Database Index Design and Idempotency Crucial for Application Performance

Database indexes can speed up queries but come with trade-offs, as each index adds overhead to write operations like inserts and updates. An index only benefits a query if it can be used, with conditions like wrapping a column in a function negating its advantage. The order of columns in a composite index is critical; filter columns used with equality should come before those used for ordering. Idempotency, where repeating an operation yields the same result, solves reliability issues in distributed systems by making retries safe after network failures.