云和恩墨技术总监,Oracle ACE Director,ACOUG 核心专家 只要增加了DISTINCT关键字,Oracle就会对随后跟着的所有字段进行排序去重。以前也经常发现由于开发人员对SQL不是很理解,在SELECT列表的20多个字段前面添加了DISTINCT,造成查询的执行异常缓慢,基本上很难在ORA-1555错误出现之前得到查询的结果,甚至有些SQL会产生ORA-7445...
但一看SQL才发现,DISTINCT居然是在查询的最外层。...对于不加DISTINCT的情况:由于使用IN子查询,Oracle对第二个连接采用了HASH JOIN SEMI,这种方式相对于普通的HASHJOIN来说代价要大一些。...这篇文章并不是在介绍一种优化SQL的方法,严格意义上讲,加上DISTINCT和不加DISTINCT是两个完全不同的SQL语句。
select string_agg(sq.name) ,sq.id from ( select distinct a.name ,...
acl c cst distinct group IN IS nc OR oracle rac st 优化2020-12-16 上传大小:47KB 所需:34积分/C币 oracle性能优化总结 oracle数据库性能优化的一些总结,供大家参考学习。 上传者:zhenjinsuo时间:2018-10-23 oracle数据库sql优化 针对oracle数据库的sql语句优化。
“Unable to enlist in the transaction” with Oracle linked server from MS SQL Server [<Name of Missing Index, sysname,>] in non clustered index [Execute SQL Task] Error: The value type (__ComObject) can only be converted to variables of type Object. [ODBC Driver Manager] Data source nam...
“Unable to enlist in the transaction” with Oracle linked server from MS SQL Server [<Name of Missing Index, sysname,>] in non clustered index [Execute SQL Task] Error: The value type (__ComObject) can only be converted to variables of type Object. [ODBC Driver Manager] Data source nam...
SQL 语句可返回唯一不同的值 A. UNIQUE是完整性约束里的一种,如果某列的值需要是唯一的那么就添加UNIQUE约束 Distinct是在查询时用的,若在SELECT的列 分享1赞 java吧 万马奔骋 oracle字段去重复oracle 怎么给除了主键之外的其他字段去重复,得到的结果就是除了主键之外,其他字段一起不重复,但是 分享1赞 niit吧 ...
Typically in SQL database engines, the use ofCOUNT(DISTINCT [field])within a query is used to count the exact number ofDISTINCTitems within the specified field. In Google BigQuery, however,COUNT(DISTINCT [field])functions slightly differently due to the massive quantities of data that are often...
Description: SELECT DISTINCT returns "empty set" when SELECT using primary key values in an IN() condition statement when a unique index of two non-primary columns is defined for the InnoDB table. It does not change the behavior if the two columns in the UNIQUE KEY are BIGINT/VARCHAR or ...
我们现在的sql:select count(distinct nick) from user_access;则是直接从nick1开始一条条扫描下来,直到扫描到最后一个nick_n, 那么中间过程会扫描很多重复的nick,如果我们能够跳过中间重复的nick,则性能会优化非常多(在oracle中,这种扫描技术为loose index scan,但在5.1的版本中,mysql中还不能直接支持这种优化技术...