利用函数索引优化<>

SQL> select count(*),ID from test_2 group by id;

  COUNT(*)	   ID
---------- ----------
    131072	    1
   1179648	    2

select count(*) from test_2 where id <>2;------利用函数索引优化:

select count(*) from test_2 where id <>2;SQL> 


SQL> select count(*) from test_2 where id <>2;


Execution Plan
----------------------------------------------------------
Plan hash value: 1548797762

-----------------------------------------------------------------------------
| Id  | Operation	   | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |	    |	  1 |	  3 |	801   (6)| 00:00:10 |
|   1 |  SORT AGGREGATE    |	    |	  1 |	  3 |		 |	    |
|*  2 |   TABLE ACCESS FULL| TEST_2 |	655K|  1920K|	801   (6)| 00:00:10 |
-----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("ID"<>2)


Statistics
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
       3463  consistent gets
       1182  physical reads
	  0  redo size
	413  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



创建函数索引:
create index idx2 on test_2(case when ID=2 then null else '@' end);


select /*+ index(test_2 idx2) */ count(*) from test_2 where (case when ID=2  THEN NULL ELSE '@' END)='@';

select /*+ index(test_2 idx2) */ count(*) from test_2 where (case when ID=2  THEN NULL ELSE '@' END)='@';
SQL> 

Execution Plan
----------------------------------------------------------
Plan hash value: 2399724746

--------------------------------------------------------------------------
| Id  | Operation	  | Name | Rows  | Bytes | Cost (%CPU)| Time	 |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |	 |     1 |     3 |   242   (2)| 00:00:03 |
|   1 |  SORT AGGREGATE   |	 |     1 |     3 |	      | 	 |
|*  2 |   INDEX RANGE SCAN| IDX2 |   131K|   384K|   242   (2)| 00:00:03 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access(CASE "ID" WHEN 2 THEN NULL ELSE '@' END ='@')


Statistics
----------------------------------------------------------
	  8  recursive calls
	  0  db block gets
	241  consistent gets
	  0  physical reads
	  0  redo size
	413  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

逻辑读大大降低  

使用不等于优化 必须保证<>过滤后的数据占表比例数据少才可以。