You have 50 million orders. How would you efficiently fetch the latest 20 orders for a user?

Short answer

Query { userId } sorted by createdAt descending, limit 20, served by a compound index { userId: 1, createdAt: -1 }. Do not skip() through older pages; for page two use a cursor on createdAt/_id.

Why?

Equality on userId plus sort on createdAt is a textbook ESR index. userId must be the same BSON type you store — ObjectId vs string is a common miss that causes COLLSCAN or empty results. Project only the list fields. If the UI only needs the last 20, do not $lookup every line item's product here.

Example

db.orders.find({ userId: ObjectId("64a1b2c3d4e5f6a7b8c90001") })
  .sort({ createdAt: -1 })
  .limit(20)

db.orders.createIndex({ userId: 1, createdAt: -1 })

Interview tip

Say the index keys out loud, then mention ObjectId type and cursor pagination. That is a complete senior answer.

Common mistake

Indexing only createdAt, then filtering userId — the sort index cannot help the equality on a different leading field.