A repeatable loop: connect → pick database → inspect collections → run find with filter → explain slow queries → index → validate in staging.
Exploration commands
use practice
db.getCollectionNames()
db.orders.find().limit(5)
db.orders.find({ status: 'shipped' }).limit(10)
db.orders.find({ status: 'shipped' }).explain('executionStats')Practice: Copy queries into mongosh, MongoDB Atlas Data Explorer, or Compass. Use a database named practice you create for learning.
Safe habits
- Use
.limit()when exploring large collections - Preview updates with
findusing the same filter beforeupdateMany - Name indexes and document the fields they support
- Keep dev/staging/prod connection strings separate
Sample collection for later lessons
db.products.insertMany([
{ sku: 'A1', name: 'Widget', price: 9.99, tags: ['sale', 'new'] },
{ sku: 'B2', name: 'Gadget', price: 24.5, tags: ['premium'] }
])Run once in practice—many lessons reference products and orders.
Important interview questions and answers
- Q: Why explain executionStats?
A: Shows docs examined vs returned—spot missing indexes and COLLSCAN. - Q: limit() purpose?
A: Caps result size during exploration and API prototyping.
Self-check
- What should you run before updateMany on a new filter?
- What does explain executionStats help you find?
Challenge
Explore practice database
use practice- Seed
productsfrom the lesson. - Run
db.products.find().limit(3).explain('executionStats')
Done when: you see IXSCAN or COLLSCAN with docs examined counts.
Interview prep
- Why explain?
- Reveals COLLSCAN vs IXSCAN and documents examined.
- Before updateMany?
- Run find with the same filter to verify row count.