近期有朋友对于单个表上的index各种情况比较模糊,这里对于单个表上,单个index出现的大多数情况进行了总结性测试,给出了测试结果,至于为什么出现这样的试验结果未做过多解释,给读者留下思考的空间.本篇文章仅仅是为了测试hint对index的影响,而不是说明走各种index方式的好坏.参考: INDEX FULL SCAN vs INDEX FAST FULL SCAN
创建表模拟测试

SQL> create table t_xifenfei as select object_id,object_name from dba_objects;
 
Table created.
 
 
SQL>  create index i_t_object_id on t_xifenfei(object_id);
 
Index created.
 
SQL> exec dbms_stats.gather_table_stats(USER,'T_XIFENFEI',cascade=>true);
 
PL/SQL procedure successfully completed.
 
SQL> desc t_xifenfei
 Name                                      Null?    Type
 ----------------------------------------- -------- ----------------------------
 OBJECT_ID                                          NUMBER
 OBJECT_NAME                                        VARCHAR2(128)

TABLE ACCESS FULL

SQL> SET AUTOT TRACE EXP STAT
SQL> SELECT OBJECT_ID FROM T_XIFENFEI;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 548923532
 
--------------------------------------------------------------------------------
| Id  | Operation         | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |            | 49838 |   243K|    57   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| T_XIFENFEI | 49838 |   243K|    57   (2)| 00:00:01 |
--------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3544  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed
 
SQL> SELECT /*+ INDEX(T i_t_object_id) */ OBJECT_ID FROM T_XIFENFEI;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 548923532
 
--------------------------------------------------------------------------------
| Id  | Operation         | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |            | 49838 |   243K|    57   (2)| 00:00:01 |
|   1 |  TABLE ACCESS FULL| T_XIFENFEI | 49838 |   243K|    57   (2)| 00:00:01 |
--------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3544  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

从上面的执行计划中可知,此时走了全表扫描. 由于我们需要查询的列为object_id,因此理论上只需要读取索引就应该可以返回所有数据,而此时为什么是全表扫描呢？ 这是因为NULL值与索引的特性所决定的.即null值不会被存储到B树索引.因此应该为表 t_xifenfei 的列 object_id 添加 not null 约束.

INDEX FAST FULL SCAN

SQL> alter table t_xifenfei modify(object_id not null);  
 
Table altered.
 
SQL> SELECT  object_id from t_xifenfei;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2036340805
 
--------------------------------------------------------------------------------------
| Id  | Operation            | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |               | 49838 |   243K|    27   (4)| 00:00:01 |
|   1 |  INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 |   243K|    27   (4)| 00:00:01 |
--------------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3432  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

INDEX FAST FULL SCAN:当在高速缓存中没有找到所需的索引块时,则根据db_file_multiblock_read_count的值进行多块读操作.对于索引的分支结构只是简单的获取,然后扫描所有的叶结点.其结果是导致索引结构没有访问,获取的数据没有根据索引键的顺序排序.INDEX FAST FULL SCAN使用multiblock_read,故产生db file scattered reads 事件.

INDEX RANGE SCAN

SQL> select object_id from t_xifenfei where object_id<10;
 
8 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2197008162
 
----------------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               |     2 |    10 |     2   (0)| 00:00:01 |
|*  1 |  INDEX RANGE SCAN| I_T_OBJECT_ID |     2 |    10 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - access("OBJECT_ID"<10)
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          3  consistent gets
          0  physical reads
          0  redo size
        499  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed
 
SQL> select /*+ index_ffs(t i_t_object_id) */ object_id from t_xifenfei t where object_id<10;
 
8 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2036340805
 
--------------------------------------------------------------------------------------
| Id  | Operation            | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |               |     2 |    10 |    27   (4)| 00:00:01 |
|*  1 |  INDEX FAST FULL SCAN| I_T_OBJECT_ID |     2 |    10 |    27   (4)| 00:00:01 |
--------------------------------------------------------------------------------------
 
Predicate Information (identified by operation id):
---------------------------------------------------
 
   1 - filter("OBJECT_ID"<10)
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        118  consistent gets
          0  physical reads
          0  redo size
        499  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          8  rows processed
1
这里可以看出index_ffs已经生效,但是对于这样的情况hint index_ffs效率一般来说不会太高.
<br>
<strong>INDEX FULL SCAN</strong>
1
SQL> SELECT /*+ INDEX(T i_t_object_id) */ object_id  from t_xifenfei t;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 431110666
 
----------------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               | 49838 |   243K|   113   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN | I_T_OBJECT_ID | 49838 |   243K|   113   (2)| 00:00:02 |
----------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3426  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed

INDEX FULL SCAN:完全按照索引存储的顺序依次访问整个索引树.当访问到叶结点之后,按照双向链表方式读取相连节点的值.换言之,对于索引上所有的数据是按照有序的方式来读取的.如果索引块没有在高速缓存中被找到时,则需要从数据文件中单块进行读取.对于需要读取大量数据的全索引扫描而言,这将使其变得低效.INDEX FULL SCAN使用single read,故产生db file sequential reads事件.新版的Oracle支持db file parallel reads方式.
HINT INDEX不会使用INDEX FAST FULL SCAN功能.

INDEX列ORDER BY

SQL> SELECT OBJECT_ID FROM T_XIFENFEI order by object_id ;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 431110666
 
----------------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               | 49838 |   243K|   113   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN | I_T_OBJECT_ID | 49838 |   243K|   113   (2)| 00:00:02 |
----------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3426  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed
 
SQL> SELECT OBJECT_ID FROM T_XIFENFEI order by object_id  desc;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2808014233
 
--------------------------------------------------------------------------------------------
| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |               | 49838 |   243K|   113   (2)| 00:00:02 |
|   1 |  INDEX FULL SCAN DESCENDING| I_T_OBJECT_ID | 49838 |   243K|   113   (2)| 00:00:02 |
--------------------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
       3427  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
      49838  rows processed
 
SQL> SELECT  /*+ index_ffs(t i_t_object_id) */ object_id from t_xifenfei t order by object_id;
 
49838 rows selected.
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2527678987
 
-----------------------------------------------------------------------------------------------
| Id  | Operation             | Name          | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |               | 49838 |   243K|       |   185   (4)| 00:00:03 |
|   1 |  SORT ORDER BY        |               | 49838 |   243K|  1192K|   185   (4)| 00:00:03 |
|   2 |   INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 |   243K|       |    27   (4)| 00:00:01 |
-----------------------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        117  consistent gets
          0  physical reads
          0  redo size
     721203  bytes sent via SQL*Net to client
      36927  bytes received via SQL*Net from client
       3324  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
      49838  rows processed

对于index 列排序,默认情况下会使用INDEX FULL SCAN/INDEX FULL SCAN DESCENDING而不选择使用INDEX FAST FULL SCAN,因为INDEX FAST FULL SCAN获得数据后,还需要做一次SORT ORDER BY操作

INDEX FAST FULL SCAN+SORT AGGREGATE

SQL> SELECT  count(object_id) FROM T_XIFENFEI;
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 3095383276
 
-------------------------------------------------------------------------------
| Id  | Operation             | Name          | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |               |     1 |    27   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE       |               |     1 |            |          |
|   2 |   INDEX FAST FULL SCAN| I_T_OBJECT_ID | 49838 |    27   (4)| 00:00:01 |
-------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        117  consistent gets
          0  physical reads
          0  redo size
        421  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
 
SQL> SELECT  /*+ INDEX(T i_t_object_id) */ count(object_id) FROM T_XIFENFEI t;
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 3079973526
 
--------------------------------------------------------------------------
| Id  | Operation        | Name          | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT |               |     1 |   113   (2)| 00:00:02 |
|   1 |  SORT AGGREGATE  |               |     1 |            |          |
|   2 |   INDEX FULL SCAN| I_T_OBJECT_ID | 49838 |   113   (2)| 00:00:02 |
--------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
        111  consistent gets
          0  physical reads
          0  redo size
        421  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

sort aggregate通常发生在使用一些聚合函数的时候,sum(),avg(),min(),max(),count()等等,实际上sort aggregate不做真正的sort,并不会用到排序空间,而是通过一个全局变量＋全表或全索引扫描来实现.这样的操作在默认情况下使用INDEX FAST FULL SCAN

INDEX FULL SCAN (MIN/MAX)

SQL> SELECT  max(object_id) FROM T_XIFENFEI;
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2939893782
 
--------------------------------------------------------------------------------------------
| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |               |     1 |     5 |     2   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE            |               |     1 |     5 |            |          |
|   2 |   INDEX FULL SCAN (MIN/MAX)| I_T_OBJECT_ID | 49838 |   243K|     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        419  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
 
SQL>  SELECT /*+ index_ffs(t i_t_object_id) */ max(object_id) FROM T_XIFENFEI t;
 
 
Execution Plan
----------------------------------------------------------
Plan hash value: 2939893782
 
--------------------------------------------------------------------------------------------
| Id  | Operation                  | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |               |     1 |     5 |    27   (4)| 00:00:01 |
|   1 |  SORT AGGREGATE            |               |     1 |     5 |            |          |
|   2 |   INDEX FULL SCAN (MIN/MAX)| I_T_OBJECT_ID | 49838 |   243K|    27   (4)| 00:00:01 |
--------------------------------------------------------------------------------------------
 
 
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          2  consistent gets
          0  physical reads
          0  redo size
        419  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

对于这样的查询INDEX FULL SCAN (MIN/MAX)明显是最优,但是此处奇怪的是使用了index_ffs提示无效,如果有知道的朋友,麻烦告知原因.

http://www.xifenfei.com/2968.html

执行计划中常见index访问方式(转)的更多相关文章

  1. MySQL 执行计划中Extra(Using where,Using index,Using index condition,Using index,Using where)的浅析

      关于如何理解MySQL执行计划中Extra列的Using where.Using Index.Using index condition,Using index,Using where这四者的区别 ...

  2. Sql Server中的表访问方式Table Scan, Index Scan, Index Seek

    1.oracle中的表访问方式 在oracle中有表访问方式的说法,访问表中的数据主要通过三种方式进行访问: 全表扫描(full table scan),直接访问数据页,查找满足条件的数据 通过row ...

  3. 转：Sql Server中的表访问方式Table Scan, Index Scan, Index Seek

    0.参考文献 Table Scan, Index Scan, Index Seek SQL SERVER – Index Seek vs. Index Scan – Diffefence and Us ...

  4. SQL Server 执行计划中的扫描方式举例说明

    SQL Server 执行计划中的扫描方式举例说明 原文地址:http://www.cnblogs.com/zihunqingxin/p/3201155.html 1.执行计划使用方式 选中需要执行的 ...

  5. [转载] EXPLAIN执行计划中要重点关注哪些要素

    原文: https://mp.weixin.qq.com/s?__biz=MjM5NzAzMTY4NQ==&mid=400738936&idx=1&sn=2910b4119b9 ...

  6. EXPLAIN执行计划中要重点关注哪些要素（叶金荣）

    原文:http://mp.weixin.qq.com/s/CDKN_nPcIjzA_U5-xwAE5w 导读 EXPLAIN的结果中,有哪些关键信息值得注意呢? MySQL的EXPLAIN当然和ORA ...

  7. EXPLAIN执行计划中要重点关注哪些要素

    MySQL的EXPLAIN当然和ORACLE的没法比,不过我们从它输出的结果中,也可以得到很多有用的信息. 总的来说,我们只需要关注结果中的几列: 列名 备注 type 本次查询表联接类型,从这里可以 ...

  8. EXPLAIN执行计划中要重点关注哪些要素(转)

    EXPLAIN的结果中,有哪些关键信息值得注意呢? MySQL的EXPLAIN当然和ORACLE的没法比,不过我们从它输出的结果中,也可以得到很多有用的信息. 总的来说,我们只需要关注结果中的几列: ...

  9. SQL Server 2000中的并行处理和执行计划中的位图运算符

    SQL Server 2000中的并行处理和执行计划中的位图运算符 摘抄自:SQLServer 2000并行处理和位图简介 刘志斌 并行查询介绍Degree of Parallelism(并行度) 一 ...

随机推荐

  1. NSNotificationCenter消息机制的介绍

    转载自http://www.cnblogs.com/pengyingh/articles/2367374.html NSNotificationCenter的作用是专门提供程序中不同类之间的消息通讯而 ...

  2. 如何学习一门新技术-iOS开发

    如何快速学习一门新技术 以CoreBluetooth 蓝牙开发为例.我们可以从官方获得的资源有:SampleCode, Documentation,API Reference. 先从Documenta ...

  3. Linux 关闭及重启方式

    一.shutdown 命令 作用:关闭或重启系统 使用权限:超级管理员使用 常用选项 1. -r 关机后立即重启 2. -h关机后不重启 3. -f快速关机,重启时跳过fsck(file system ...

  4. GET和POST本质上有什么区别

    如果有人问你,GET和POST,有什么区别?你会如何回答? 我的经历 前几天有人问我这个问题.我说GET是用于获取数据的,POST,一般用于将数据发给服务器之用. 这个答案好像并不是他想要的.于是他继 ...

  5. javascript动画效果

    之前工作项目中,运用了缓动动画的效果,在网上看到其他大牛写的相关公式,结合工作需要,进行了整理,拿出来跟大家分享下,js代码中,只运用了一个小功能进行了测试 <!DOCTYPE html> ...

  6. asp.net模态窗口返回值

    个人感觉模态窗口在做网站的时候,使用到的比较少,前段时间在做项目时要实现以模态窗口传值和接收返回值, 模态窗口传值实现比较简单,但是做好后发现在Chrome浏览器中接收不到返回值,修改好Chrome浏 ...

  7. 完全卸载oracle

    今天在网上看到有位网友写的篇日志,感觉蛮好的,一般卸载oracle有4个地方需求注意:1)Services,2)software,3eventlog,4)path. 1.关闭 oracle 所有的服务 ...

  8. 在treeview外加一个滚动条的实现

    前台代码: <div style="overflow:auto;width:190px;height:280px;border:1px solid #336699;padding-le ...

  9. exp/imp 有很多弊端

    弊端1. 空表 无法执行导出操作弊端2. 高版本的导出文件  无法使用 低版本的 oracle软件 导入 环境准备:create table test0707(n1 date); 认证弊端1 案例1. ...

  10. svn的初级使用

    首先呢 你需要下载一个软件  比如说是 Cornerstone. 进行安装好之后 然后 然后输入账号密码 就可以了 然后去xcode去进行相关的配置 点击第二个进入 偏好设置 点击最下边的+ 点击第二 ...