unique_subquery is just an index lookup function that replaces the subquery completely for better efficiency. unique_subquery是一个索引查找函数,代替子查询提高效率。 index_subquery This join type is similar to unique_subquery. It replaces IN subqueries, but it works for nonunique indexes in subquer...
DELIMITER//CREATEDEFINER=`root`@`localhost`FUNCTIONrand_string2(nINT)RETURNSVARCHAR(255)CHARSETutf8mb4COLLATEutf8mb4_general_ci #该函数会返回一个字符串BEGINDECLAREchars_strVARCHAR(100)DEFAULT'abcdefghijklmnopqrstuvwxyzABCDEFJHIJKLMNOPQRSTUVWXYZ';DECLAREreturn_strVARCHAR(255)DEFAULT'';DECLAREiINTDEFAULT0...
If this function returns true, explain_into_variable_name() returns the name of the variable. Return values trueEXPLAIN output should be stored in a user variable. falseEXPLAIN output should be sent to the client. virtual bool Explain_format::is_hierarchical()const ...
The INTO clause for EXPLAIN introduced in MySQL 8.1 allows storing the output of an EXPLAIN query in a user variable, instead of returning it to the client. It is currently only supported for the JSON format, but that should be sufficient for most use cases. With the EXPLAIN output stored ...
SELECT @@log_bin_trust_function_creators variable; SET GLOBAL log_bin_trust_function_creators = 1; 存储过程 向s1、s2表添加数据的存储过程 DELIMITER // CREATE PROCEDURE insert_s1 (IN min_num INT (10), IN max_num INT(10)) BEGIN DECLARE i INT DEFAULT 0; SET autocommit = 0; REPEAT...
MySQL Explain计划中“选择表被优化掉”的含义 在MySQL中使用Explain命令可以查看SQL语句在执行过程中的执行计划,通过分析执行计划可以查看SQL语句的性能问题和优化建议。在Explain计划中,我们有时会发现一个奇怪的现象——”Select tables optimized away”,那么它到底代表什么意义呢?
设置参数 log_bin_trust_function_creators 创建函数,假如报错,需开启如下命令:允许创建函数设置: set global log_bin_trust_function_creators=1; # 不加global只是当前窗口有效。 创建函数 DELIMITER // CREATE FUNCTION rand_string1(n INT) RETURNS VARCHAR(255) #该函数会返回一个字符串 ...
SET GLOBAL log_bin_trust_function_creators=1; #创建存储过程: DELIMITER // CREATE PROCEDURE insert_s1 (IN min_num INT (10),IN max_num INT (10)) BEGIN DECLARE i INT DEFAULT 0; SET autocommit = 0; REPEAT SET i = i + 1; INSERT INTO s1 VALUES( ...
ref can be usedforindexed columns that are compared using the = or <=> operator. In the following examples, MySQL can use a ref jointo process ref_table: 注意这里索引使用了最左侧前缀,因此值是ref。表结构如下: | dept_manager |CREATE TABLE `dept_manager` ( ...
--MySQL 执行计划的输出选项如下表所示: 下面具体看看每项的含义: 2.1.1 id The SELECTidentifier. This is the sequential number of the SELECT within the query. --Query Optimizer 所选定的执行计划中查询的序列号; 2.1.2 select_type The type of SELECT,which can be any of those shown in the foll...