Allow a cache index to be define. A cache index allow singleResult queries to obtain a cache hit when querying on the indexed fields. resultList queries cannot obtain cache hits, as it is unknown if all of the objects are in memory, (unless the cache usage query hint is used). The index should be unique, but if not unique, the first indexed object will be returned. Cache indexes are only relevant when caching is enabled. The @CacheIndex can be defined on a Entity class, or on an attribute. The column is defaulted when defined on a attribute.