Why should you care?

Ultra-hot lookups (session token → userId) become much faster and cheaper when covered.

See it live — copy this example

Open mongosh or MongoDB Compass, select database nosqlverse, then run the example. Change one field and run again.

db.users.createIndex({ email: 1, username: 1 })
db.users.insertOne({ email: "c@nosqlverse.dev", username: "cj", city: "Kochi" })
db.users.find(
  { email: "c@nosqlverse.dev" },
  { email: 1, username: 1, _id: 0 }
).explain("executionStats")

Run Example »

Edit the code below and click Run to see the result in Toolliyo’s live editor.

What happened?

• The compound index holds email and username.
• The query filters on email and returns only those fields with _id: 0, so totalDocsExamined can be 0 while totalKeysExamined > 0 — a covered plan.

Practice next

1. Create the index and insert a user.
2. Run the projected find with explain.
3. Add city to the projection and see document fetch return.
4. Cover a token → userId map collection.
5. Compare latency with and without city in projection.

Remember

Covered queries read only from indexes. Projection must match indexed fields. Great for tiny hot lookups.

API key validation

Gateway checks apiKeys index for key hash → tenantId without loading the full key document.

Outcome: Auth overhead stays tiny at high QPS.