google it http://stackoverflow.com/questions/24127932/mysql-query-clause-execution-order The actual execution of SQL statements is a bit tricky. However, the standard does specify the order of interpretation of
ORDER BY 或 GROUP BY 和 LIMIT 联合使用优化器默认使用有序索引 For a query with an ORDER BY or GROUP BY and a LIMIT clause, the optimizer tries to choose an ordered index by default when it appears doing so would speed up query execution. Prior to MySQL 5.7.33, there was no way to ...
ORDER BY 或 GROUP BY 和 LIMIT 联合使用优化器默认使用有序索引 For a query with an ORDER BY or GROUP BY and a LIMIT clause, the optimizer tries to choose an ordered index by default when it appears doing so would speed up query execution. Prior to MySQL 5.7.33, there was no way to ...
// 将select * 转换为 select 每个列 "expanded_query": "/* select#1 */ select `test_index`.`id` AS `id`,`test_index`.`key1` AS `key1`,`test_index`.`key2` AS `key2`,`test_index`.`key3` AS `key3`,`test_index`.`key_part1` AS `key_part1`,`test_index`.`key_part2`...
MySQL官方文档地址:8.8 Understanding the Query Execution Plan 引言 MySQL 优化器会根据 SQL 语句中的表, 列, 索引和 WHERE 子句中的条件的详细信息, 使用许多技术来有效地执行 SQL 查询. 可以在不读取所有行的情况下对一个巨大的表执行查询; 可以在不比较每个行组合的情况下执行涉及多个表的连接. 优化器选择执...
query is called the “query execution plan”, also known as the EXPLAIN plan. Your goals are to recognize the aspects of the EXPLAIN plan that indicate a query is optimized well, and to learn the SQL syntax and indexing techniques to improve the plan if you see some inefficient operations....
QEP: Query Execution Plan,查询执行计划。 语法: EXPLAIN [explain_type] {explainable_stmt } explain_type: { EXTENDED | PARTITIONS | FORMAT = format_name} format_name: { TRADITIONAL | JSON} explainable_stmt: { SELECT statement | DELETE statement | INSERT statement | REPLACE statement | UPDATE ...
执行计划(execution plan,也叫查询计划或者解释计划)是 MySQL 服务器执行 SQL 语句的具体步骤。例如,通过索引还是全表扫描访问表中的数据,连接查询的实现方式和连接的顺序,分组和排序操作的实现方式等。 负责生成执行计划的组件就是优化器,优化器利用表结构、字段、索引、查询条件、数据库的统计信息和配置参数决定 SQL...
It is very important to have ORDER BY with LIMIT executed without scanning and sorting the full result set, so it is important for it to use index – in this case, index range scan will be started, and query execution stopped as soon as the required amount of rows generated....
默认情况下，MySQL并没有开启慢日志，可以通过修改slow_query_log参数来打开慢日志。与慢日志相关的参数介绍如下：slow_query_log：是否启用慢查询日志，1表示开启，0表示关闭，默认为关闭。slow_query_log_file：指定慢查询日志位置及名称，默认值为host_name-slow.log，可指定绝对路径。long_query_time：慢查询执行...