SQL indexes and their costs
An index can speed up locating rows. It uses storage and must be maintained on writes. Inspect actual queries, data distribution and query plans instead of indexing every column.
A concrete example
For frequent queries for a user’s recent orders, consider a composite (user_id, created_at) index. A planner may prefer a table scan for a small table or a broad result.
SELECT * FROM orders WHERE user_id = 7 ORDER BY created_at DESC;Explain it yourself
How would you measure whether a new index helps?
How to study this
Write your reasoning first. Check the original documentation, identify an assumption, then try a different example. Reading this page does not count as verified skill evidence.
Source and review
Original RealDeveloper explanation; last source check: .