slow_query_log
WARNINGOK
Rule IDs: lg_001, lg_008
Overview
- Purpose
- Documented in the MySQL 8.4 manual as a server system variable (scope: Global). Purpose and semantics are described at the linked manual page.
- Dynamic (MySQL 8.4 reference)
- MySQL 8.4 marks this variable as dynamic (
Dynamic= Yes). Runtime changes useSET GLOBAL(global scope) orSET SESSION(session scope) — confirm syntax and persistence (SET PERSIST) in the manual. - Default value
- OFF (MySQL 8.4)
- Version and product notes
- MariaDB and Percona Server may use different names, defaults, or dynamic behavior; verify their documentation.
- Documentation
- https://dev.mysql.com/doc/refman/8.4/en/server-system-variables.html#sysvar_slow_query_log
- Other vendors
What is checked
Rules that reference this variable, with their severity and what each rule detects:
- WARNING
lg_001: Enable slow_query_log=ON. This is the most important diagnostic tool for query performance. - OK
lg_008: Positive check — confirms configuration meets expected thresholds.
Tuning guidance
- Recommended actions:
- Enable slow_query_log=ON. This is the most important diagnostic tool for query performance.
- Trade-offs: Enabling slow query log and performance_schema adds observability but consumes some I/O and CPU. General log should never be enabled in production due to extreme I/O overhead.
Example
SET GLOBAL slow_query_log = ON;
SET GLOBAL long_query_time = 1; -- Log queries > 1 second
Always validate on a non-production instance first. Use SET PERSIST (MySQL 8.0+) for changes that should survive restarts.