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:

MySQLoffersvariousstorageengines,eachsuitedfordifferentusecases:1)InnoDBisidealforapplicationsneedingACIDcomplianceandhighconcurrency,supportingtransactionsandforeignkeys.2)MyISAMisbestforread-heavyworkloads,lackingtransactionsupport.3)Memoryengineis

Common security vulnerabilities in MySQL include SQL injection, weak passwords, improper permission configuration, and unupdated software. 1. SQL injection can be prevented by using preprocessing statements. 2. Weak passwords can be avoided by forcibly using strong password strategies. 3. Improper permission configuration can be resolved through regular review and adjustment of user permissions. 4. Unupdated software can be patched by regularly checking and updating the MySQL version.

Identifying slow queries in MySQL can be achieved by enabling slow query logs and setting thresholds. 1. Enable slow query logs and set thresholds. 2. View and analyze slow query log files, and use tools such as mysqldumpslow or pt-query-digest for in-depth analysis. 3. Optimizing slow queries can be achieved through index optimization, query rewriting and avoiding the use of SELECT*.

To monitor the health and performance of MySQL servers, you should pay attention to system health, performance metrics and query execution. 1) Monitor system health: Use top, htop or SHOWGLOBALSTATUS commands to view CPU, memory, disk I/O and network activities. 2) Track performance indicators: monitor key indicators such as query number per second, average query time and cache hit rate. 3) Ensure query execution optimization: Enable slow query logs, record and optimize queries whose execution time exceeds the set threshold.

The main difference between MySQL and MariaDB is performance, functionality and license: 1. MySQL is developed by Oracle, and MariaDB is its fork. 2. MariaDB may perform better in high load environments. 3.MariaDB provides more storage engines and functions. 4.MySQL adopts a dual license, and MariaDB is completely open source. The existing infrastructure, performance requirements, functional requirements and license costs should be taken into account when choosing.

MySQL uses a GPL license. 1) The GPL license allows the free use, modification and distribution of MySQL, but the modified distribution must comply with GPL. 2) Commercial licenses can avoid public modifications and are suitable for commercial applications that require confidentiality.

The situations when choosing InnoDB instead of MyISAM include: 1) transaction support, 2) high concurrency environment, 3) high data consistency; conversely, the situation when choosing MyISAM includes: 1) mainly read operations, 2) no transaction support is required. InnoDB is suitable for applications that require high data consistency and transaction processing, such as e-commerce platforms, while MyISAM is suitable for read-intensive and transaction-free applications such as blog systems.

In MySQL, the function of foreign keys is to establish the relationship between tables and ensure the consistency and integrity of the data. Foreign keys maintain the effectiveness of data through reference integrity checks and cascading operations. Pay attention to performance optimization and avoid common errors when using them.


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