优化器根据解析树可能会生成多个执行计划,然后选择最优的的执行计划。 代码语言:javascript 复制 SHOWVARIABLESLIKE'optimizer_trace';--启用优化器的追踪SEToptimizer_trace='enabled=on';--执行一条查询语句SELECT*FROMinformation_schema.optimizer_trace;--用完关闭SEToptimizer_trace="enabled=off";SHOWVARIABLESLIKE'o...
代码语言:javascript 复制 --生成随机 UserIdCREATEDEFINER=`root`@`localhost`FUNCTION`generateCode`(n int)RETURNSvarchar(20)CHARSETutf8DETERMINISTICBEGINDECLAREchars_strVARCHAR(100)DEFAULT'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789';DECLAREreturn_strVARCHAR(255)DEFAULT'';DECLAREiINTDEFAULT0;W...
WHERE条件中含子查询,select_type也会出现PRIMARY与SUBQUERY: 代码语言:javascript 复制 EXPLAINSELECTidFROMstudentWHEREid=(SELECTs_id idFROMclass_studentWHEREc_id=3) 2、DERIVED FROM中包含的子查询被标记为DERIVED,最外层查询被标记为PRIMARY,如下面这个SQL: 代码语言:javascript 复制 EXPLAINSELECT*FROM(SELECT*FR...
2mysql> SHOW variables like '%tmp_table_size%'; 3 4# 2. 查看 max_heap_table_size 大小, max_heap_table_size: 用户可以创建的内存表 (memory table) 的大小. 这个值用来计算内存表的最大行数值 5mysql> SHOW VARIABLES LIKE '%max_heap_table_size%'; 6 7# 3. 修改 tmp_table_size 大小 8...
Variables declared with var are hoisted, meaning they are lifted to the top of their scope and initialized as undefined. let and const are also hoisted but remain in a "temporal dead zone" until the code execution reaches their declaration, preventing their use before initialization. Mutability:...
使用show variables like 'slow_query_log_file';可以查询到日志的名称: 3.2.1 mysqldumpslow 当log_output = FILE时,可使用mysql内置的mysqldumpslow工具分析: ➜ mysqldumpslow --help Usage: mysqldumpslow [ OPTS... ] [ LOGS... ] Parse and summarize the MySQL slow query log. Options are ...
TypeScript allows you to perform runtime checks (type guards) to ensure variables are not null or undefined before using them. Additionally, you can use type assertions to tell the compiler you've ensured a value is not null. function printName(name: string | null) { if (name !== null...
You can also use tuples in switch cases to handle multiple variables. When the parameters firstName="Sanjay" and lastName="Kumar" public static void UseOfSwithCaseWithTuple(string firstName, string lastName) { (string firstName, string lastName) personDetail = (firstName, lastName); switch...
eslint最初是由Nicholas C. Zakas 于2013年6月创建的开源项目。它的目标是提供一个插件化的javascript代码检测工具。eslint使用 Node.js 编写,这样既可以有一个快速的运行环境的同时也便于安装。 一、认识eslint ESLint 是一个开源的 JavaScript 代码检查工具,由 Nicholas C. Zakas 于2013年6月创建。代码检查是...
This will ensure the variables will not affect anything outside their current scope (namespace). It's similar to var Myvar; in JavaScript.A few global variables have been defined at the top of the file.(setq *Json (pack (car (file)) "lib/libparson.so") *JSONError -1 *JSONNull 1 ...