DYNAMIC SNAPSHOT SET ADJUSTMENT

A method and apparatus for determining whether a particular version of a data item may be used by a transaction are provided. To determine whether the particular version may be used by the transaction, a MUST-SEE set associated with the transaction is compared to an EXCLUDED set associated with the particular version, and a CANNOT-SEE set associated with the transaction is compared to an INCLUDED set associated with the particular version. The MUST-SEE set includes all transactions that have made updates that must be seen by the transaction. The EXCLUDED set includes all transactions that have made updates to the data item that have been removed from the particular version of the data item and all transactions that will make changes to the data item in the future. The CANNOT-SEE set includes all transactions that have made updates that cannot be seen by the transaction. The INCLUDED set includes all transactions that have made updates that are reflected in the particular version of the data item. The particular version can be supplied to the transaction if no member of the MUST-SEE set is also a member of the EXCLUDED set, and no member of the CANNOT-SEE set is also a member of the INCLUDED set.