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Oracle 反向索引(反转建索引) 理解

时间:2018-12-24 00:18:43      阅读:195      评论:0      收藏:0      [点我收藏+]

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  • 一 反向索引

    1.1 反向索引的定义

    • 反向索引作为B-tree索引的一个分支,主要是在创建索引时,针对索引列的索引键值进行字节反转,进而实现分散存放到不同叶子节点块的目的。

     

    1.2 反向索引针对的问题

    • 使用传统的B-tree索引,当索引的列是按顺序产生时,相应的索引键值会基本分布在同一个叶块中。当用户对该列进行操作时,难免会发生索引块的争用。
    • 使用反向索引,将索引列的键值进行反转,实现顺序的键值分散到不同的叶块中,从而减少索引块的争用。
    • 例如:键值1001、1002、1003,反转后1001、2001、3001,进而分散到不用的叶子节点块中。

     

    1.3 反向索引应用场景

    • 索引块成为热点块
    • rac环境
      • rac环境下中多节点访问访问数据呈现密集且集中的特点,索引热块的产生较高。
      • 在范围检索不高的rac环境中使用反向索引可有效提高性能。

     

    1.4 反向索引的优点与缺点

    • 优点:降低索引叶子块的争用问题,提升系统性能。
    • 缺点:对于范围检索,例如:between,>,<时,反向索引无法引用,进而导致全表扫面的产生,降低系统性能。

     

    1.5 反向索引示例说明

  • -- 创建两张相同结构的表,内部结构及数据均引用scott用户下的emp表SQL> select count(*) from test01;
    
      COUNT(*)
    ----------
    
    SQL> select count(*) from test02;
    
      COUNT(*)
    ----------
    
    
    
    --针对表TEST01的empno列,添加B-tree索引 
    SQL> create index PK_TEST01 on TEST01(EMPNO);
    Index created.
    
    --针对表TEST02的empno列,添加反向索引
    SQL> create index PK_REV_TEST02 on TEST02(EMPNO) REVERSE;
    Index created.
    
    
    --验证上面的索引,NORMAL/REV表明为反向索引
    SQL> select TABLE_NAME,INDEX_NAME,INDEX_TYPE from user_indexes where INDEX_NAME like %TEST%;
    
    TABLE_NAME           INDEX_NAME           INDEX_TYPE
    -------------------- -------------------- --------------------
    TEST01               PK_TEST01            NORMAL
    TEST02               PK_REV_TEST02        NORMAL/REV
    
    
    --打开会话追踪
    SQL> set autotrace traceonly
    
    
    --相同条件查询,观察两表的执行计划
    SQL> select * from TEST01 where empno=7369;
    Execution Plan
    ----------------------------------------------------------
    Plan hash value: 515586510
    
    -----------------------------------------------------------------------------------------
    | Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
    -----------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT            |           |     1 |    87 |     2   (0)| 00:00:01 |
    |   1 |  TABLE ACCESS BY INDEX ROWID| TEST01    |     1 |    87 |     2   (0)| 00:00:01 |
    |*  2 |   INDEX RANGE SCAN          | PK_TEST01 |     1 |       |     1   (0)| 00:00:01 |
    -----------------------------------------------------------------------------------------
    
    Predicate Information (identified by operation id):
    ---------------------------------------------------
    - access("EMPNO"=7369)
    
    Note
    -----
       - dynamic sampling used for this statement (level=2)
    
    
    Statistics
    ----------------------------------------------------------
     recursive calls
     db block gets
     consistent gets
     physical reads
     redo size
     bytes sent via SQL*Net to client
     bytes received via SQL*Net from client
     SQL*Net roundtrips to/from client
     sorts (memory)
     sorts (disk)
     rows processed
    
    
    
    
    SQL> select * from TEST02 where empno=7369;
    
    Execution Plan
    ----------------------------------------------------------
    Plan hash value: 1053012716
    
    ---------------------------------------------------------------------------------------------
    | Id  | Operation                   | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
    ---------------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT            |               |     1 |    87 |     2   (0)| 00:00:01 |
    |   1 |  TABLE ACCESS BY INDEX ROWID| TEST02        |     1 |    87 |     2   (0)| 00:00:01 |
    |*  2 |   INDEX RANGE SCAN          | PK_REV_TEST02 |     1 |       |     1   (0)| 00:00:01 |
    ---------------------------------------------------------------------------------------------
    
    Predicate Information (identified by operation id):
    ---------------------------------------------------
    - access("EMPNO"=7369)
    
    Note
    -----
       - dynamic sampling used for this statement (level=2)
    
    
    Statistics
    ----------------------------------------------------------
     recursive calls
     db block gets
     consistent gets
     physical reads
     redo size
     bytes sent via SQL*Net to client
     bytes received via SQL*Net from client
     SQL*Net roundtrips to/from client
     sorts (memory)
     sorts (disk)
     rows processed
    
    
    
    
    -- 相同范围条件查询,观察两表的执行计划
    SQL> select * from TEST01 where empno between 7350 and 7500;
    
    Execution Plan
    ----------------------------------------------------------
    Plan hash value: 515586510
    
    -----------------------------------------------------------------------------------------
    | Id  | Operation                   | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
    -----------------------------------------------------------------------------------------
    |   0 | SELECT STATEMENT            |           |     2 |   174 |     2   (0)| 00:00:01 |
    |   1 |  TABLE ACCESS BY INDEX ROWID| TEST01    |     2 |   174 |     2   (0)| 00:00:01 |
    |*  2 |   INDEX RANGE SCAN          | PK_TEST01 |     2 |       |     1   (0)| 00:00:01 |
    -----------------------------------------------------------------------------------------
    
    Predicate Information (identified by operation id):
    ---------------------------------------------------
    - access("EMPNO">=7350 AND "EMPNO"<=7500)
    
    Note
    -----
       - dynamic sampling used for this statement (level=2)
    
    
    Statistics
    ----------------------------------------------------------
     recursive calls
     db block gets
     consistent gets
     physical reads
     redo size
     bytes sent via SQL*Net to client
     bytes received via SQL*Net from client
     SQL*Net roundtrips to/from client
     sorts (memory)
     sorts (disk)
     rows processed
    
    
    
    
    
    SQL> select * from TEST02 where empno between 7350 and 7500;
    
    Execution Plan
    ----------------------------------------------------------
    Plan hash value: 3294238222
    
    ----------------------------------------------------------------------------
    | Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
    ----------------------------------------------------------------------------
    |   0 | SELECT STATEMENT  |        |     2 |   174 |     3   (0)| 00:00:01 |
    |*  1 |  TABLE ACCESS FULL| TEST02 |     2 |   174 |     3   (0)| 00:00:01 |
    ----------------------------------------------------------------------------
    
    Predicate Information (identified by operation id):
    ---------------------------------------------------
    - filter("EMPNO">=7350 AND "EMPNO"<=7500)
    
    Note
    -----
       - dynamic sampling used for this statement (level=2)
    
    
    Statistics
    ----------------------------------------------------------
     recursive calls
     db block gets
     consistent gets0  redo size
     bytes sent via SQL*Net to client
     bytes received via SQL*Net from client
     SQL*Net roundtrips to/from client
     sorts (memory)
     sorts (disk)
     rows processed

    通过上面的示例可以看到,当使用between条件进行范围查询时,采用反向索引的表,并没有使用索引,而是采用了全表扫面的方式进行检索。

Oracle 反向索引(反转建索引) 理解

标签:环境   查询   ndt   range   color   net   系统   select   ace   

原文地址:https://www.cnblogs.com/lucifa/p/10165940.html

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