mysql - varchar(4000) 列的唯一性约束

Question

我将 URL 存储在具有 varchar(4000) 数据类型的列中。我想对该列强制执行唯一性约束，但无法这样做，因为 4000 远远超出了最大限制。

有没有办法在这些列上强制执行唯一性？

Answer 1

The uniqueness of that long fields is not enforcable by use of indices.

The right way is likely to perform a select inside a trigger before insert/update and failing for a value that is already contained in the column.

A practicable workaround at the application level or inside a stored procedure may be to perform a select for the new value followed by an insert (if that value is not already present) inside a transaction.

In any case, part of the column in question shoud be (non-uniquely) indexed to speed the select.