Mysql explain ref null. This is a MySQL restriction that is lifted in MySQL 8.

Mysql explain ref null 14, a derived table cannot contain outer references. Don't try decoding the EXPLAIN output! Feb 4, 2014 · MySQL reads all rows (not indexes) from lean_users because there is no WHERE condition in his query. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them Aug 12, 2023 · 1. 5k 阅读 Optimize MySQL queries with easy-to-understand Visual Explains. 等值匹配 -- 1. `system`。当表中只有一条记录 The EXPLAIN statement provides information about how MySQL executes statements. In MySQL 8. EXPLAIN EXTENDED can be used to provide additional information. MySQL Explain 使用explain关键字可以模拟优化器执行SQL查询语句，从而知道MySQL是如何处理你的SQL语句的，分析你的查询语句或是表结构的性能瓶颈。 Mar 17, 2024 · はじめに 記事の内容が分かりやすくなるようにあらかじめ記事で出てくる語句の説明をします。 クエリ：特定の情報をデータベースに問い合わせて取得するためのリクエストです。SELECT When you precede a SELECT, UPDATE or a DELETE statement with the keyword EXPLAIN, MariaDB displays information from the optimizer about the query execution plan. This restriction is lifted in MySQL 5. 12, extended information is available only for SELECT statements. 总结sql -- 结果值从最好到最坏依次是：system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL。SQL 性能优化的目标：至少要达到 range 级别，要求是 ref 级别，最好是 consts级别。（阿里巴巴开发手册要求） -- 1. 7. 8ref1. 7, EXPLAIN works as EXPLAIN EXTENDED and EXPLAIN EXTENDED has been 10. 12, extended information is available for SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. Prior to 8. MySQL 튜닝에서 가장 중요한 것은 쿼리와 스키마 설계인데, 스키마 설계는 한번 진행되면 그 테이블을 사용하는 모든 쿼리에 영향을 주기 때문에 좀처럼 변경하기 힘들지만 The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. ref_or_null (나쁘지 않음) 이 접근 방법은 ref 접근 방법과 같은데, NULL 비교가 추가된 형태다. One useful tool for query optimisation is the EXPLAIN statement, which provides insight The DESCRIBE and EXPLAIN statements are synonyms. What is MySQL EXPLAIN? The DESCRIBE and EXPLAIN statements are synonyms. So key_len of 107 (ms_id, status, sync_time) = (99 + 5 + 3). In 5. Sep 27, 2023 · ref_or_null：类似于ref，但是当查询语句的连接条件或者查询条件包含的列有NULL值时，MySQL会进行额外查询，经常被用于解析子查询。 典型的场景为 is null index_merge: 当查询语句使用索引合并优化时，type的取值为index_merge。 Dec 25, 2013 · I ran a query in Mysql like below: EXPLAIN SELECT * FROM( SELECT * # Select Number 2 FROM post WHERE parentid = 13 ORDER BY time, id LIMIT 1, 10 ) If you need only a specified number of rows from a result set, use a LIMIT clause in the query, rather than fetching the whole result set and throwing away the extra data. The fix for this bug in 5. 14 by the fix for Bug#28086754. 6. I provided the Tagged with mysql, sql, database. explain关键字可以模拟MySQL优化器执行SQL语句，可以很好的分析SQL语句或表结构的性能瓶颈。 Prior to MySQL 8. Querying the Information Schema PARTITIONS table. That is, MariaDB explains how it would process the SELECT, UPDATE or DELETE, including information about how tables are joined and in which order. Aug 4, 2025 · 文章浏览阅读1. For example, MySQL can use indexes and ranges to search for NULL with IS NULL. For example, the derived table dt in the following query contains a reference t1. Jul 30, 2019 · MySQL Explain. It works with SELECT, INSERT, UPDATE, or DELETE statements. Nov 14, 2020 · It is quite common to use an equality condition and an IS NULL condition on the same columns. Sep 1, 2017 · fulltext 使用全文索引的時候才會出現 ref_or_null 查詢類型和ref很像，但是 MySQL 會做一個額外的查詢，來看哪些行包含了NULL。 這種類型常見於解析子查詢的優化 index_merge 在一個查詢裡面很有多索引用被用到，可能會觸發 index_merge 的優化機制 unique_subquery Jun 29, 2020 · MySQL EXPLAIN命令详解：全面解析SQL执行计划，包含type、Extra等字段的完整取值说明，帮助优化查询性能。适用于MySQL 5. The EXPLAIN statement provides information about how MySQL executes statements. Feb 27, 2023 · In my last blog, I wrote about how to read the output of the explain query in MySQL. 0及以上版本，提供JSON、TRADITIONAL等多种输出格式，助你深入理解索引使用、连接类型和查询优化策略。 The DESCRIBE and EXPLAIN statements are synonyms. 19 and later, it also works with TABLE statements. When it executes the query, the flag cases it to return information about each step in the execution plan, instead of Jul 23, 2025 · The EXPLAIN statement provides insights into how MySQL executes queries and helps identify potential bottlenecks or inefficiencies in the query execution plans. Mar 29, 2023 · Learn how to read the output in MySQL EXPLAIN plans so you can utilize them to improve query performance. 2 EXPLAIN 输出格式 该 EXPLAIN 语句提供有关 MySQL 如何执行语句的信息。 EXPLAIN 适用于 SELECT, DELETE, INSERT, REPLACE, 和 UPDATE 语句。 EXPLAINSELECT 为语句中使用的每个表返回一行信息 。 它按照 MySQL 在处理语句时读取它们的顺序列出输出中的表。 Mar 25, 2023 · UNION RESULT MySQL 选择使用临时表来完成 UNION 查询的去重工作，针对该临时表的查询的 select_type 就是 UNION RESULT，例子上面有，就不赘述了。 SUBQUERY Sep 21, 2025 · MySQLのEXPLAINの見方を初心者向けにわかりやすく解説。出力結果の読み方やクエリ最適化の方法を具体例付きで紹介します。 Feb 23, 2022 · おっしーさんによる記事MySQLのEXPLAINについてざっくりとしたメモ EXPLAINを使うことでSQLの実行計画を確認できる 実行クエリ例 Sep 27, 2023 · ref_or_null：类似于ref，但是当查询语句的连接条件或者查询条件包含的列有NULL值时，MySQL会进行额外查询，经常被用于解析子查询。 典型的场景为 is null index_merge: 当查询语句使用索引合并优化时，type的取值为index_merge。 You cannot use arithmetic comparison operators such as =, <, or <> to test for NULL. More Information See the MySQL documentation for more information about the EXPLAIN statement. Address Unused Indexes Caused by Pattern Matching key = NULL means MySQL isn’t leveraging an index, even if one does exist. 0. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them 10. Dec 17, 2024 · MySQL 中的 `EXPLAIN` 语句用于分析和优化 SQL 查询，帮助你了解查询优化器的执行计划。本文详细介绍了 `EXPLAIN` 输出的各项指标，如 `id`、`select_type`、`table`、`type`、`key` 等，并提供了如何利用这些指标优化索引结构和 SQL 语句的具体方法。通过实战案例，展示了如何通过创建合适索引和调整查询语句来 EXPLAIN 作为MySQL的性能分析神器，读懂其结果是很有必要的，然而我在各种搜索引擎上竟然找不到特别完整的解读。都是只有重点，没有细节（例如type的取值不全、Extra缺乏完整的介绍等）。 所以，我肝了将近一个星期，整理了一下。这应该是全网最全面、最细致的EXPLAIN解读文章了，下面是全文 Feb 27, 2018 · 文章浏览阅读2. When EXPLAIN is used with an explainable statement, MySQL displays information from the optimizer about the statement execution plan. As of MySQL 8. 14, not a restriction of the SQL standard. ALL, index 두 가지는 테이블 또는 특정 인덱스가 전체 행에 접근하기 때문에 테이블 크기가 크면 효율이 떨어진다. Explain执行计划1. b to the table t1 in the outer query: The EXPLAIN statement provides information about how MySQL executes statements. MySQL sometimes optimizes a query that has a LIMIT row_count clause and no HAVING clause: The EXPLAIN statement provides information about how MySQL executes statements. The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. 4type1. By mastering the interpretation of the EXPLAIN output users can optimize queries, improve database performance, and enhance overall application efficiency. Using the statement EXPLAIN SELECT to see which partitions are used by a given SELECT. Unfortunately, this could make queries very slow. In effect, you’ve given MySQL a table of contents so it can jump to relevant rows instead of searching the entire table. To see the current set of optimizer flags Prior to 5. EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. Extended information is available for SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. To do this, you just need to convert the statement into an equivalent SELECT that accesses all the same columns. MySQL Workbench provides all of the EXPLAIN formats for executed queries including the raw extended JSON, traditional format, and visual query plan. The rows count decreases from 5 to 1, indicating that InnoDB should need to examine fewer rows to produce the result. This understanding is key for developers and database administrators who are aiming to optimize performance. Also EXPLAIN … FOR CONNECTION requires the PROCESS privilege if the specified connection belongs to a different user. If the key_len tells me that the index is being used they why the REF column is NULL? Why the TYPE is RANGE? This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. 7key_len1. However, you can rewrite some non-SELECT queries to be EXPLAIN-able. In practice, the DESCRIBE keyword is more often used to obtain information about table structure, whereas EXPLAIN is used to obtain a query execution plan (that is, an explanation of how MySQL would execute a query). It lists the tables in the output in the order that MySQL would read them while processing the statement. Explain执行计划EXPLAIN是MySQL的一条关键字，用于分析查询语句的执行计划。通过执行EXPLAIN语句，可以 The EXPLAIN statement provides information about how MySQL executes statements. Using the SHOW TABLE STATUS statement to determine whether a table is partitioned. Apr 12, 2024 · mysql 执行计划ref null mysql执行计划详解，执行计划是什么?执行计划，简单的来说，是SQL在数据库中执行时的表现情况,通常用于SQL性能分析,优化等场景。 在MySQL使用 explain 关键字来查看SQL的执行计划。 The visual explain feature generates and displays a visual representation of the MySQL EXPLAIN statement by using extended information available in the extended JSON format. Invoking EXPLAIN To use EXPLAIN, simply add the word EXPLAIN just before the SELECT keyword in your query. This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. Summary The EXPLAIN statement in MySQL can be used to obtain information about query execution. MySQL 튜닝에서 가장 중요한 것은 쿼리와 스키마 설계인데, 스키마 설계는 한번 진행되면 그 테이블을 사용하는 모든 쿼리에 영향을 주기 때문에 좀처럼 변경하기 힘들지만 The optimizer_switch system variable enables control over optimizer behavior. 3table1. This join type optimization is used most often in resolving subqueries. Examples: Dec 26, 2024 · mysql执行计划 ref是空的，#MySQL执行计划中的ref字段为何为空？在使用MySQL数据库时，查询的性能是一个非常重要的因素。为了优化查询性能，我们通常需要分析SQL语句的执行计划。执行计划提供了对SQL查询的成本估算，并给出了响应的执行路径与策略。在MySQL的执行计划中，有一个重要的字段叫做`ref Nov 25, 2015 · The type column of EXPLAIN output describes how tables are joined. It is advantageous when designing schemas or indexes and to ensure that our database can use the EXPLAIN ステートメントは、MySQL がステートメントを実行する方法に関する情報を提供します。 EXPLAIN は、 SELECT, DELETE, INSERT, REPLACE および UPDATE ステートメントで動作します。 EXPLAIN は SELECT ステートメントで使用される各テーブルに関する情報の行を返します。 これは、MySQL がステートメントの Apr 11, 2025 · With EXPLAIN, you can figure out exactly why a query is underperforming. The Message value in SHOW Jun 20, 2025 · mysql explain ref null_MySQL Explain使用详解 原创 最新推荐文章于 2025-06-20 17:32:05 发布 · 2. 2. This 107 key_len can be calcuated from the below EXPLAINs. Examples: Sep 24, 2022 · EXPLAIN是sql提供的一個用來分析語法的工具，可以使用在 SELECT, DELETE, INSERT, REPLACE, and UPDATE ，一般來說，會拿來分析SELECT 語句的效能。 Apr 3, 2023 · 当你的数据里只有几千几万，那么 SQL 优化并不会发挥太大价值，但当你的数据里去到了几百上千万，SQL 优化的价值就体现出来了！因此稍微有些经验的同学都知道，怎么让 MySQL 查询语句又快又好是一件很重要的事情。要让 SQL 又快又好的前提是，我们知道它「病」在哪里，而 explain 关键字就是 The DESCRIBE and EXPLAIN statements are synonyms. as explain here :- From where does the MySQL Query Optimizer read index statistics? for the second execution plan, it seems information_schema. 7 was only shown when doing EXPLAIN EXTENDED. system 表中只有一条数据，特殊的const类型 const 针对主键或这唯一索引查询，最多返回一条数据。 eq_ref 多表join查询，表示前表都能匹配到后表的结果 ref 使用了最左前缀规则索引的查询（针对于非唯一或者非主键索引） range 表示使用索引范围查询, 当 type 是 range 时, 那么 EXPLAIN 输出的 ref 字段为 NULL The MySQL 5. Jun 8, 2020 · Interpreting EXPLAIN will also help you learn how MySQL’s optimizer works. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them Feb 3, 2025 · 本文详细介绍了MySQL中EXPLAIN命令的'ref'列显示NULL的含义及其可能出现的场景，帮助读者更好地优化数据库查询。 Mar 20, 2023 · When working with MySQL databases, developers often need to optimise their queries to improve performance. That is, rows shows the estimated number of r The DESCRIBE and EXPLAIN statements are synonyms. Sep 17, 2023 · EXPLAINについて 普段はフロントばかりですがAPIを作成することがあったので、MySQLのEXPLAIN（実行プラン）について、忘れないようにまとめます。 EXPLAINは、クエリがどのように実行されるかを確認することができます。 結果を分析することで、パフォーマン May 7, 2023 · MySQL 실행계획 정리 쿼리 튜닝 검토 전, 실행계획(explain)을 보고 실행할 SQL이 DB에서 어떻게 처리될지 파악할 수 있습니다. A hash join can also be used when there are one or more indexes that can be used for single-table predicates. Here is one for which i don’t understand why the ref is NULL when i run the explain comamnd: mysql> explain SELECT clone. To demonstrate this for yourself, try the following query: Oct 13, 2019 · 이들 접근 방식 가운데도 주의가 필요한 것은 ALL, index, ref_or_null이다. Nov 13, 2023 · 文章浏览阅读4. 16, a foreign key constraint cannot reference a secondary index defined on a virtual generated column. 10Extra2. The global default can be set at server startup. Its value is a set of flags, each of which has a value of on or off to indicate whether the corresponding optimizer behavior is enabled or disabled. ref is used if the join uses only a leftmost prefix of the key or if the key is not a PRIMARY KEY or 10. Aug 12, 2023 · SQL 性能优化的目标：至少要达到 range 级别，要求是 ref 级别，最好是 consts级别。 （阿里巴巴开发手册要求） 是针对在一些包含 IN 子查询的查询语句中，如果查询优化器决定将 IN 子查询转换为 EXISTS 子查询，而且子查询可以使用到唯一索引进行等值匹配的话。 是针对在一些包含 IN 子查询的查询语句中，如果查询优化器决定将 IN 子查询转换为 EXISTS 子查询，而且子查询可以使用到普通索引进行等值匹配的话。 May 12, 2025 · To use EXPLAIN, simply place it before your query. Explain执行计划的局限性1. 15 IS NULL Optimization MySQL can perform the same optimization on col_name IS NULL that it can use for col_name = constant_value. Nov 3, 2024 · mysql 执行计划ref 为null，文章目录1. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them The DESCRIBE and EXPLAIN statements are synonyms. 6k次，点赞2次，收藏9次。本文详细介绍了MySQLEXPLAIN命令的使用方法及各参数的意义，包括id、select_type、table、type等列的含义，帮助理解MySQL如何执行SQL语句。 10. 2select_type1. The following list describes the join types, ordered from the best type to the worst: system const eq_ref ref All rows with matching index values are read from this table for each combination of rows from the previous tables. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. 1k次。本文介绍了MySQL性能分析的方法及Explain用法，包括如何使用Explain进行查询分析、组合索引的工作原理、慢查询分析技巧、MyISAM与InnoDB的锁定差异以及事务配置项的影响。 Oct 19, 2015 · When it comes to the query tuning, EXPLAIN is one the most important tool in the DBA’s arsenal. Jun 8, 2020 · MySQL explain only SELECT queries, not stored routine calls or INSERT, UPDATE, DELETE, or any other statements. The DESCRIBE and EXPLAIN statements are synonyms. statistics already catch up and fix the NULL cardinality issue. 6key1. Maybe it’s looking at too many rows, ignoring indexes, or struggling with joins. MySQL will set a flag on the query. In other words, I think this bug will only appear in 5. Explain执行计划EXPLAIN是MySQL的一条关键字，用于分析查询语句的执行计划。通过执行EXPLAIN语句，可以 Jul 29, 2021 · EXPLAIN can be used in several different scenarios, but as you can probably tell by now, it's most useful when optimizing the performance of queries or evaluating the results of query optimization. When you precede a SELECT statement with the keyword EXPLAIN, MySQL explains how it would process the SELECT, providing information about how tables are joined and in which order. 10. The Message value in SHOW Sep 29, 2021 · I think this issue was fixed in 8. 14. 13 and earlier, InnoDB does not permit defining a foreign key constraint with a cascading referential action on the base column of an indexed generated virtual column. In MySQL 5. The Message value in SHOW Dec 17, 2024 · MySQL 中的 `EXPLAIN` 语句用于分析和优化 SQL 查询，帮助你了解查询优化器的执行计划。本文详细介绍了 `EXPLAIN` 输出的各项指标，如 `id`、`select_type`、`table`、`type`、`key` 等，并提供了如何利用这些指标优化索引结构和 SQL 语句的具体方法。通过实战案例，展示了如何通过创建合适索引和调整查询语句来 . l… 8. 25 is slightly different and does not include the call to can_switch_from_ref_to_range() in test_if_skip_sort_order(). 4k次，点赞2次，收藏2次。本文深入解析MySQL的Explain工具，详细介绍了ref和rows列的含义及其在查询优化中的作用。ref列显示哪些列或常量被用作索引列上的值，而rows列则估算查询所需的读取记录行数。 Aug 11, 2023 · 背景 先日、「EXPLAINコマンドのどこを見るか、どう判断するか」といった質問に答えることができず 悔しい思いをしたので調べて記事にしてみようと思いました いっぱい項目あったのでまずはtype編。 と言っても全部やるかはわからないです （わからないんかい） ty Dec 15, 2006 · | 1 | SIMPLE | quote_invdtl | ALL | NULL | NULL | NULL | NULL | 257474 | | +----+-------------+--------------+------+---------------+------+---------+------+--------+-------+ Apr 12, 2024 · mysql 执行计划ref null mysql执行计划详解，执行计划是什么?执行计划，简单的来说，是SQL在数据库中执行时的表现情况,通常用于SQL性能分析,优化等场景。 在MySQL使用 explain 关键字来查看SQL的执行计划。 The visual explain feature generates and displays a visual representation of the MySQL EXPLAIN statement by using extended information available in the extended JSON format. 1. Dec 7, 2017 · The "warnings" is the Notes that before 5. For instance: This will return the query execution plan in table format. unique_subquery (나쁘지 않음) WHERE 조건절에서 사용될 수 있는 ÌN(subquery) 형태의 쿼리를 위한 방법이다. MySQL uses the ref_or_null strategy always when an index is used with an additional NULL condition: First, the index is searched for the requested value and then again with another step to find all values matching NULL. 3 Extended EXPLAIN Output Format The EXPLAIN statement produces extra (“extended”) information that is not part of EXPLAIN output but can be viewed by issuing a SHOW WARNINGS statement following EXPLAIN. Examples: The EXPLAIN statement provides information about how MySQL executes statements. How to improve type ref extra NULL Ask Question Asked6 years, 3 months ago Modified 6 years, 3 months ago Viewed 1k times 10. ref_or_null의 경우 NUL이 들어있는 행은 인덱스의 맨 앞에 모아서 저장하지만 그 건수가 많으면 MySQL 서버의 작업량이 ⑤ ref_or_null：该连接类型如同 ref，但是添加了 MySQL 可以专门搜索包含NULL值的行。 在解决子查询中经常使用该连接类型的优化。 Jun 9, 2008 · Hello, i’m trying to accelerate some queries from the tool est2uni freely downloadable online. Please tell me what is the "type" column. 2 EXPLAIN 输出格式 该 EXPLAIN 语句提供有关 MySQL 如何执行语句的信息。 EXPLAIN 适用于 SELECT, DELETE, INSERT, REPLACE, 和 UPDATE 语句。 EXPLAINSELECT 为语句中使用的每个表返回一行信息 。 它按照 MySQL 在处理语句时读取它们的顺序列出输出中的表。 Sep 21, 2025 · MySQLのEXPLAINの見方を初心者向けにわかりやすく解説。出力結果の読み方やクエリ最適化の方法を具体例付きで紹介します。 Feb 23, 2022 · おっしーさんによる記事MySQLのEXPLAINについてざっくりとしたメモ EXPLAINを使うことでSQLの実行計画を確認できる 実行クエリ例 Sep 7, 2023 · 一、前言 上周老周的一个好朋友让我出一篇教你读懂 SQL 执行计划，和我另一位读者反馈的面试题如何排查慢 SQL 的强相关，索性先出一篇一文读懂 MySQL Explain 执行计划。Explain 执行计划你一定得会看，不然你简历上就别去写什么你会 SQL 调优，不然面试官会觉得，Explain 执行计划你都不会看，那你还 This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. Nov 16, 2022 · MySQL EXPLAIN命令用于查询性能优化，解析SQL执行计划，涵盖id、select_type、table等关键参数，指导索引使用与查询优化，提升数据库查询效率。 Jun 30, 2025 · EXPLAIN 是用于分析 SQL 查询的执行计划。它能揭示 MySQL 如何执行查询（如是否使用索引、表连接顺序等），帮助我们定位性能瓶颈。 Jun 29, 2020 · MySQL EXPLAIN命令详解：全面解析SQL执行计划，包含type、Extra等字段的完整取值说明，帮助优化查询性能。适用于MySQL 5. Examples: Dec 22, 2010 · When we prefix an SQL query with the keyword "explain" we get a table with some columns. What does eq_ref and ref mean in that context. 2. This is a MySQL restriction that is lifted in MySQL 8. Aug 9, 2024 · As various sources explain the key_len can be determined to check which columns are being used. 1id1. Nov 26, 2024 · MySQL Explain详解 简介 执行计划 (query Execution plan) 语法 explain select * from table explain 中的列 expain出来的信息有10列， 分别是id,select_type,table、type,partitions,possible_keys,key,key_len,ref,rows,Extra,下面对这些字段出现的可能进行解释： 一、ID SQL 执行的顺序的标识,SQL从大到小的执行 ID相同时，执行顺序由上至下 EXPLAIN tbl_name or EXPLAIN SELECT select_options EXPLAIN tbl_name is a synonym for DESCRIBE tbl_name or SHOW COLUMNS FROM tbl_name. 3k次，点赞9次，收藏23次。文章介绍了MySQL查询的不同访问方法，包括const、ref、ref_or_null、range、index和all，详细解释了各种方法的执行机制和效率，并提供了查询优化的视角。通过理解这些访问方法，可以更好地优化SQL查询性能。 EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. That is, MySQL explains how it would process the statement, including information about how tables are joined and in which order. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them The EXPLAIN statement provides information about how MySQL executes statements. 8. Sep 26, 2018 · MySQL Explain详解涵盖执行计划、语法及10列字段含义，包括id、select_type等，解析各字段对查询性能影响，助你优化SQL查询。 The EXPLAIN statement provides information about how MySQL executes statements. This variable has global and session values and can be changed at runtime. He could have forced users to be leading, in which case MySQL would have to read all rows from users which might or might not be more efficient. Oct 3, 2010 · The ref value changes from const to const,const because the key lookup uses two key parts, not one. In the example just shown and the remaining examples in this section, we assume that the three tables t1, t2, and t3 have been created using the following statements: Feb 20, 2024 · Privileges EXPLAIN requires the same privileges required to execute the explained statement. It also requires the SHOW VIEW privilege for any explained view. Why is a given query slow, what does the execution plan look like, how will JOINs be processed, is the query using the correct indexes, or is it creating a temporary table? In this blog post, we’ll […] EXPLAIN works with SELECT, DELETE, INSERT, REPLACE, and UPDATE statements. EXPLAIN returns a row of information for each table used in the SELECT statement. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them Dec 24, 2022 · SQL Execution Plan: Analyse MySQL Performance This article aims to be able to read and interpret the Execution Plan in MySQL. That is, MySQL explains how it would process the statement, including information about how tables are joined and in which May 12, 2025 · When you rerun EXPLAIN, type should change and rows should have dropped significantly. If you want to test the below examples, you can get test data from this … Aug 4, 2025 · 文章浏览阅读1. 7. 7 documentation states: The filtered column indicates an estimated percentage of table rows that will be filtered by the table condition. ALL ref：通常为 NULL，因为这是全表扫描。 结论 EXPLAIN 语句中的 type 和 ref 字段是理解查询执行计划的重要部分： type 字段表示 MySQL 在查询过程中使用的访问类型，从最优到最差的顺序为： system 、 const 、 eq_ref 、 ref 、 range 、 index 、 ALL。 Sep 7, 2025 · 文章浏览阅读8. 9rows1. 9w次，点赞23次，收藏56次。本文详细解析了MySQL EXPLAIN执行计划中type字段的不同类型及其性能表现，包括全表扫描、索引全扫描、索引范围扫描等，并通过具体实例说明了每种类型的使用场景。 Jun 4, 2024 · 通常、MySQLのクエリチューニングするときに、EXPLAIN構文を使って実行計画を確認することがあると思います。 Nov 26, 2024 · 文章浏览阅读2. The following discussion uses the DESCRIBE and EXPLAIN keywords in accordance with those uses, but the MySQL parser treats them Dec 26, 2023 · ANDPAD アドベントカレンダー 2023 の 21 日目の記事です。 DBRE として活動している まみー です。 今回は 以前の登壇資料 や個人的にメモしていた「EXPLAIN を読むときの勘所」を解説します。 EXPLAIN を読む理由 アプリケーション開発の現場 Using the SHOW CREATE TABLE statement to view the partitioning clauses used in creating a partitioned table. 5possible_keys1. For a more comprehensive output, try the JSON format: This provides a structured output with extra details, such as cost estimates, which can deepen your analysis. Sep 1, 2017 · fulltext 使用全文索引的時候才會出現 ref_or_null 查詢類型和ref很像，但是 MySQL 會做一個額外的查詢，來看哪些行包含了NULL。 這種類型常見於解析子查詢的優化 index_merge 在一個查詢裡面很有多索引用被用到，可能會觸發 index_merge 的優化機制 unique_subquery ⑤ ref_or_null：该连接类型如同 ref，但是添加了 MySQL 可以专门搜索包含NULL值的行。 在解决子查询中经常使用该连接类型的优化。 Jun 9, 2008 · Hello, i’m trying to accelerate some queries from the tool est2uni freely downloadable online. 6k次，点赞2次，收藏9次。本文详细介绍了MySQLEXPLAIN命令的使用方法及各参数的意义，包括id、select_type、table、type等列的含义，帮助理解MySQL如何执行SQL语句。 When you precede a SELECT, UPDATE or a DELETE statement with the keyword EXPLAIN, MariaDB displays information from the optimizer about the query execution plan. ykhuekw zjjqt wfxw aqsbn dqcgznrw sbufnq ngdm hgfsh mzooz nsmyd nxv rmpv yea dnrq kleeq