AI代码解释 drop databaseifexists`_index`;create databaseifnot exists`_index`defaultcharactersetutf8;use`_index`;--构建一个8000000条记录的数据--构建的海量表数据需要有差异性,所以使用存储过程来创建, 拷贝下面代码就可以了,暂时不用理解--产生随机字符串 delimiter $$ createfunctionrand_string(nINT)return...
Indexes are less important for queries on small tables, or big tables where report queries process most or all of the rows. When a query needs to access most of the rows, reading sequentially is faster than working through an index. Sequential reads minimize disk seeks, even if not all the...
The most comprehensive set of advanced features, management tools and technical support to achieve the highest levels of MySQL scalability, security, reliability, and uptime. Learn More » MySQL for OEM/ISV Over 2000 ISVs, OEMs, and VARs rely on MySQL as their products' embedded database to...
An index is best used when you need to select a small number of rows in comparison to the total rows. 这句话我摘自stackoverflow上《MySQL: low selectivity columns = how to index?》下面一个人的回答。(详情见:https://stackoverflow.com/questions/2386852/mysql-low-cardinality-selectivity-columns-...
Accessing data in a hash index is very quick, unless there are many collisions(multiple values with the same hash). When there are collisions, the storage engine must follow each row pointer in the linked list and compare their values to the lookup value to find the right row(s). ...
When I use the query 'show indexes from test_table' I see the results in two rows as: test_table 0 PRIMARY 1 Sl_Num A 10000524 BTREE test_table 1 Period 1 Period A 18 YES BTREE Does it confirm successful creation of index on the column Period?
Currently when user specifies FORCE INDEX, optimizer still always calculates cost in order to pick access method and join order using index dives. Under some circumstances it's possible to avoid doing this and this could speed up execution by 40%. The goal of this WL is to detect if access...
('MySQL Security','When configured properly, MySQL ...'); 1.4、唯一索引 见名知义,索引列中的值必须是唯一的,但是允许为空值。d表中name就是唯一索引,相比主键索引,主键字段不能为null,也不能重复 create table d( id int primary key auto_increment , ...
use 是建议,实际使用哪个索引 MySQL 还会自己权衡运行速度去更改,force就是无论如何都强制使用该索引。 覆盖索引&回表查询 尽量使用覆盖索引(查询使用了索引,并且需要返回的列,在该索引中已经全部能找到),减少 select *。 explain 中 extra 字段含义: using index condition:查找使用了索引,但是需要回表查询数据 using...
CREATE TABLE employees ( data JSON, INDEX ((CAST(data->>'$.name' AS CHAR(30))) ); The hidden generated column is assigned the VARCHAR(30) data type, which can be indexed. But this approach produces a new issue when trying to use the index: CAST() returns a string with the colla...