union 与 union all 的表现 [英] performance of union versus union all

问题描述

我必须在多个表中运行一个 select 语句.我确信这些表返回不同的记录.无论如何，我都在使用 UNION ALL.

I have to run a select statement across several tables. I am sure the tables return different records. I am anyway using UNION ALL.

当我确定表返回不同的记录时，在性能方面使用 UNION 还是 UNION ALL 更好?

Is it better to use UNION or of UNION ALL in performance terms when I am sure the tables return different records?

推荐答案

当您不担心消除重复记录时，UNION ALL 的性能将比 UNION 更好，因为您避免了昂贵的 不同排序 操作.参见:SQL SERVER – Union 与 Union All 的区别 – 最佳性能比较

UNION ALL will perform better than UNION when you're not concerned about eliminating duplicate records because you're avoiding an expensive distinct sort operation. See: SQL SERVER – Difference Between Union vs. Union All – Optimal Performance Comparison

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