Projection, sorting, and pagination

Why this matters

Returning full documents when you only need two fields is one of the most common accidental performance problems in MongoDB applications. A document with 40 fields sent over the wire 1,000 times per second is dramatically more expensive than projecting to 3 fields — and it's a one-line fix. Pagination matters because skip(n) performs a full scan of the first n documents every time; cursor-based pagination using a range filter on _id or a timestamp scales to billions of documents without degrading, which is why every high-volume API uses it.

Demo

Project specific fields, sort results, and implement both offset and cursor-based pagination.

Try it yourself

1. Insert 20 documents with a score field. Implement cursor-based pagination using score as the cursor field (instead of _id). Fetch 3 pages of 5 and verify no document appears twice and none are skipped.
2. Use explain('executionStats') on a query with skip(1000).limit(10). Note the docsExamined value. Then implement cursor pagination for the same result. Compare docsExamined between the two.
3. Project with { password: 0 } (exclusion projection) on a users collection. Verify the field is absent. Then try mixing inclusion and exclusion in one projection — observe the error MongoDB returns.

Prompt your AI

Use these three in order. Each builds on the one before.

1. Basics & terminology

Explain the difference between offset pagination (skip/limit) and cursor-based pagination. Why does offset pagination become slower as the offset grows?

2. Why it works (the mechanism)

When I project `{ title: 1, _id: 0 }`, at what stage in query execution does MongoDB discard the other fields? Does it read them from storage first?

3. Advanced — application & what's next

I need to paginate a feed sorted by a non-unique `score` field. If two documents have the same score, cursor-based pagination on score alone will skip documents. How do I implement stable cursor pagination on a non-unique sort field?

References

• Cursor-based pagination in MongoDB
• MongoDB projection docs