Does Filter Order Affect MongoDB Query Optimization with `bson.D` vs `bson.M`?

Query Optimization with bson.D vs bson.M in MongoDB

When querying MongoDB using the Go package mongo-driver, developers have the option to specify filter criteria using either bson.M or bson.D types. According to the documentation, bson.D should be used for ordered data, while bson.M is suitable for unordered data.

The question arises as to whether the order of the filter elements has any impact on the query plan generated by MongoDB's query optimizer. In traditional SQL databases, the order of query elements generally doesn't affect optimization, as indexing and summary statistics provide guidance for the optimizer.

Can the Same Assumption Be Made for MongoDB?

The answer is yes. The use of ordered vs. unordered filter structures does not interfere with MongoDB's query optimization process. The server is capable of identifying and utilizing appropriate indexes regardless of the filter order. Thus, bson.M can be used without concern.

Ordered vs. Unordered in Different Contexts

While the order is inconsequential for filters, it does hold significance in other contexts:

• Sorting: bson.D is necessary to specify the order of sorting fields.
• Document Insertion: bson.D ensures that the order of fields in the inserted document matches the order specified in the bson.D structure.

Conclusion

For query filters in MongoDB, both bson.M and bson.D are acceptable choices based on preference and clarity. The order of elements is not a factor in query optimization, allowing developers to use the most convenient structure for their specific needs.

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