Build a tiny query planner: choose index vs full scan, push down predicates, estimate cost

Executing a query is the easy half; deciding HOW to execute it is the staff-level half. The interesting failure is choosing an index that returns 90% of the table, where a full scan is faster, and a planner that knows that beats one that always uses the index.

Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.