Reviewed by Aditya Kumar · Last reviewed 2026-03-24
**Index**: B-tree/hash/bitmap—fast lookup without full scan. **Use**: Filter columns, join keys. **Trade-off**: Faster reads; slower writes (maintain index). ** selectivity**: High-cardinality columns benefit more. **Composite**: (a, b) helps (a) and (a, b); not (b) alone....
Index: B-tree/hash/bitmap—fast lookup without full scan. Use: Filter columns, join keys. Trade-off: Faster reads; slower writes (maintain index). selectivity: High-cardinality columns benefit more. Composite: (a, b) helps (a) and (a, b); not (b) alone. Over-indexing: Every column indexed = write penalty. Statistics: Planner needs current stats for index selection.
Red Flag: Index on every column. Pro-Move: 'We analyzed query patterns, added 2 composite indexes—coverage for 90% of queries; write latency increased 5%, acceptable.'
According to DataEngPrep.tech, this is one of the most frequently asked SQL interview questions, reported at 1 company. DataEngPrep.tech maintains an editor-reviewed database of 1,863 data engineering interview questions across 7 categories.