bson.D vs. bson.M in MongoDB Find Queries: Does Order Matter for Query Optimization?

bson.D vs bson.M for Find Queries in MongoDB

Overview

In MongoDB, find queries can be constructed using either bson.M (unordered) or bson.D (ordered) documents. The documentation suggests using bson.D when the order of elements matters.

Effects on Query Optimizer

The primary question is whether the use of ordered or unordered structures influences the query plan generated by MongoDB's query optimizer.

Answer

In most cases, using bson.M or bson.D will not impact the query plan optimization. MongoDB's optimizer is intelligent enough to identify and use appropriate indexes regardless of the order of fields in the filter. This is true even for compound indexes that span multiple fields.

Exceptions

However, there are some exceptions where the order of fields in the filter may matter:

• Sorting: If the find query involves sorting by multiple fields, using bson.D to specify the sort order is crucial. The order in which you list the fields in bson.D determines the order in which the results will be sorted.
• Document Insertion: When inserting new documents, the order of fields in a bson.M is not guaranteed to be maintained in the database. However, using bson.D ensures that the order you specify in the document will be reflected in the database.

Conclusion

For find queries where the order of fields does not matter, bson.M is typically preferred due to its simplicity and conciseness. For sorting or when maintaining the field order in inserted documents is essential, bson.D should be used.

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