1. A computer system in which a plurality of transactions are concurrently executing, said computer system comprising: memory containing an index that corresponds to a set of data to which a uniqueness constraint applies, said index containing a plurality of entries, each entry of said plurality of entries being associated with an occurrence of a data value in said set of data; and a mechanism for preventing more than one transaction from concurrently modifying index entries associated with different occurrences of the same data value by causing all transactions to obtain exclusive ownership of one invariant resource associated with said data value prior to modifying an index entry associated with an occurrence of the data value, wherein said invariant resource is a slot that would contain a particular fake entry that is associated with the data value.