| 查询级写法 | SELECT … SETTINGS max_threads = 4, max_memory_usage = 10000000000 |
|---|---|
| 会话级 | SET max_threads = 8;（当前会话内生效） |
| 常用设置 | max_threads、max_memory_usage、max_execution_time、max_bytes_before_external_group_by、join_algorithm |
示例
示例
SELECT count() FROM events SETTINGS max_threads = 2;
SET max_execution_time = 60;
-- 查看设置当前值与说明
SELECT name, value, description
FROM system.settings
WHERE name IN ('max_threads', 'max_memory_usage');用户 profile 里可以限定单查询内存与并发，防单条大查询拖垮整机；查询级设置不能突破 profile 的硬限制。