(1)When youprecede a SELECT statement with the keyword EXPLAIN, MySQL displays informationfrom the optimizer about the query execution plan. That is, MySQL explains howit would process the statement, including information about how tables are joinedand in which order. EXPLAIN EXTENDED can be used ...
执行EXPLAIN 命令: 使用你要分析的查询替换 query,然后执行 EXPLAIN 命令。例如: 复制 EXPLAIN SELECT * FROM table_name WHERE column_name = 'value'; 1. 解读查询计划输出: 查询计划输出包含多个部分,每个部分描述了查询计划中的一个操作。以下是一些常见的关键信息: QUERY PLAN:整个查询计划的概述。 ->:表示...
可以使用带有-dynamic选项的db2pd命令来获取标识。 -statementquery-statement 要动态准备并说明的 SQL 或 XQuery 查询语句。 要解释多个语句,请使用-stmtfile选项提供包含要说明的查询语句的文件,或者使用-terminator选项定义可用于在-statement选项中分隔语句的终止字符。 -stmtfilequery-statement-file 指定一个文件,其中...
Thisinformation displays how the optimizer qualifies table and column names in theSELECT statement, what the SELECT looks like after the application of rewritingand optimization rules, and possibly other notes about the optimization process.EXPLAIN EXTENDED also displays the filtered column. Note You ...
/* If this process has done JIT in parallel workers, merge stats */ if (queryDesc->estate->es_jit_worker_instr) InstrJitAgg(&ji, queryDesc->estate->es_jit_worker_instr); ExplainPrintJIT(es, queryDesc->estate->es_jit_flags, &ji, -1);}/* * ExplainPrintJIT - * Append information...
When EXPLAIN is used with an explainable statement, MySQL displays information from the optimizer about the statement execution plan. That is, MySQL explains how it would process the statement, including information about how tables are joined and in which order. For information about using EXPLAIN ...
The FOREIGN KEY relationship plays the main part in this process as tables must be related to each other. Let us have a look at the scenario for MySQL EXPLAIN with example: we created external keys for a table we wanted to execute a query for. Then, we ran the query with the Query ...
This join type is likeref, but with the addition that MySQL does an extra search for rows that containNULLvalues. This join type optimization is used most often in resolving subqueries. In the following examples, MySQL can use aref_or_nulljoin to processref_table: ...
EXPLAIN is used to obtain a query execution plan (that is, an explanation of how MySQL would execute a query). 简单翻译一下,就是explain用于获取查询执行计划(即MySQL是如何执行一个查询的)。 工作中,我们会遇到慢查询,这个时候我们就可以在select语句之前增加explain关键字,模拟MySQL优化器执行SQL语句,从而...
https://help.kingbase.com.cn/v8/perfor/sql-optimization/sql-optimization-13.html?highlight=enable_nestloop 我们可以关闭Nested Loop Left Join这种两表连接方式进行验证(优化器会自动采用其他连接方式)。 具体操作就是在INSERT INTO...SELECT语句之前之后分别添加set enable_nestloop to off;set enable_nestloop...