A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns. 联合索引是多列按照次序一列一列比较大小,拿idx_book_id_hero_name这个联合索引来说,先比较book_id,book_id小的排在左边,book_id大的排在...
As an alternative to a composite index, you can introduce a column that is“hashed”based on information from other columns. If this column is short, reasonably unique, and indexed, it might be faster than a“wide”index on many columns. In MySQL, it is very easy to use this extra colu...
The numberofdifferent valuesina table column.When queries refer to columns that have an associated index,the cardinalityofeach column influences which access method is most efficient.For example,fora columnwitha unique constraint,the numberofdifferent values is equal to the numberofrowsinthe table.If...
In practice this makes a unique constraint on multiple columns completely useless once any of those columns is allowed to have null values. Nevertheless there are still a lot of valid use cases for these kind of situations. That's why I would still consider this a bug....
官方的文档中明确说明在唯一索引中是允许存在多行值为NULL的数据存在的。A UNIQUE index permits multiple NULL values for columns that can contain NULL. 因为在扫描唯一索引树来确定是否会发生冲突的时候,源码有这段注释: If the secondary index is unique, but one of the fields in the n_unique first fi...
唯一索引(Unique Index)要求列的数据必须是唯一的,唯一索引具有唯一性约束,在插入数据时,如果有列中有相同的数据就会报错。唯一索引可以允许多个列的值为NULL,如果列是字符串类型的话,空字符串值只能有一个。 全文索引 全文索引(Full-Text Index)只有在MyISAM和InnoDB存储引擎中支持,全文索引只能创建在基于文本的列...
unique_subquery unique_subquery 是针对在一些包含IN子查询的查询语句中,如果查询优化器决定将IN子查询转换为EXISTS子查询,而且子查询可以使用到主键进行等值匹配的话,那么该子查询执行计划的type列的值就是unique_subquery。 range 如果使用索引获取某些~范围区间`的记录,那么就可能使用到`range`访问方法 ...
If you wish to create a composite primary key you can select multiple columns and check the PK check box. However, there is an additional step that is required, you must click the Indexes tab, then in the Index Columns panel you must set the desired order of the primary keys. ...
If it is important to ensure the same row order with and without LIMIT, include additional columns in the ORDER BY clause to make the order deterministic. For example, if id values are unique, you can make rows for a given category value appear in id order by sorting like this: ...
The error message “MySQL 1503: A Unique Index Must” is usually encountered when trying to create a unique index on a column or set of columns that contains duplicate values. A unique index ensures that each value in the indexed column(s) is unique, allowing efficient retrieval and preventin...