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. Qt 学习之路：线程和 QObject

    前面两个章节我们从事件循环和线程类库两个角度阐述有关线程的问题.本章我们将深入线程间得交互,探讨线程和QObject之间的关系.在某种程度上,这才是多线程编程真正需要注意的问题. 现在我们已经讨论过事 ...

  2. viewController启动方法分析

    viewController启动方法分析 转载:http://blog.csdn.net/dizzthxl/article/details/14170047 首先理清楚一个概念: 创建一个类和实例化一 ...

  3. ipad itunes 恢复

    http://jingyan.baidu.com/article/a3aad71aa58efbb1fa00967c.html http://act.feng.com/wetools/index.php ...

  4. Java基础知识强化之集合框架笔记32：集合之可变参数的概述和使用

    1. 可变参数的概述和使用: (1)可变参数:定义方法的时候不知道该定义多少个参数(2)格式:     修饰符  返回值类型  方法名(数据类型… 变量名){   }  注意: 这里的变量其实是一个数 ...

  5. windows下安装redis和php的redis扩展

    1.redis简介 redis是一个key-value存储系统.和Memcached类似,它支持存储的value类型相对更多,包括string(字符串).list(链表).set(集合).zset(s ...

  6. 给Sublime Text2安装轻量级代码提示插件：SublimeCodeIntel

    步骤: 1.下载SublimeCodeIntel(地址https://github.com/SublimeCodeIntel/SublimeCodeIntel): 2.将下载的压缩包解压,并放置在Pa ...

  7. How to customize authentication to my own set of tables in asp.net web api 2?

    ssuming your table is called AppUser, convert your own AppUser domain object to IUser(using Microsof ...

  8. C++ Reference 的“三位一体”诠释

    C++ 是介于汇编语言与高级语言之间的一种“全能”语言.它的能力是其他任何基于VMA(冯-诺曼架构)计算机的高级程序设计语言无法望其项背的,而性能也只有C语言可与之伯仲. 然而长期以来,喜欢C++和憎 ...

  9. nginx插件ngx_lua

    ngx_lua是淘宝的维护的产品,真心不错.配置文件包含可以做很多事情的lua脚本. 公司有个产品对注册的广告盒子进行反向代理,这样可以在盒子上做很多事情:和服务器通信,远程控制盒子等等.nginx反 ...

  10. OC 字符和日期的互转

    需要用到NSDateFormatter这个类. 1.字符串转换为日期 [plain]NSDateFormatter* dateFormat = [[NSDateFormatter alloc] ini ...