Oracle 反向索引（反转建索引） 理解

Posted 思则有备

tags:

篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了Oracle 反向索引（反转建索引） 理解相关的知识，希望对你有一定的参考价值。

  • 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。

以上是关于Oracle 反向索引（反转建索引） 理解的主要内容，如果未能解决你的问题，请参考以下文章

Oracle 反向主键

Oracle11G_索引

技术感悟:我对Oracle索引的理解

Python仅枚举反向索引

如何用反向二进制表示重新索引一个numpy数组？

没索引oracle无唯一索引交换分区会丢失数据?