Query Execution and Optimization

A query optimizer turns your SQL into a physical plan: which tables to scan, in what join order, and whether to use a hash join, sort, or index lookup. Reading an EXPLAIN plan tells you why a query is slow (a full scan on a huge table, a bad join order that explodes intermediate rows, a sort that spilled to disk) and which lever fixes it. Applied-AI interviews probe this because the difference between a 30-second and a 0.3-second query is usually understanding the plan, not rewriting the logic.