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字典

字典是一个映射 (键 -> 属性）, 是方便各种类型的参考清单。

ClickHouse支持一些特殊函数配合字典在查询中使用。 将字典与函数结合使用比将 JOIN 操作与引用表结合使用更简单、更有效。

NULL 值不能存储在字典中。

ClickHouse支持:

• 内置字典 ,这些字典具有特定的 函数集.
• 插件（外部）字典 ,这些字典拥有一个 函数集.