Programming Google App Engine

Chapter 5. Datastore Queries

Inevitably, an application that manages data must do more than store and retrieve that data one record at a time. It must also answer questions about that data: which records meet certain criteria, how records compare to one another, what a set of records represents in aggregate. Web applications in particular are expected not only to know the answers to questions about large amounts of data, but to provide them quickly in response to web requests.

Most database systems provide a mechanism for executing queries, and the App Engine datastore is no exception. But App Engine’s technique differs significantly from that of traditional database systems. When the application asks a question, instead of rifling through the original records and performing calculations to determine the answer, App Engine simply finds the answer in a list of possible answers prepared in advance. App Engine can do this because it knows which questions are going to be asked.

This kind of list, or index, is common to many database technologies, and some relational databases can be told to maintain a limited set of indexes to speed up some kinds of queries. But App Engine is different: it maintains an index for every query the application is going to perform. Since the datastore need only do a simple scan of an index for every query, the application gets results back quickly. And for large amounts of data, App Engine can spread the data and the indexes across many machines, and get results ...