Advanced MongoDB Interview Questions

Design an Aggregation Pipeline

Create an aggregation pipeline that calculates the average order amount per customer for orders placed in the last month.

Expected output: A pipeline array with $match and $group stages returning average order amounts.

Hint: Use $match to filter by date, then $group to calculate averages.

Analyze Index Usage

Use explain() on a query to determine if it uses an index and suggest improvements.

Expected output: Explanation of index usage and recommendations for index creation.

Hint: Look for COLLSCAN in the explain output indicating a collection scan.

A replica set provides data redundancy and high availability by replicating data across multiple nodes, while a sharded cluster distributes data across multiple servers to scale horizontally and handle large datasets.

MongoDB's aggregation framework uses a pipeline of stages allowing complex transformations, filtering, grouping, and joining, whereas SQL GROUP BY is limited to grouping and aggregation functions in a single query.

Choose a shard key that provides high cardinality, evenly distributes data, supports your query patterns, and avoids creating hotspots to ensure balanced load across shards.