tmp_table_size
WARNING (2 rules)
Rule IDs: mem_004, mem_009
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
- 16 MB (16777216) (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_tmp_table_size
- Other vendors
What is checked
Rules that reference this variable, with their severity and what each rule detects:
- WARNING
mem_004: Increase tmp_table_size and max_heap_table_size together. But first check if queries use BLOB/TEXT columns — those always go to disk. - WARNING
mem_009: Reduce max_connections, per-thread buffer sizes, or innodb_buffer_pool_size to keep total possible memory under 80% of RAM.
Tuning guidance
- Recommended actions:
- Increase tmp_table_size and max_heap_table_size together. But first check if queries use BLOB/TEXT columns — those always go to disk.
- Reduce max_connections, per-thread buffer sizes, or innodb_buffer_pool_size to keep total possible memory under 80% of RAM.
- 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 tmp_table_size = 67108864; -- 64 MB
SET GLOBAL max_heap_table_size = 67108864;
Always validate on a non-production instance first. Use SET PERSIST (MySQL 8.0+) for changes that should survive restarts.