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Primary keys and foreign keys on fact tables

EXPERT RESPONSE FROM: Ian Abramson

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QUESTION POSED ON: 10 November 2003
I've taken over a data warehouse/reporting database. All the fact tables have primary keys and foreign keys. The fact tables have a mixture of measures (inventory, shipments, etc.). Aren't PK and FK unnecessary if all I'm doing for the load is summing from the individual base tables. I would guess all I would need are individual indexes on the keys.

The inclusion of primary and foreign keys should not be seen as an optional add-on to a warehouse, it is a standard option. I understand that there is an impact during the loading of the warehouse, but the potential data cost may be higher at a later time. So I always recommend that all warehouses include referential integrity. At times we disable this during loading cycles, but they are always restored after a load is complete. However, if these constraints are not re-enabled to fail to re-enable, this is a clear indication that your load routines did not perform as expected. Of course the choice is yours, but if this was my warehouse I would recommend that you retain your constraints. As for indexes, you definitiely want to keep these if the data utilizes these indexes during data retrieval.

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