How do indexes work in MongoDB and what is a compound index?
Assesses fundamental understanding of MongoDB conventions, runtime behavior, and memory/performance considerations.
Hiring managers look for precision, avoidance of ambiguous jargon, and ability to explain trade-offs under real production conditions.
An index is a B-tree structure storing a sorted subset of a collection's fields, letting the planner find documents without scanning everything. Without an index a query does a COLLSCAN; with one it can do an IXSCAN.
A compound index covers multiple fields in a defined order:
db.orders.createIndex({ customerId: 1, createdAt: -1 });
Order matters. This index supports queries on customerId alone and on customerId plus createdAt, but not on createdAt alone, because of the leftmost-prefix rule. Direction matters for sorts: sorting customerId ascending and createdAt descending uses this index directly.
Every index speeds reads but slows writes and consumes memory, so create only what your queries need. Use explain("executionStats") to confirm an index is used and to see how many documents were examined versus returned.
Candidate Response Strategy & Interview Tips
- Start with a concise one-sentence summary: Deliver a direct, confident answer first before expanding into nuances.
- Demonstrate real-world trade-offs: Discuss where this approach excels and when you would avoid it in production systems.
- Discuss complexity & edge cases: Proactively explain time/space complexity or boundary conditions (null values, scale limits).
- Prepare for interviewer follow-ups: Technical hiring panels frequently probe deeper into concurrency, backward compatibility, or alternative libraries.