Lets you define queries over some data set declaratively, and instead of recomputing the query over the entire data set every time you want an updated answer, it uses Differential Dataflow <https://github.com/frankmcsherry/differential-dataflow> to efficiently(^1) calculate the new results by updating the results of the previous query execution in response to new updates to the data set.

^1: I'm not an expert on Differential Dataflow, so I don't know what "efficiently" means in this context, other than "should be faster than running the query from scratch."