7. A method performed by a computing device with a processor and a memory for identifying objects of a collection that are similar to a target object, the method comprising: for each of a plurality of features, identifying, from a cluster index data structure for that feature, candidate objects that are feature similar to the target object based on that feature, the cluster index data structure defining, for each of the plurality of features of objects, clusters of objects that are feature similar based on that feature, wherein each cluster index data structure provides a separate clustering of the objects in the collection based on a different feature; and for candidate objects, indicating by the processor similarity of the candidate object to the target object based on the features for which the candidate object is feature similar to the target object.
7. A method performed by a computing device with a processor and a memory for identifying objects of a collection that are similar to a target object, the method comprising: for each of a plurality of features, identifying, from a cluster index data structure for that feature, candidate objects that are feature similar to the target object based on that feature, the cluster index data structure defining, for each of the plurality of features of objects, clusters of objects that are feature similar based on that feature, wherein each cluster index data structure provides a separate clustering of the objects in the collection based on a different feature; and for candidate objects, indicating by the processor similarity of the candidate object to the target object based on the features for which the candidate object is feature similar to the target object. 11. The method of claim 7 wherein each feature represents a characteristic of the object.