Does the Order of Fields in MongoDB Queries Affect Query Optimization?

Ordered vs Unordered Query Structures in MongoDB

When querying MongoDB collections, developers have two options for specifying filter criteria: bson.M (unordered) and bson.D (ordered). The documentation suggests using bson.D for elements with specified order and bson.M for unordered elements.

However, the question arises whether the use of ordered or unordered structures affects the query plan generated by MongoDB's optimizer. In classic SQL databases, query order often matters, as the optimizer uses indexes and other factors to optimize execution.

This question examines whether the same applies to MongoDB and whether using ordered structures interferes with the optimizer or provides similar functionality to optimizer hints. Additionally, it explores how indexing affects the impact of ordered structures.

Answer:

For filter criteria, the order of fields does not matter. Developers can use bson.M for brevity and clarity. The MongoDB server effectively utilizes indexes regardless of the order specified. Using bson.D or bson.M in filter declarations will yield similar results.

However, for sort criteria, the order matters significantly. Using bson.D is essential when specifying multiple sort fields, as the order in the document will dictate the sorting order.

In terms of document insertion, the order of fields is guaranteed when using bson.D, while it is not in bson.M. This distinction may be important for certain use cases where the order of fields is relevant.

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