Configuration·MySQL

MySQL Slow Query Log

Off by default, on in one statement, and the field that matters is not Query_time but Rows_examined - 400,000 rows read to return 1 is the signature of a missing index.

Performance Guide 33 of 45 Intermediate

OSUbuntu 26.04 LTS
MySQL8.4.10-0ubuntu0.26.04.1
Test data400,000 rows, 2 core / 4 GB host
TimeAbout 14 min

long_query_time accepts fractions. The default of 10 seconds is far too high to catch anything on a healthy system - a query taking 10 seconds is already an incident.

Every command on this page ran on db-a01.
Server Name	IP Address	OS	Roles	CPU	RAM	HDD
db-a01	192.168.0.81	Ubuntu 26.04 LTS	Primary / Source / Replica Set Member 1	2 Core	4 GB	50 GB

This guide includes

Use this when you need to find which queries are genuinely slow. This matters because the log is off by default and goes on in one statement - and the field worth reading is not the one most people look at.

checking whether it is on, and what the default threshold actually is
turning it on with a threshold that will catch something
running two ordinary-looking queries, and reading what got recorded
turning it off again, since it costs a write per slow query and grows without limit

Before you start

The events table, and sudo to read the log file.

Check whether it is on
Turn it on with a threshold that catches something
Run two queries and read what was recorded
Turn it off again

Official sources

← All MySQL guides

MySQL Slow Query Log

This guide includes

Before you start

Check whether it is on

Turn it on with a threshold that catches something

Run two queries and read what was recorded

Turn it off again

Official sources