Is there a way to do aggregate functions on Google App Engine?

Question

One of the nice things relational databases support are the aggregate functions like count, sum, avg etc. But it seems that if you are using GAE, when inserting or updating a record you must calculate and store the count, sum, avg, etc. values of the whole table. But what if you have many conditional groupings? Given a Person:

class Person {
    @Id
    Integer age;
    String city;
}

If I want

1. the total number of persons and
2. the average age

Is it correct that everytime I create, update or delete a person I should also calculate both aggregates and store them as separate columns in the same table. If I also want the total and average values per city, should I store these values for each city also as separate columns in the same table?

Answer 1

Right: to use GAE storage properly, you need to selectively de-normalize some aspects of your model, keeping "redundant" data which, in a DB in normal form, you'd recompute on the fly, such as aggregates (overall and "grouped by" ones).

However, don't add such fields to the Person table in your case -- that makes little sense! Make another PersonAggregates table with columns such as City (null/missing for overall totals), Count, TotalAges (easier to maintain: compute the average at any time as total divided by count).

Answer 2

For frequently used aggregates the best is to update them on every update/insert/delete.

If you haven't designed such aggregates into your application from the start, you can run a script via Remote DataStore API or set up a server-side cron job that will process all entities and calculate the aggregates. It is fairly easy, just keep in mind per-request CPU quota.