From “How many documents are in my collection?” to “What state has the highest percentage of people living in its most populous city?”, there are many questions MongoDB can answer about your data. In this talk, we’ll see what sorts of questions can be asked, and how MongoDB finds the most efficient way to answer them. Determining the best way to answer a query is a challenging and complex subject. We’ll show how to express your questions using MongoDB’s query and aggregation language, then explore how MongoDB considers the various options and attempts to choose the most efficient way to answer these queries. We’ll show some of the interesting optimizations MongoDB takes advantage of and explore how we might be able to improve performance in the future.