long_query_time
INFO
Rule IDs: lg_002
Overview
- Purpose
- Documented in the MySQL 8.4 manual as a server system variable (scope: Both). 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
- 10 (seconds) (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_long_query_time
- Other vendors
What is checked
Rules that reference this variable, with their severity and what each rule detects:
- INFO
lg_002: Set long_query_time to 1s or lower for active tuning. Start at 1s and reduce to 0.5s or 0.1s as you optimize.
Tuning guidance
- Recommended actions:
- Set long_query_time to 1s or lower for active tuning. Start at 1s and reduce to 0.5s or 0.1s as you optimize.
- 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 long_query_time = 1; -- 1 second threshold
Always validate on a non-production instance first. Use SET PERSIST (MySQL 8.0+) for changes that should survive restarts.