执行计划生成(Execution Plan Generation):在优化过程中,MySQL会生成一个执行计划,描述了如何执行查询。执行计划通常是一个查询执行树,它包括了表的访问顺序、连接方法、过滤条件等信息。 执行(Execution):一旦生成了执行计划,MySQL就会按照执行计划的步骤开始执行查询。这包括打开表、读取数据、应用过滤条件、进行连接等操...
一、假设我们仅仅是对于在某个程序中的应用是须要依照例如以下的方式排序,我们仅仅需在SQL语句级别设置排序方式: 1、依照oracled的默认方式排序:select * from table_name order by col_name (desc|asc);(默觉得升序或无序对于升降仅仅有在数字字段); 2、依照自己定义的顺序排序: select * from table_name orde...
1、按照oracled的默认方式排序:select * from table_name order by col_name (desc|asc);(默认为升序或无序对于升降只有在数字字段); 2、按照自定义的顺序排序: select * from table_name order by decode(col_name,'value1',1,value2',2,value3',3,value4',4,...valueN',N); 二、如果我们只是对...
{"join_execution":{"select#":1,"steps":[{"creating_tmp_table":{// 创建临时表"tmp_table_info":{"table":"intermediate_tmp_table","row_length":13530,"key_length":0,"unique_constraint":false,"location":"memory (heap)",// 临时表位于内存"row_limit_estimate":1240}}},{"converting_tmp_...
Easy!SQL随手一写: 代码语言:javascript 代码运行次数:0 运行 AI代码解释 select city,name,age from citizen where city='上海'order by name limit1000; 为避免全表扫描,给city加个索引,再explain验证: Extra的Using filesort表示需要排序,MySQL会给每个线程分配一块内存(sort_buffer)用于排序。
indexes are built according to a binary order of keys. Thus the optimizer can use an index to satisfy the ORDER BY clause when NLS_SORT is set to BINARY. If NLS_SORT is set to any linguistic sort, the optimizer must include a full table scan and a full sort in the execution plan. ...
One frequent “gotcha” in SQL is trying to use a where statement to filter aggregations, which will violate SQL order of execution rules. This is because when the where statement is being evaluated, the “group by” statement has yet to be executed and aggregate values are unknown. Thus, ...
"filesort_execution": [],"filesort_summary": {"rows":4000"examined_rows":4000,"number_of_tmp_files":10,"sort_buffer_size":32728,"sort_mode":"<sort_key, rowid>" select @b-@a结果变成5000 因为这时除了排序过程,在排序完成后,还要根...
SQL Lesson 12: Order of execution of a Query Now that we have an idea of all the parts of a query, we can now talk about how they all fit together in the context of a complete query. Complete SELECT query SELECT DISTINCT column, AGG_FUNC(column_or_expression), … FROM mytable JOIN...
二、常见sql深入优化 2.1、Order by与Group by优化 在开始一点先补充一点 Extra列里面如果出现了using filesort,则表示没有用到索引排序,用的是文件排序。 接下来先来几个案例看下各种场景下的区别 CASE1 EXPLAIN SELECT * FROM employees WHERE name = 'LiLei' AND position ='dev' order by age; ...