all distinct 1-1 mapping) the number of entries in a subscription-store will still be small. One row will always map to only one Foreign Key.
3. The user might want a different type (or differently configured) store for the subscription-store: This would be for users fine-tuning the access pattern of the store.

...