MongoDB Query Planner

SQL databases use query planners (often cost-based optimizers) so developers don’t worry about physical data access. Many NoSQL systems like DynamoDB and Redis drop this layer, making developers act as the query planner by querying indexes directly. MongoDB keeps a query planner—an empirical, trial-based multi-planner—that chooses the best index and reuses the winning plan until it’s no longer optimal. Here is how it works: