SQL开发实战系列(五):从执行计划看IN、EXISTS 和 INNER JOIN效率,我们要分场景不要死记网上结论 - 一、组合相关的行相对查询单表中的数据来说,平时更常见的需求是要在多个表中返回数据。比如,显示部门10的员工编码、姓名及所在部门名称和工作地址。select a.empno,a.d
--this statement needs to check the entire table select count(*) from [table] where ...--thi...
SQL>select /*+ monitor */ SQL_ID, SQL_PLAN_HASH_VALUE, SQL_PLAN_LINE_ID, count(*)2 from T1 3 where T1.sql_id in (select T2.sql_id from T2) 4 group by SQL_ID, SQL_PLAN_HASH_VALUE, SQL_PLAN_LINE_ID 5 order by 1; SQL_ID SQL_PLAN_HASH_VALUE SQL_PLAN_LINE_ID COUNT(*)...
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=363 Card=1 Bytes=3) 1 0 SORT (AGGREGATE) 2 1 FILTER 3 2 TABLE ACCESS (FULL) OF 'BIG_TABLE' (Cost=363 Card=1011 2 Bytes=30336) 4 2 INDEX (RANGE SCAN) OF 'IDX_SMALL_TABLE' (NON-UNIQUE) ( Cost=1 Card=1 Bytes=3) Statistics --- 0 r...
环境:Oracle 19.16 多租户架构 经常会在网上看到有人写exists和in的效率区别,其实在新版本的数据库中,是不存在这个问题的,优化器会自己判断选择最优的执行计划。 为了直观的说明,我在PDB中构造如下测试用例: vi 1.sql 代码语言:javascript 代码运行次数:0 ...
Hello, select v_R_SYSTEM.ResourceID from v_R_System WHERE NOT EXISTS( select v_R_SYSTEM.ResourceID /* ,v_R_SYSTEM.ResourceType,v_R_SYSTEM.Name0,v_R_SYSTEM.SMS_Unique_Identifier0, v_R_SYSTEM.Resource_Domain_OR_Workgr0,v_R_SYSTEM.Client0,…
syntaxsql EXISTS( subquery ) Arguments subquery Is a restricted SELECT statement. The INTO keyword is not allowed. For more information, see the information about subqueries inSELECT (Transact-SQL). Result Types Boolean Result Values Returns TRUE if a subquery contains any rows. ...
1、关于在 Oracle8i 时代中in和exists的区别这里有条SQL语句:select * from A where id in(select id from B) 以上查询使用了in语句,in()只执行一次,它查出B表中的所有id字段并缓存起来.之后,检查A表的id是否与B表中的id相等,如果相等则将A表的记录加入结果集中,直到遍历完A表的所有记录;它的查询过程类似...
If strict SQL mode is enabled and any of these initial columns do not have an explicit default value, the statement fails with an error. 2.如何在数据表存在的时候不创建也不插入重复的数据呢 目前我没有搜索到好的办法,知道的网友也希望不吝赐教,留言告知。一个解决的办法就是先drop table,再执行CRE...
There are multiple ways in which a sub select or lookup can be framed in a SQL statement. PostgreSQL optimizer is very smart at optimizing queries, and many of the queries can be rewritten/transformed for better performance. Let’s discuss the topic with an example, for which I am using ...