Does MongoDB Query Order Affect the Query Plan?

Ordered vs. Unordered Data Structures in MongoDB Find Queries

When using the Find method in MongoDB queries, developers have the option of specifying the filter using either the bson.M or bson.D data structures. As mentioned in the documentation, bson.D should be used if the order of elements matters, while bson.M should be used otherwise.

Query Plan Generation

One common question is whether the choice of ordered (bson.D) or unordered (bson.M) data structures affects the query plan generated by the MongoDB query optimizer.

Optimizer Intelligence

In classic SQL databases, the query order typically doesn't matter due to the optimizer's ability to leverage indexes and summary statistics to optimize execution. Does MongoDB exhibit similar behavior?

MongoDB's Handling of Filter Order

Answer: No, in general, the order of fields in the filter does not affect the query plan generated by the MongoDB optimizer. The MongoDB server can effectively identify and utilize existing indexes regardless of the order in which fields are specified. This applies to both simple and compound indexes. Therefore, you can use bson.M for the filter for simplicity and clarity.

When Order Matters

However, the order of fields does become relevant in specific scenarios:

• Sorting: When specifying sort fields, the order matters because the order of fields in the bson.D structure will determine the sort order of the results.
• Document Insertion: When using bson.D to insert a new document, the order of fields in the structure will be preserved in the inserted document.

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