What is a database index and when should you add one?
Reported in Rivian USA engineering loops. SQL performance question covering B-trees, composite indexes, and trade-offs.
Interview scenario
Often asked in Rivian on-site or virtual loops at US offices (Bay Area, Seattle, NYC, Austin, and remote US). Prepare a clear spoken answer plus key trade-offs.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Rivian: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
An index is a separate data structure (usually B-tree or B+ tree) enabling fast lookup by key without full table scan. CREATE INDEX on columns used in WHERE, JOIN, ORDER BY.
Composite index column order matters—index (status, created_at) helps WHERE status = ? ORDER BY created_at but may not help filtering only on created_at (left-prefix rule).
Trade-offs: indexes accelerate reads but slow writes (maintain structure) and consume disk. Over-indexing hurts insert throughput.
Advanced: covering indexes include all selected columns (index-only scan), partial indexes for hot subsets, and EXPLAIN ANALYZE to verify planner choices. Mention when hash vs btree indexes apply.
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