MySQL：无索引的UNIQUE约束 [英] MySQL: UNIQUE constraint without index

问题描述

可以添加一个约束，例如

Is it possible to add a constraint like

ALTER TABLE `t1` ADD UNIQUE(`col1`, `col2`);

索引不会用于任何查询，所以这将是一个浪费空间。

without creating an index? The index wouldn't be used for any queries so it would be a waste of space.

如果插入和更新会更慢，这不会是一个问题，因为表不会频繁更新。

It wouldn't be a problem if inserts and updates would be way slower, because the table doesn't get updated very often.

推荐答案

不，这是不可能的。 UNIQUE约束包含索引定义和I几乎没有想象如何实现而不创建索引（在DBMS术语中）。

No, this is not possible. A UNIQUE constraint contains an index definition and I barely imagine how it might be implemented without creating an index (in DBMS terms).

您应该意识到，索引不仅仅是精灵 - 它们是一个真实的数据结构，它需要空间，特殊的过程待处理等唯一约束本身是指唯一索引值，而不是唯一列值。

You should realize that indexes are not just 'wizardy' - they are a real data structure, which takes space to be placed, special procedures to be handled e.t.c. A unique constraint, itself, means unique index values, not unique column values.

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