deptno in (select d.deptno from dept d where d.dname = 'SALES'); select_type 用于分辨查询类型,比如普通查询,连表查询等。官网的解释如下: select_type Value Meaning SIMPLE Simple SELECT (not using UNION or subqueries) PRIMARY Outermost SELECT UNION Second or later SELECT statement in a UNION...
This join type is like ref, but with the addition that MySQL does an extra search for rows that contain NULL values. This join type optimization is used most often in resolving subqueries. In the following examples, MySQL can use a ref_or_null join to process ref_table: 这个查询类型和ref...
2个全表扫描。 首先,Mysql通过依赖的子查询,建立一个attached_subqueries函数, 然后,主表的每行数据作为函数参数,函数返回一个结果。 最后,直到主表所有行被执行完成,返回的结果集合被排序order。 Visual Explain Plan https://dev.mysql.com/doc/workbench/en/wb-performance-explain.html 标准框--红色:全表扫描(...
简单的 select,即没有用 union 或子查询 simple select,not using union or subqueries primary 外层select outermost select union 在一个 union 的第二或后面的 select second or later select statement in a union dependent union 依靠外层查询的第二或后面的 select 在一个 union second or later select stat...
explain可用来分析SQL的执行计划。格式如下: {EXPLAIN|DESCRIBE|DESC} tbl_name [col_name|wild] {EXPLAIN|DESCRIBE|DESC} [explain_type] {explainable_stmt|FORCONNECTION connection_id} {EXPLAIN|DESCRIBE|DESC} ANALYZE select_statement explain_type: { ...
SIMPLE Simple SELECT (not using UNION or subqueries) PRIMARY Outermost SELECT UNION Second or later SELECT statement in a UNION UNION RESULT Result of a UNION SUBQUERY First SELECT in subquery DEPENDENT SUBQUERY First SELECT in subquery, dependent on outer query DEPENDENT UNION Second or later SELEC...
半连接操作。inner_tables展示未拉出的表。详见“Optimizing Subqueries, Derived Tables, and View References with Semijoin Transformations”[17] 14 表示创建了内部临时表而缓存中间结果 当某些表是const或system类型时,这些表中的列所涉及的表达式将由优化器尽早评估,并且不属于所显示语句的一部分。但是,当使用FORM...
半连接操作。inner_tables展示未拉出的表。详见 “Optimizing Subqueries, Derived Tables, and View References with Semijoin Transformations”[17] 14 <temporary table> 表示创建了内部临时表而缓存中间结果 当某些表是const或system类型时,这些表中的列所涉及的表达式将由优化器尽早评估,并且不属于所显示语句的一...
如上面描述当使用 explain format=tree可以展示sql执行的树状结构 MySQL执行计划是火山模型,执行计划树自顶向下调用Read()接口,数据则自底向上被拉取处理。因此FORMAT=TREE展示的树形计划,数据获取顺序是最底层的先执行获取。对于并列的operator,上面的operator是树的左节点,优先执行。 mysql> explain format=tree select...
MATERIALIZED:被物化的子查询,MySQL5.6 引入的一种新的 select_type,主要是优化 FROM 或 IN 子句中的子查询,更多详情请查看:Optimizing Subqueries with Materialization UNCACHEABLE SUBQUERY:对于外层的主表,子查询不可被缓存,每次都需要计算 UNCACHEABLE UNION:类似于 UNCACHEABLE SUBQUERY,只是出现在 UNION 操作中...