Indexes are the single most impactful performance lever in any database. A missing index turns a millisecond query into a multi-second table scan. But indexes aren't free — they slow down writes, consume storage, and can mislead the query planner if poorly designed.

Reading EXPLAIN Output

Always verify with EXPLAIN ANALYZE. Look for Seq Scan on large tables — that's usually the problem. Check the estimated vs. actual row counts — large discrepancies mean stale statistics. A good composite index follows the 'equality first, range last' rule: columns with = conditions go first, columns with range conditions go last.