SET optimizer_switch='mrr=on,mrr_cost_based=off,batched_key_access=on'; 1. 我们再来看执行计划 tbl_user_login_log 查询到的 user_name 的值先放到 join buffer,当 join buffer 满了或者数据查完了,再对 join buffer 里面的值进行排序,然后再去关联 tbl_user ,此时就会顺序匹配索引 i_aaa 如果需要...
-- 左外连接不包含内连接 select customers.cust_id,orders.order_num from customers left outer join orders on customers.cust_id = orders.cust_id where orders.cust_id is null; -- 右外连接不包含内连接 select customers.cust_id,orders.order_num from customers right outer join orders on customers...
【2】为了从考虑的条件中消除行。 【3】如果表有一个multiple-column索引,任何一个索引的最左前缀可以通过使用优化器来查找行。 【4】查询中与其它表关联的字,字段常常建立了外键关系 【5】查询中统计或分组统计的字段 select max(hbs_bh) from zl_yhjbqk select qc_bh,count(*) from zl_yhjbqk group by ...
SET optimizer_switch='mrr=on,mrr_cost_based=off,batched_key_access=on'; 我们再来看执行计划 从tbl_user_login_log 查询到的user_name 的值先放到 join buffer,当 join buffer 满了或者数据查完了,再对 join buffer 里面的值进行排序,然后再去关联tbl_user ,此时就会顺序匹配索引i_aaa ,类似下图 如果...
join_buffer_size默认大小是:262144 B=256 KB,这个大小是分配给每个join的 One buffer is allocated for each join that can be buffered, so a given query might be processed using multiple join buffers. mysql> show variables like 'join_buffer_size%'; ...
If the table has a multiple-column index, any leftmost prefix of the index can be used by the optimizer to look up rows. For example, if you have a three-column index on (col1, col2, col3), you have indexed search capabilities on (col1), (col1, col2), and (col1, col2, co...
left join multiple Posted by:Jon Wojkowski Date: May 09, 2011 11:40AM CREATE TABLE t1 (a INT, b INT, c INT, d INT); CREATE TABLE t2 (a INT, b INT, e INT, f INT); CREATE TABLE t3 (a INT, b INT, g INT, h INT);...
A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns. Note As an alternative to a composite index, you can introduce a column that is “hashed” based on information from other columns....
6、Join优化 MySQL的join语句连接表使用的是nested-loop join算法，这个过程类似于嵌套循环，简单来说，...
INNER JOIN lastclock AS lastclock ON lastclock.emplid = w.emplid AND lastclock.tsdt = w.tsdt WHERE w.shift_no IS NOT NULL AND w.shift_no < 99 AND w.relevantdt IS NOT NULL GROUP BY w.emplid, w.tsdt; I recognize that this query will take a while to execute, as it will return...