The answer
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 }.
Compound Indexes in MongoDB Explained
How should I order fields in a compound index?
More FAQs in Compound Indexes in MongoDB Explained
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.
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.
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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