SQL> set linesize 200
SQL> set pagesize 200
SQL> set autot trace
SQL> select distinct department_name
from hr.departments dept, hr.employees emp
where dept.department_id = emp.department_id; 2 3 11 rows selected. Execution Plan
----------------------------------------------------------
Plan hash value: 93782236 -----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 27 | 513 | 4 (25)| 00:00:01 |
| 1 | HASH UNIQUE | | 27 | 513 | 4 (25)| 00:00:01 |
| 2 | NESTED LOOPS | | 106 | 2014 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| DEPARTMENTS | 27 | 432 | 3 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 4 | 12 | 0 (0)| 00:00:01 |
----------------------------------------------------------------------------------------- Predicate Information (identified by operation id):
--------------------------------------------------- 4 - access("DEPT"."DEPARTMENT_ID"="EMP"."DEPARTMENT_ID") Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
10 consistent gets
0 physical reads
0 redo size
622 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
11 rows processed 走的是hash unique:
Oracle10g在distinct操作时作了算法改进,使用Hash Unique 代理了以前的Sort Unique.该行为由隐藏参数” _gby_hash_aggregation_enabled”决定,optimizer_features_enable设置为10.2.0.1时默认为TRUE. SQL> set linesize 200
SQL> set pagesize 200
SQL> ALTER SESSION SET "_gby_hash_aggregation_enabled" = FALSE; Session altered. SQL> set autot trace
SQL> select distinct department_name
from hr.departments dept, hr.employees emp
where dept.department_id = emp.department_id; 2 3 11 rows selected. Execution Plan
----------------------------------------------------------
Plan hash value: 1155849093 -----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 27 | 513 | 4 (25)| 00:00:01 |
| 1 | SORT UNIQUE | | 27 | 513 | 4 (25)| 00:00:01 |
| 2 | NESTED LOOPS | | 106 | 2014 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| DEPARTMENTS | 27 | 432 | 3 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | EMP_DEPARTMENT_IX | 4 | 12 | 0 (0)| 00:00:01 |
----------------------------------------------------------------------------------------- Predicate Information (identified by operation id):
--------------------------------------------------- 4 - access("DEPT"."DEPARTMENT_ID"="EMP"."DEPARTMENT_ID") Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
10 consistent gets
0 physical reads
0 redo size
622 bytes sent via SQL*Net to client
419 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
11 rows processed

Hash unique和Sort unique的更多相关文章

  1. SORT UNIQUE|AGGREGATE|GROUP BY|ORDER BY|JOIN

    相信做oracle开发和管理的朋友对sort肯定不会陌生,大家通常都遇到这样那样的排序性能问题,所以我写这一系列关于sort的文章告诉大家在oracle里面sort是怎么一回事以及如果调整sort获得 ...

  2. 去重函数unique,sort,erase的应用

    std::unique 一.总述 unique函数属于STL中比较常用函数,它的功能是元素去重.即"删除"序列中所有相邻的重复元素(只保留一个).此处的删除,并不 是真的删除,而是 ...

  3. Nested Loops,Hash Join 和 Sort Merge Join. 三种不同连接的不同:

    原文:https://blog.csdn.net/tianlesoftware/article/details/5826546 Nested Loops,Hash Join 和 Sort Merge ...

  4. vector 去除重复元素(sort + unique)

    struct SIndexData { string sCode; int iDate; int iTime; double f1; SIndexData(): iDate(0), iTime(0) ...

  5. PAT L2-021. 点赞狂魔 /// sort+unique去重

    https://www.patest.cn/contests/gplt/L2-021 题目大意: 微博上有个“点赞”功能,你可以为你喜欢的博文点个赞表示支持.每篇博文都有一些刻画其特性的标签,而你点赞 ...

  6. 62. Unique Paths && 63 Unique Paths II

    https://leetcode.com/problems/unique-paths/ 这道题,不利用动态规划基本上规模变大会运行超时,下面自己写得这段代码,直接暴力破解,只能应付小规模的情形,当23 ...

  7. php Redis函数使用总结(string,hash,list, set , sort set )

    对于:string, set , sort set , hash 的增,改操作,是同一个命令,但是把它当改操作时,及时成功返回值依旧为0 对于:list结构来说,增删改查自有一套方法.   <? ...

  8. Nested loops、Hash join、Sort merge join(三种连接类型原理、使用要点)

    nested loop 嵌套循环(原理):oracle从较小结果集(驱动表.也可以被称为outer)中读取一行,然后和较大结果集(被侦查表,也可以叫做inner)中的所有数据逐条进行比较(也是等值连接 ...

  9. Differences between INDEX, PRIMARY, UNIQUE, FULLTEXT in MySQL?

    487down vote Differences KEY or INDEX refers to a normal non-unique index.  Non-distinct values for ...

随机推荐

  1. Objective-C--@property,@synthesize关键字介绍

    Objective-C–@property,@synthesize关键字介绍 转载:http://www.cnblogs.com/QM80/p/3576282.html /** 注意:由@proper ...

  2. WisDom.Net 框架设计(二) 服务总线

    WisDom.Net 框架设计--服务总线 1.Soa 简介     soa 就是面向服务的体系结构 是一个组件模型,不同的组件之间通过定义良好的接口联系起来.就像盖房子一块砖头一块砖头的砌墙,一片一 ...

  3. Creating a web application.

    About creating web GIS applications As you learn and use ArcGIS for Server, you'll probably reach th ...

  4. Sprite Kit教程:初学者

    作者:Ray Wenderlich 原文出处:点击打开链接 http://www.raywenderlich.com/42699/spritekit-tutorial-for-beginners 转自 ...

  5. 段落排版--缩进(text-indent)

    中文文字中的段前习惯空两个文字的空白,这个特殊的样式可以用下面代码来实现: p{text-indent:2em;} <p>1922年的春天,一个想要成名名叫尼克卡拉威(托比?马奎尔Tobe ...

  6. Android开发系列----学习伊始

    因为对移动端开发开始感兴趣,开始学习App开发,没有苹果环境的我,只好先选择Android来玩一玩了~~ 找了一套视频,买了几本java.android开发的书,开始搞起~~

  7. 一些javascript免费中文书籍

    在这里谢谢那些无私的人~~这些内容来自这里:我只把js的链接粘到这里了~ 还有其它技术文档, 实在是太多了, 多的看都看不完!!! Google JavaScript 代码风格指南 Google JS ...

  8. wordpress version

    version info /readme.html /wp-includes/version.php remove copyright 1.wp-login.php //<h1><a ...

  9. nodejs初体验

    安装好nodejs之后 在命令行中直接运行:node -v //若安装成功则显示版本号 var http = require('http'); http.createServer(function ( ...

  10. CodeIgniter框架——介绍

    CodeIgniter 是一个应用程序框架 CodeIgniter 是一个为用 PHP 编写网络应用程序的人员提供的工具包.它的目标是实现让你比从零开始编写代码更快速地开发项目,为此,CI 提供了一套 ...