EXPLAIN语句能够被用于获取一些关于SQL执行时的相关信息,比如表的连接顺序,对表的方式方式等等。通过对该相关信息进行进一步的
EXPLAIN语句能够被用于获取一些关于SQL执行时的相关信息,比如表的连接顺序,对表的方式方式等等。通过对该相关信息进行进一步的分析,我们可以通过对表添加适当的索引,以及优化连接顺序,使用提示等等手段来达到使SQL高效运行的目的。本文描述了EXPLAIN的用法并给出了相关示例。
一、EXPLAIN概述 EXPLAIN 语句主要是用于解析SQL执行计划,通过分析执行计划采取适当的优化方式提高SQL运行的效率。 EXPLAIN 语句输出通常包括id列,select_type,table,type,possible_keys,key等等列信息 MySQL 5.6.3后支持SELECT, DELETE, INSERT,REPLACE, and UPDATE. EXPLAIN EXTENDED支持一些额外的执行计划相关的信息 EXPLAIN PARTITIONS支持基于分区表查询执行计划的相关信息 二、EXPLAIN输出列描述 -- 下面通过示例来展示EXPLAIN输出列 (root@localhost) [sakila]> explain select sum(amount) from customer a, -> payment b where 1=1 and a.customer_id=b.customer_id and -> email='JANE.BENNETT@sakilacustomer.org'\G *************************** 1. row *************************** id: 1 select_type: SIMPLE table: a type: ALL possible_keys: PRIMARY key: NULL key_len: NULL ref: NULL rows: 590 Extra: Using where *************************** 2. row *************************** id: 1 select_type: SIMPLE table: b type: ref possible_keys: idx_fk_customer_id key: idx_fk_customer_id key_len: 2 ref: sakila.a.customer_id rows: 14 Extra: 1、各列表示的意义 Column Meaning ------ ------------------------------------ id The SELECT identifier select_type The SELECT type table The table for the output row partitions The matching partitions type The join type possible_keys The possible indexes to choose key index actually chosen key_len The length of the chosen key ref The columns compared to the index rows Estimate of rows to be examined filtered Percentage of rows filtered by table condition Extra Additional information 2、各列上的具体描述 id: 包含一组数字,表示查询中执行select子句或操作表的顺序 id相同,执行顺序由上至下,否则id值越大(通常子查询会产生)优先级越高,越先被执行 id如果相同,可以认为是一组,从上往下顺序执行;在所有组中,id值越大,优先级越高,越先执行 select_type: 表示查询中每个select子句的类型(简单 OR复杂) select_type Value Meaning ------------- ----------------------------------------------- SIMPLE Simple SELECT (not using UNION or subqueries) PRIMARY Outermost SELECT 最外层select UNION Second or later SELECT statement in a UNION DEPENDENT UNION Second or later SELECT statement in a UNION, dependent on outer query UNION RESULT Result of a UNION. SUBQUERY First SELECT in subquery DEPENDENT SUBQUERY First SELECT in subquery, dependent on outer query(通常为相关子查询) DERIVED Derived table SELECT (subquery in FROM clause) MATERIALIZED Materialized subquery UNCACHEABLE SUBQUERY A subquery for which the result cannot be cached and must be reevaluated for each row of the outer query UNCACHEABLE UNION The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY) table: 从哪个表(表名)上输出行记录,也可能是下列值: •本文永久更新链接地址:

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