Also thanks a lot, Steinar, for your response. EXPLAIN. Como EXPLICA el Manual de Referencia de MySQL: Cuando se utiliza EXPLAIN MySQL muestra información del optimizador sobre el plan de ejecución de la sentencia. zhuxineli：写的啥玩意，完全看不懂 . xclltssun：《1》搜索最新的100条留言 举得例子不恰当，而且两条sql结果不一样。 Our Reference Manual states it clearly: " When some tables are of const or system type, expressions involving columns from these tables are evaluated early by the optimizer and are not part of the displayed statement. How to use EXPLAIN to profile query performance in Azure Database for MySQL. MYSQL explain详解.
By default, MySQL 8.0.18 and later employs hash joins whenever possible. It is possible to control whether hash joins are employed using one of the BNL and NO_BNL optimizer hints. 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。 explain为select语句中使用的每个表返回一行信息。 EXPLAIN은 MySQL 서버가 어떠한 쿼리를 실행할 것인가, 즉 실행 계획이 무엇인지 알고 싶을 때 사용하는 기본적인 명령어이다. Going back to our simple query: it is a SIMPLE type of select with a const type of join. At the end of my talk “Troubleshooting MySQL Performance” at the LinuxPiter conference, a user asked me a question: “What does the EXPLAIN ‘filtered’ field mean, and how do I use it?” I explained that this is the percentage of rows that were actually needed, against the equal or bigger number of resolved rows. MYSQL explain详解. This column indicates the estimated percentage of table rows that will be filtered by the table condition. (MySQL 8.0.18 supported hash_join=on or hash_join=off as part of the setting for the optimizer_switch server system variable as well as the optimizer hints HASH_JOIN or NO_HASH_JOIN. The value of filtered is 100% because all records of the table is retrieved. 问题 执行Mysql的explain extended的输出会比单纯的explain多一列filtered(MySQL 5.7缺省就会输出filtered)，它指返回结果的行占需要读到的行(rows列的值)的百分比。 EXPLAIN is a handy tool to optimize queries.
1. Hi Øystein, Steinar, First of all, thank you Øystein for the bug report. When EXPLAIN is used with the EXTENDED keyword, the output includes a filtered column not otherwise displayed. EXPLAIN statement can be used to get information about how SQL statements are executed. Use of EXPLAIN in SELECT query with JOIN: The following EXPLAIN statement is applied in a SELECT query of two tables with the JOIN clause and a WHERE condition. Its purpose is to explain that what the optimizer predicts is the best “plan” and describe that to us. It tells us where data is gathered from, how it is filtered, and how it is combined or aggregated and much more:
The venerable EXPLAIN command has been with us a long time as a built-in MySQL utility statement.
The full documentation on the explain output format may be found on the official MySQL page.. EXPLAIN funciona con las instrucciones SELECT, DELETE, INSERT, REPLACE y UPDATE.