title: How filtered document list look up work in nearest neighbour search prefiltering
body: <p>In pre-filter based ANN, once we have list of documents after applying pre-filter, vespa starts hsnw algorithm to find nearest neighbours. In hsnw algorithm, vespa starts with a node and look for the neighbours which are present in pre-filter list, How search of neighbour in document list is implemented in vespa ? it's linear search or hashing ?</p>