Oracle 反向索引（反转建索引） 理解

• 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。