If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix length. A UNIQUE index permits multiple NULL values for columns that can contain NULL. 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。 当然我们会认为这是My...
(4) 当查询语句的where条件或group by、order by含多列时,可根据实际情况优先考虑联合索引(multiple-column index),这样可以减少单列索引(single-column index)的个数,有助于高效查询。 If you specify the columns in the right order in the index definition, a single composite index can speed up several ...
比如INDEX idx_book_id_hero_name (book_id, hero_name) USING BTREE,即对book_id, hero_name两列建立了一个联合索引。 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...
MySQL can create composite indexes (that is, indexes on multiple columns). An index may consist of up to 16 columns. For certain data types, you can index a prefix of the column (seeSection 8.3.4, “Column Indexes”). MySQL can use multiple-column indexes for queries that test all the...
The best way to improve the performance of SELECT operations is to create indexes on one or more of the columns that are tested in the query. But unnecessary indexes waste space and waste time for MySQL to determine which indexes to use. Indexes also add to the cost of inserts, updates,...
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...
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...
多列索引(Multiple-Column Index,又称联合索引、复合索引)顾名思义就是几个列共同组成一个索引。多列索引最多由16个列组成。多列索引遵守最左前缀原则。 最左前缀原则就是在查询数据时,以最左边的列为基准进行索引匹配。例如,有个索引mul_index(col1, col2, col3),在进行索引查询的时候,只有(col1)、(col...
多列索引(Multiple-Column Index,又称联合索引、复合索引)顾名思义就是几个列共同组成一个索引。多列索引最多由16个列组成。多列索引遵守最左前缀原则。 最左前缀原则就是在查询数据时,以最左边的列为基准进行索引匹配。例如,有个索引mul_index(col1, col2, col3),在进行索引查询的时候,只有(col1)、(col...
error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix length. A UNIQUE index permits multiple NULL values for columns that can contain ...