要启用derived condition pushdown,需要打开optimizer_switch中的系统变量——derived_condition_pushdown。如果optimizer_switch禁用了derived_condition_pushdown,可以使用优化器hint——DERIVED_CONDITION_PUSHDOWN为特定查询启用它。也可以使用优化hint——NO_DERIVED_CONDITION_PUSHDOWN来禁用给定查询的优化。以下约束和限制被应用...
SHOWVARIABLESLIKE'derived_condition_pushdown'; 1. 该命令将返回当前的derived_condition_pushdown设置值。 2. 设置 derived_condition_pushdown 为 OFF 要将derived_condition_pushdown设置为OFF,我们可以使用以下 SQL 命令: SETGLOBALderived_condition_pushdown=OFF; 1. 请注意,设置 GLOBAL 可能需要管理员权限,并且更...
mysql> set optimizer_switch="derived_merge=off,derived_condition_pushdown=off"; Query OK, 0 rows affected (0.00 sec) 既不合并,又不下推,此时要对t1全表进行物化,再对物化结果集进行条件过滤。这种情况效率是最低的。 从上面的实验可以看出使用derived_merge, 可以避免不必要的物化,合并后,相当于将外查询...
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): SELECT *...
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
所以会利⽤ sesson 已有的@r的值进⾏ where 条件匹配,结果不符合预期。 当derived_condition_pushdown=OFF 时: 测试语句1: set optimizer_switch='derived_condition_pushdown=off'; set @r=0;select * from (select @r := 603014203924416,@i := 0) vars where @r<>0 ;explain select * from (sele...
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
Version:8.1.0, 8.0.34OS:Ubuntu (22.04) Assigned to:CPU Architecture:x86 (x86_64) Tags:Derived condition push down [30 Jul 2023 5:35] Pedro Ferreira Description:Run these queries: SET SESSION optimizer_switch='derived_condition_pushdown=on'; SELECT 1 FROM (SELECT b'1') y(y) WHERE (NUL...
release) must be set toon, which is the default setting. If this optimization is disabled byoptimizer_switch, you can enable it for a specific query using theDERIVED_CONDITION_PUSHDOWNoptimizer hint. To disable the optimization for a given query, use theNO_DERIVED_CONDITION_PUSHDOWNoptimizer hint...
pushdown=off"; Variable 'optimizer_switch' can't be set to the value of 'derived_condition_pushdown=off' select * from (select c1, (@rownum_r := @rownum_r + 1) as r from t1, (select @rownum_r := 0) as b) as q where q.c1 = 300; c1 r 300 3...