What is a compound index?
PICTURE THIS: DATABASE INDEX
Simple meaning
An index on multiple fields matching common filters and sorts.
WHY — MongoDB instead of guessing?
Why interviewers care about MongoDB:
who only read docs from people who shipped.
and tied to Full Stack work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1An index on multiple
fields matching common filters and sorts.
- 2Why it exists
Order of fields matters.
- 3How it works
Explain plans confirm usage.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
An index on multiple fields matching common filters and sorts. Order of fields matters.