在 ClickHouse Cloud 中查询此系统表中的数据分别保存在 ClickHouse Cloud 各节点的本地。因此，如需查看所有数据的完整情况，需要使用
clusterAllReplicas 函数。更多详情请参见此处。说明
config.xml 中指定的服务器全局设置相关信息。
该表还包含结构固定的受支持嵌套设置；不包含列表等动态部分。
列
name(String) — 服务器设置名称。value(String) — 服务器设置值。default(String) — 服务器设置默认值。changed(UInt8) — 表示是否在 config.xml 中指定了该设置。description(String) — 服务器设置的简短说明。type(String) — 服务器设置值的类型。changeable_without_restart(Enum8(‘No’ = 0, ‘IncreaseOnly’ = 1, ‘DecreaseOnly’ = 2, ‘Yes’ = 3)) — 表示该设置是否可在运行时更改。is_obsolete(UInt8) — 表示该设置是否已废弃。
示例
thread_pool 的服务器设置相关信息。
SELECT *
FROM system.server_settings
WHERE name LIKE '%thread_pool%'
┌─name──────────────────────────────────────────┬─value─┬─default─┬─changed─┬─description─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┬─type───┬─changeable_without_restart─┬─is_obsolete─┐
│ max_thread_pool_size │ 10000 │ 10000 │ 0 │ The maximum number of threads that could be allocated from the OS and used for query execution and background operations. │ UInt64 │ No │ 0 │
│ max_thread_pool_free_size │ 1000 │ 1000 │ 0 │ The maximum number of threads that will always stay in a global thread pool once allocated and remain idle in case of insufficient number of tasks. │ UInt64 │ No │ 0 │
│ thread_pool_queue_size │ 10000 │ 10000 │ 0 │ The maximum number of tasks that will be placed in a queue and wait for execution. │ UInt64 │ No │ 0 │
│ max_io_thread_pool_size │ 100 │ 100 │ 0 │ The maximum number of threads that would be used for IO operations │ UInt64 │ No │ 0 │
│ max_io_thread_pool_free_size │ 0 │ 0 │ 0 │ Max free size for IO thread pool. │ UInt64 │ No │ 0 │
│ io_thread_pool_queue_size │ 10000 │ 10000 │ 0 │ Queue size for IO thread pool. │ UInt64 │ No │ 0 │
│ max_active_parts_loading_thread_pool_size │ 64 │ 64 │ 0 │ The number of threads to load active set of data parts (Active ones) at startup. │ UInt64 │ No │ 0 │
│ max_outdated_parts_loading_thread_pool_size │ 32 │ 32 │ 0 │ The number of threads to load inactive set of data parts (Outdated ones) at startup. │ UInt64 │ No │ 0 │
│ max_unexpected_parts_loading_thread_pool_size │ 32 │ 32 │ 0 │ The number of threads to load inactive set of data parts (Unexpected ones) at startup. │ UInt64 │ No │ 0 │
│ max_parts_cleaning_thread_pool_size │ 128 │ 128 │ 0 │ The number of threads for concurrent removal of inactive data parts. │ UInt64 │ No │ 0 │
│ max_backups_io_thread_pool_size │ 1000 │ 1000 │ 0 │ The maximum number of threads that would be used for IO operations for BACKUP queries │ UInt64 │ No │ 0 │
│ max_backups_io_thread_pool_free_size │ 0 │ 0 │ 0 │ Max free size for backups IO thread pool. │ UInt64 │ No │ 0 │
│ backups_io_thread_pool_queue_size │ 0 │ 0 │ 0 │ Queue size for backups IO thread pool. │ UInt64 │ No │ 0 │
└───────────────────────────────────────────────┴───────┴─────────┴─────────┴─────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┴────────┴────────────────────────────┴─────────────┘
WHERE changed 很有用，例如，当你想检查配置文件中的设置是否已正确加载并生效时。
SELECT * FROM system.server_settings WHERE changed AND name='max_thread_pool_size'