核心数据结构是Condition_pushdown: classCondition_pushdown{public:boolmake_cond_for_derived();Item*make_remainder_cond(Item*cond);Item*get_remainder_cond(){returnm_remainder_cond;}private:/// Condition that needs to be checked to push down to the derived table.Item*m_cond_to_check;/// Deriv...
When a derived table cannot be merged into the outer query (for example, if the derived table uses aggregation), pushing the outer WHERE condition down to the derived table should decrease the number of rows that need to be processed and thus speed up execution of the query. 也就是说合并...
在MySQL8.0.21 版本及之前版本,还没有参数 optimizer_switch,还没有derived_condition_pushdown变量,等价于 derived_condition_pushdown=off。 在MySQL8.0.22 版本及之后版本,参数 optimizer_switch 引⼊了 derived_condition_pushdown 变量,在开启的时候,会导致下⾯的 SQL 语句及类似语句得到⾮预期的结果: select ...
set global optimizer_switch=‘derived_condition_pushdown=on’; – 设置全局值(影响后续新建的会话) set persist optimizer_switch=‘derived_condition_pushdown=on’; – 设置全局值,并固化到配置⽂件mysqld-auto.cnf; 测试MySQL 版本: MySQL8.0.23 当derived_condition_pushdown=ON时: 测试语句1: set optimiz...
Mysql 8.0.22之后的版本支持派生条件回移(Derived Condition Pushdown)优化。该优化可以减少派生表处理的行数从而提高查询执行的效率。比方说有如下查询: SELECT * FROM (SELECT i, j FROM t1) AS dt WHERE i > constant 1. 2. 3. 通过派生条件回移优化后类似如下形式(WHERE条件拿到派生表的里面): ...
derived_condition_pushdown 按字⾯意思就是派⽣条件下推; MySQL8.0.22 开始对⽀持符合条件的子查询进⾏派⽣条件下推,derived_condition_pushdown=ON 后, 对于查询: SELECT * FROM (SELECT i, j FROM t1) AS dt WHERE i > constant 在许多情况下可能将外部的 where 条件下推到派⽣表,这会导致语...
derived_condition_pushdown 按字⾯意思就是派⽣条件下推; MySQL8.0.22 开始对⽀持符合条件的子查询‘’进⾏派⽣条件下推,derived_condition_pushdown=ON 后, 对于查询: SELECT * FROM (SELECT i, j FROM t1) AS dt WHERE i > constant
5 Derived Condition Pushdown Optimization MySQL 8.0.22 and later supports derived condition pushdown for eligible【ˈelɪdʒəbl有资格的;合格的;(指认为可做夫妻的男女)合意的,合适的,中意的;具备条件的;】 subqueries. For a query such as SELECT * FROM (SELECT i, j FROM t1) AS dt WHERE...
Description:Result mismatch when a condition is pushed down to inner tables. Set "derived_condition_pushdown=off" will solve this problem, but it will bring a huge performance loss. ### Have result, which is wrong. set optimizer_switch="derived_condition_pushdown=on"; select 1 as c0, ref_...
derived merge有限制的时候,往往是derived_condition_pushdown发挥作用的时候,但是也有一些限制对这两者都有影响。 1.派生表中含有max(),min(),count(),sum()等聚合函数,或者含有DISTINCT,GROUP BY,HAVING这些分组子句, 此时不会发生合并,但是外层查询的条件会推入派生表。这一点是derived_condition_pushdown主要发挥...