MySQL:使用 UNION 与多个查询 [英] MySQL: using UNION vs multiple queries

问题描述

我很想知道 UNION 是否比运行多个查询更好.

I'm interested to know if a UNION is better than running multiple queries.

如果是这样，是否有时出于其他原因多个查询会更快或更受欢迎?

If so, are there times that multiple queries would be faster or preferred for other reasons?

推荐答案

• 如果您使用 UNION ALL 并且不排序输出，则UNION 的性能应该或多或少与假设查询是多个单独查询的性能与您 UNION 结合在一起的那些相同.
• 如果你使用 UNION ALL 和 sort 数据，显然你会施加一些开销(虽然可能比您在其中对您的应用程序进行排序要少).
• 如果你省略了 ALL 关键字，MySQL 会做额外的工作DISTINCT - 你的结果.同样，这会带来额外的开销虽然可能比自己做的要少.
• If you use UNION ALL and don't sort the output, then the performance of UNION should be more-or-less the same as the performance of multiple separate queries assuming the queries are identical to the ones you're UNION-ing together.
• If you use UNION ALL and sort the data, obviously you're imposing some overhead (although probably less than if you sorted in it your application).
• If you leave out the ALL keyword, MySQL will do the extra work of DISTINCT-ing your results. Again, this imposes extra overhead although probably less than doing it yourself.

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