Support for aggregating queries

[ak0rz]

Let's think about adding groupBy functionality for query

[incubos]

We'd better start with specific use-cases.

[ak0rz]

We'd better start with specific use-cases.

Use case for groupBy:
Consider you want to count documents for each distinct value of a field in a fetch. Now you may only do it using a Map in a DocumentProcessor to collect all of the documents and process them all. On the other hand, groupBy may process it as distinct fetches for each value of a field using a function as (value, subFetch) -> T.

Next, consider you want to find the maximum value for field field1 on the specific fetch from the database. Now you have the only option - run over all of the documents of fetch and find the maximum. But if we evolve querying API, we may provide a way to find the maximum value of a field in a fetch with only O(log N).