Surrogate keys are used to implement the primary keys of almost all dimension tables. In addition, single column surrogate fact keys can be useful, albeit not required. Fact table surrogate keys, which are not associated with any dimension, are assigned sequentially during the ETL load process and are used 1) as the single column primary key of the fact table; 2) to serve as an immediate identiļ¬er of a fact table row without navigating multiple dimensions for ETL purposes; 3) to allow an interrupted load process to either back out or resume; 4) to allow fact table update operations to be decomposed into less risky inserts plus deletes.