Why Your MySQL Queries Are Slow (And How to Fix Them)

The Real Cost of Slow Queries

Every slow query is a compounding problem. It holds locks longer, consumes more buffer pool pages, and cascades into connection pool exhaustion. At Etsy, we learned that a single unoptimized query running thousands of times per minute could bring an entire cluster to its knees.

Start With EXPLAIN, Not Guessing

The most common mistake we see is developers guessing at performance fixes. Adding random indexes, increasing buffer sizes, or throwing hardware at the problem rarely works. Instead, start with EXPLAIN ANALYZE on your problematic queries and read the execution plan carefully.

The Three Most Common Culprits

After analyzing thousands of slow query logs across dozens of production systems, the pattern is remarkably consistent: missing or incorrect indexes account for roughly 60% of issues, followed by suboptimal JOIN ordering at about 25%, and unnecessary data retrieval (SELECT *) making up most of the remainder.

Each of these has straightforward fixes once properly diagnosed. The key is systematic analysis rather than ad-hoc tuning.