Queries can be executed in many different ways. All paths lead to the same query result. The Query optimizer evaluates the possibilities and selects the efficient plan. Efficiency is measured in latency and throughput, depending on the workload. The cost of Memory, CPU, disk usage is added to the cost of a plan in a cost-based optimizer.

Now, most NoSQL databases have SQL-like query language support. So, a good optimizer is mandatory. When you don't have a good optimizer, developers have to live with feature restrictions and DBAs have to live with performance issues.