I'm happy with using UNION but if a longer more convoluted query is faster than the simple join type, I'm wondering if there are alternative methods that might speed things up. Here's the long winded but faster
outer join分为left outer join、right outer join和full outer join。 left outer join是以左表驱动,右表不存在的key均赋值为null; right outer join是以右表驱动,左表不存在的key均赋值为null; full outer join全表关联,将两表完整的进行笛卡尔积操作,左右表均可赋值为null。 create table if not exists ba...
4 查看表中索引的个数(仅仅在MYSQL8 中作用,information_schema) 代码语言:javascript 代码运行次数:0 运行 AI代码解释 select it.name,ii.index_count from innodb_tablesasit innerjoin(select table_id,count(*)asindex_countfromINNODB_INDEXESgroup byTABLE_ID)asii on it.table_id=ii.table_id where it....
左连接(Left Join)是一种关联查询方式,它返回左表中的所有记录,以及右表中与左表关联字段匹配的记录。简单来说,左连接可以用来获取左表中的所有数据,无论是否有匹配的右表数据。 下面是一个示例,展示了如何在Django中进行两个表之间的左连接: 假设我们有两个模型,一个是Order(订单)模型,另一个是Product(产品...
Simple nested loop join算法执行的逻辑是:顺序取出驱动表中的每一行数据,到被驱动表中做全表扫描匹配,匹配成功则作为结果集的一部分返回。Simple nested loop join算法其实也是把数据读入内存里,然后按照条件进行判断,为什么性能差距胡这么大--解释这个问题,需要用到mysql中索引结构和buffer pool的相关知识...
(use 2 windows) : in window 1 run : top Then monitor the mysql processes in window 2 run : mysql -u test -ptest use test; select t1.*, t2.* from t1 left join t2 on t1.id=t2.id and t1.did=2; Then watch in the top window as the resident memory size of the mysqld ...
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Bug #22367 Optimizer uses ref join type instead of eq_ref for simple join on strings Submitted: 14 Sep 2006 20:52Modified: 26 Oct 2006 3:31 Reporter: jocelyn fournier (Silver Quality Contributor) Email Updates: Status: Closed Impact on me: None Category: MySQL Server: OptimizerSeverity:...
Re: simple JOIN with sub SELECT strangeness 2080 Rick James October 21, 2009 08:52PM Sorry, you can't reply to this topic. It has been closed. Content reproduced on this site is the property of the respective copyright holders. It is not reviewed in advance by Oracle and does not neces...