| -> Limit: 100 row(s) (cost=14.62 rows=1) (actual time=1.252..1.252 rows=0 loops=1) -> Gather (merge sort; slice: 1; workers: 1; nodes: 2) (cost=14.62 rows=1) (actual time=1.250..1.250 rows=0 loops=1) -> Limit: 100 row(s) (cost=4.52 rows=1) (actual time=0.084,0....
system是const的特例,表里只有一条数据时为 system; (使用主键或者唯一索引的时候会出现) mysql> set session optimizer_switch='derived_merge=off'; #关闭mysql5.7新特性对衍生表的合并优化mysql> explain extended select * from (select * from film where id = 1) tmp;mysql> set session optimizer_switch=...
Using index for limit:表示查询使用了索引来执行 LIMIT 操作。 Using index for merge:表示查询使用了索引合并进行连接操作。 Using where with pushed condition:表示查询的 WHERE 条件中的部分条件被推迟执行。 Loose index scan: 表示在一些情况下,MySQL 可能对索引的列进行宽松扫描,而不是精确匹配。 Range checke...
In C++, describe an iterative version of mergeSort. Explain a buffer overflow. How does input output work? Explain 1's complement and 2's complement of a binary number. Explain briefly with example. In C++, what is "tail recursion" and what is it mainly used for? Provide an example. ...
Sort sort表示将数据按顺序排序(升序ASC或者降序DESC),通常是order by子句的结果。 示例:对TPC-H lineitem表的l_shipdate列进行排序输出。 EXPLAIN SELECT l_shipdate FROM public.lineitem ORDER BY l_shipdate; 返回结果: 调优建议:如果order by的数据量较大,将会消耗较多的资源,需要尽量避免大数据量的排序查...
如果我们想看看某个查询的执行计划的话,可以在具体的查询语句前边加一个 EXPLAIN ,就像这样:在select 语句之前增加 explain 关键字,它会返回一行或多行信息,显示出执行计划中的每一部分和执行的顺序。 EXPLAIN select * from actor; image-20240329211315166 ...
The following example illustrates an LF SecureScan node, which you can use to view Lake Formation-RLS relations.EXPLAIN SELECT * FROM lf_db.public.t_share WHERE a > 1; QUERY PLAN --- XN LF SecureScan t_share (cost=0.00..0.02 rows=2 width=11) (2 rows)Examples Note For these exampl...
VALUES ('张三', 'zhangsan@example.com', 'password123'), ('李四', 'lisi@example.com', 'password123'), ('王五', 'wangwu@example.com', 'password123'), ('赵六', 'zhaoli@example.com', 'password123'), ('钱七', 'qianqi@example.com', 'password123'); ...
EXPLAIN is used to obtain a query execution plan (that is, an explanation of how MySQL would execute a query). 简单翻译一下,就是explain用于获取查询执行计划(即MySQL是如何执行一个查询的)。 工作中,我们会遇到慢查询,这个时候我们就可以在select语句之前增加explain关键字,模拟MySQL优化器执行SQL语句,从而...
unionM,N: The row refers to the union of the rows with id values of M and N. derivedN: The row refers to the derived table result for the row with an id value of N. A derived table may result, for example, from a subquery in the FROM clause. ...