Q15: What is the difference between a Single-Field Index and a Compound Index?
- Single-Field Index: Index constructed on a single key
{ email: 1 }. - Compound Index: Index constructed on multiple keys
{ status: 1, createdAt: -1 }. The order of fields matters significantly; queries must follow the Equality, Sort, Range (ESR) rule and match index prefix sequences.
Key Takeaways
- Compound indexes support queries on prefix subsets of fields.
- Follow the ESR Rule: Equality fields first, Sort fields second, Range fields last.
Knowledge Check
Given a compound index { a: 1, b: 1, c: 1 }, which query CANNOT utilize this index efficiently?