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...
{"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_...
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...
每日一博 - 闲聊SQL Query Execution Order --- SQL查询阶段在MySQL中,SQL查询的执行顺序可以分为以下几个阶段: 词法分析(Lexical Analysis):在这个阶段,MySQL首先将SQL查询文本分解成词法单元,例如关键字...这个过程会删除注释并将查询文本拆分成可识别的单词。 语法分析(Syntax Parsing):一旦词法分析完成,MySQL会进...
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...
freesql rownumber 多字段 sql order by多个字段 有一张流水表,未分库分表,目前的数据量为950w,分页查询使用到了limit,优化之前的查询耗时167s左右 (execution: 16s831ms, fetching: 107 ms) 按照下文的方式调整SQL后,耗时347ms (execution: 163 ms, fetching: 184 ms);优化前的SQL类似这样:...
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; ...