Introducing Dragonfly Cloud! Learn More

Question: How does MongoDBs ObjectId affect performance?


MongoDB uses the ObjectId type for its default _id field on documents, which serves as a unique identifier for each document in a collection. Understanding how ObjectId affects performance involves considering its structure, indexing, and comparison speed.

Structure of ObjectId

An ObjectId is a 12-byte BSON type, consisting of:

  • A 4-byte timestamp value, representing the ObjectId's creation, measured in seconds since the Unix epoch.
  • A 5-byte random value generated once per process. This random value is unique to the machine and process.
  • A 3-byte incrementing counter, initialized to a random value.

Performance Considerations

Index Efficiency

MongoDB automatically creates a unique index on the _id field for every collection. The structured nature of ObjectId allows these indexes to be highly efficient. Since part of the ObjectId is a timestamp, it has a roughly increasing order, which is beneficial for index insertion performance as new documents tend to be added at the end of the index.

Shard Key Potential

For sharded clusters, choosing a shard key is crucial for maintaining balance across shards and ensuring efficient query routing. While using ObjectId as a shard key isn't always recommended due to potential hotspotting (since newer IDs are always increasing), its structure can still be advantageous for certain workloads that benefit from chronological ordering.

Query Performance

Queries using the _id field are generally very fast, thanks to the automatic indexing. However, the efficiency of queries also depends on how well the overall database schema and indexing strategy align with the application's access patterns.

Storage Consideration

The size of the ObjectId is relatively small (12 bytes), but in collections with billions of documents, every byte counts towards storage and memory usage. It's important to consider this when designing your data model, especially if an alternative, smaller type of unique identifier could serve the same purpose without compromising uniqueness or performance.


In summary, MongoDB's use of ObjectId has several implications for performance, mainly positive due to its indexing efficiency and the structured nature allowing for potentially optimized query and insert operations. However, like any design choice, it comes with trade-offs that should be evaluated in the context of specific application requirements and data access patterns.

Was this content helpful?

White Paper

Free System Design on AWS E-Book

Download this early release of O'Reilly's latest cloud infrastructure e-book: System Design on AWS.

Free System Design on AWS E-Book

Start building today 

Dragonfly is fully compatible with the Redis ecosystem and requires no code changes to implement.