Typical Access Paths
来源:互联网 发布:淘宝无线分类链接 编辑:程序博客网 时间:2024/04/27 22:57
Typical Access Paths
Db2 determine an access path for every executable SQL statement. To do this, it looks at several things, such as the catalog statistics and configuration parameters. It takes into consideration how large a table is, whether there is an index on the table, and so forth.
You are capable of predicting and influencing the access path by the way you write your SQL statement.
1. Relation scan
扫描整个表(where语句中的column没建索引)
2. Matching index scan
从根索引开始找,到叶子索引,最后到相应的data pages中取数据(where语句中的column已建索引,但select语句中存在没建索引的column)
3. Index-only access
从索引中就能找到所有数据,不在access the data pages(where语句中的column已建索引,并且select语句中的column都完全覆盖在此索引中)
4. Non-matching index scan
从叶子索引开始扫描,然后在相应的data pages中取数据(where语句中的column已建索引,但不是leading part of the index key, 并且select语句中存在没建索引的column)
- Typical Access Paths
- arm:typical memory size and access time
- How to Find Suspicious Access Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- Paths
- reSIProcate:Typical DUM callflow
- typical write device driver
- typical Transactions about Position
- some typical PL/SQL
- Indicator Variables
- Brew 学习
- Scrollable Cursors
- SAP call BAPI
- 两个Cookie类
- Typical Access Paths
- wmware的winxp虚拟机添加新硬盘以及调整vmware虚拟机硬盘空间的方法
- 实体定义:每一层都不相同,对于层间实体之间映射的一点思考
- 利用GetPrivateProfileString读取配置文件
- VC++6.0 全部默认快捷键
- java常用函数
- Java 学习笔记-手机书 Rev.2 (080411)
- General Guidelines for Better Performance
- Environ 函数取得系统参数