The answer
An index on two or more fields. MongoDB can use it for queries that filter on any prefix of the indexed fields. Example: index on (fromUserId, toUserId) works for fromUserId alone or fromUserId + toUserId, but not toUserId alone.
Compound Indexes in MongoDB Explained
What is a compound index in MongoDB?
More FAQs in Compound Indexes in MongoDB Explained
A compound index on (A, B, C) can be used for A, A+B, or A+B+C. It cannot be used for B, C, or B+C. If you query on B+C, you need a separate index on (B, C) or a different compound index that starts with B.
By the ESR rule: Equality (exact match) first, Sort second, Range last. For a query like { fromUserId: X, age: { $gte: 18 } }.sort({ createdAt: -1 }), the index would be { fromUserId: 1, createdAt: -1, age: 1 }.
Call .explain('executionStats') on your query. Look at executionStats.totalDocsExamined. If it equals the collection size, the index is not used. If it is small, the index is working.
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