join_buffer_size
WARNING
Rule IDs: mem_003
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
- 262144 (256 KB) (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_join_buffer_size
- Other vendors
What is checked
Rules that reference this variable, with their severity and what each rule detects:
- WARNING
mem_003: Increasing join_buffer_size is a band-aid. The real fix is to add appropriate indexes to join columns. Check slow query log for offending queries.
Tuning guidance
- Recommended actions:
- Increasing join_buffer_size is a band-aid. The real fix is to add appropriate indexes to join columns. Check slow query log for offending queries.
- Trade-offs: Per-thread buffers are allocated per-session on demand. Large values waste memory when multiplied by max_connections. Small values cause disk spills for complex queries. Tune based on query profiles, not guesswork.
Example
SET GLOBAL join_buffer_size = 524288; -- 512 KB
Always validate on a non-production instance first. Use SET PERSIST (MySQL 8.0+) for changes that should survive restarts.