If you want to make your queries as fast as possible, look out for Extra column values of Using filesort and Using temporary, or, in JSON-formatted EXPLAINoutput, for using_filesort and using_temporary_table properties equal to true. 大概的意思就是说,如果你想要优化你的查询,那就要注意extra...
对于结果集很大的情况,merge-join需要对其排序效率并不会很高,而nested loop join是一种嵌套循环的查询方式无疑更不适合大数据集的连接,而hash-join正是为处理这种棘手的查询方式而生,尤其是对于一个大表和一个小表的情况,基本上只需要将大小表扫描一遍就可以得出最终的结果集。 不过hash-join只适用于等值连接,对...
When EXPLAIN is used with FOR CONNECTION connection_id rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 8.8.4, “Obtaining Execution Plan Information for a Named Connection”. 当EXPLAIN与FOR CONNECTION connection_id...
被驱动表的关联字段没索引为什么要选择使用 BNL 算法而不使用 Nested-Loop Join 呢? 如果上面第二条sql使用 Nested-Loop Join,那么扫描行数为 100 * 10000 = 100万次,这个是磁盘扫描。 很显然,用BNL磁盘扫描次数少很多,相比于磁盘扫描,BNL的内存计算会快得多。 因此MySQL对于被驱动表的关联字段没索引的关联查询...
for x in (select p.plan_table_output from table(dbms_xplan.display_cursor(null, null, 'advanced -bytes -PROJECTION allstats last')) p) loop dbms_output.put_line(x.plan_table_output); end loop; rollback; end; / 两种窗口: 1、SQL窗口的,执行SQL后只能去output查看; ...
| 1 | SIMPLE | s2 | NULL | ALL | NULL | NULL | NULL | NULL | 9954 | 100.00 | Using join buffer (Block Nested Loop) | +---+---+---+---+---+---+---+---+---+---+---+---+ 2 rows in set, 1 warning (0.01 sec) 可以看到这个连接查询的执行计划中有两条记录,这...
UNCACHEABLE UNION The second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY) SIMPLE 查询语句中不包含UNION或者子查询的查询都算作是SIMPLE类型,比如常见的单表查询和连接查询等。PRIMARY 对于包含UNION、UNION ALL或者子查询的大查询来说,它是由几个小查询组成...
2.10.6Using join buffer (Block Nested Loop) explain select * from user where id in(select id from user where sex='no'); mysql> explain select * from user where id in(select id from user where sex='no'); +---+---+---+---+---+---+---+---+---+---+---+---+ | ...
优先优化NestedLoop的内层循环,保证Join语句中被驱动表上Join条件字段已经被索引。当无法保证被驱动表的Join条件字段被索引且内存资源充足的前提下,不要太吝惜JoinBuffer的设置5.4 索引失效案例建表:CREATE TABLE staffs( id INT PRIMARY KEY AUTO_INCREMENT, `name` VARCHAR(24) NOT NULL DEFAULT'' COMMENT'姓名',...
---1SIMPLE a (NULL)ALL(NULL) (NULL) (NULL) (NULL)1100.00(NULL)1SIMPLE b (NULL)ALL(NULL) (NULL) (NULL) (NULL)1100.00Usingwhere; Usingjoinbuffer (Block Nested Loop)1SIMPLE c (NULL)ALL(NULL) (NULL) (NULL) (NULL)1100.00Usingwhere; FirstMatch(b); Usingjoinbuffer (Block Nested Loop)...