Why should you care?

Full collection scans on 10M accounts timeout Express requests and anger customers.

See it live — copy this example

Paste into your MeanVerse project (Angular + Express + MongoDB), then run with ng serve / node / mongosh as noted.

// schema
accountSchema.index({ tenantId: 1, accountNumber: 1 }, { unique: true });
accountSchema.index({ tenantId: 1, createdAt: -1 });

// explain
const plan = await Account.find({ tenantId: 't_acme' }).sort({ createdAt: -1 }).limit(20).explain('executionStats');
// winningPlan.inputStage.stage should be IXSCAN not COLLSCAN

What happened?

• Compound index supports filter + sort.
• unique prevents duplicate account numbers per tenant.
• explain proves index use — COLLSCAN means add index.

Practice next

1. Identify slow queries from logs.
2. Create index matching filter prefix order.
3. Run explain in staging before production.
4. Add partial index { status: 1 } where status: 'active' only.
5. Drop unused index after reviewing $indexStats.

Remember

Indexes trade write cost for read speed. Compound indexes follow equality-sort-range rule. Always explain critical MeanVerse queries.

Login by email

Auth query on tenantId + email was COLLSCAN at scale.

Outcome: Unique compound index cuts login latency from 2s to 15ms.