Google Cloud Platform for Developers

EntitesByCompositeProperty and Custom Indexes

EntitesByCompositeProperty and the Custom Indexes table are how Datastore implements complex queries based on user-defined indexes. EntitiesByCompositeProperty implements the actual indexing based on the composite key, while Custom Indexes persists information about the composite property indexes.

Indexes involving collection properties such as lists may result in very large numbers of rows in the EntitiesByCompositeProperty table. This is due to the fact that every permutation of the elements within the collection property must be indexed on a separate row, leading to a combination explosion for larger sets. Be careful when considering building indexes on these properties.