The Query That Ate 75% of Our Database CPU: A MySQL Full-Text Post-Mortem

A 'related articles' query matched 16,000 of 80,000 rows, sorted all of them by a computed expression, and returned six. How I found it by sampling the process list, why ORDER BY on an expression kills a LIMIT, and the two-stage rewrite that made it 4.5x faster without adding cache.