#query-optimization

Databases Bug Report

ORDER BY RAND() LIMIT 1 on a hot endpoint melted our MySQL primary

John Nguyen · Feb 11, 2026 · 8 min read

A 'random tip of the day' widget used ORDER BY RAND() LIMIT 1. It was fine at 5,000 rows and pinned the primary at 100 percent CPU at 1.4 million. Here is how we proved it and three random-row strategies that use an index instead.

Databases

Finding the query that is overloading MySQL: processlist, performance_schema and pt-query-digest

Le Minh Khoa · Mar 16, 2025 · 7 min read

When MySQL sits at 100% CPU, the guilty query is rarely the slowest one; it is usually a fast query run millions of times. A practical sequence for finding it: the processlist for right now, performance_schema statement digests for the last hours, the sys schema for shortcuts, and pt-query-digest for slow log history.