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数据库中查询的数量是多少？一个realtion是属性的数量。是所有输入关系中的属性数，还是新查询输出关系的属性数？例如。输入：数据库 D，查询 Q/k ∈ L，常量元组 t∈ adom(D)^k，其中 L 是语言。谢谢