组合索引和单列索引效率对比

SQL> create table  test as select * from dba_objects;

Table created.


select * from test where object_id=10 and OWNER='SYS';


SQL> select count(*) from test where object_id=10;

  COUNT(*)
----------
	 1

SQL> select count(*) from test;

  COUNT(*)
----------
     74443


模拟插入object_id=10 and OWNER='SYS'的记录

SQL> select count(*) from test where object_id=10 and owner='SYS';

  COUNT(*)
----------
   4194304




create table test4 as (select * from (select * from test where object_id=10 and owner='SYS') where  rownum<100001);

SQL> select count(*) from test4;

  COUNT(*)
----------
    100000


update test4 set owner='SCOTT';


SQL> insert into test select * from test4;

100000 rows created.

SQL> commit;

Commit complete.


SQL>  select object_id,owner,count(*) from test group by  object_id,owner
  2  having object_id=10;

 OBJECT_ID OWNER			    COUNT(*)
---------- ------------------------------ ----------
	10 SYS				     4194304
	10 SCOTT			      100000


SQL> create index test_idx1 on test(object_id);

Index created.

SQL> create index test_idx2 on test(object_id,owner);

Index created.


对比单列索引和组合索引效率:

SQL> select /*+index(test test_idx1)*/ * from test where object_id=10 and owner='SYS';

4194304 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2624864549

-----------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|  4412K|   871M|   202K  (1)| 00:40:30 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| TEST	|  4412K|   871M|   202K  (1)| 00:40:30 |
|*  2 |   INDEX RANGE SCAN	    | TEST_IDX1 |  4412K|	|  8701   (1)| 00:01:45 |
-----------------------------------------------------------------------------------------

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

   1 - filter("OWNER"='SYS')
   2 - access("OBJECT_ID"=10)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
     616418  consistent gets
      56597  physical reads
	  0  redo size
   60679059  bytes sent via SQL*Net to client
    3076239  bytes received via SQL*Net from client
     279622  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
    4194304  rows processed

此时table access by index rowid  能过滤掉的数据为 100000 行 


对比走组合索引的效率:
SQL> select /*+index(test test_idx2)*/ * from test where object_id=10 and owner='SYS';

4194304 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2533302371

-----------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|  4412K|   871M|   205K  (1)| 00:41:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST	|  4412K|   871M|   205K  (1)| 00:41:01 |
|*  2 |   INDEX RANGE SCAN	    | TEST_IDX2 |  4412K|	| 11214   (1)| 00:02:15 |
-----------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"=10 AND "OWNER"='SYS')

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
	  0  recursive calls
	  0  db block gets
     617237  consistent gets
      58752  physical reads
	  0  redo size
  384479179  bytes sent via SQL*Net to client
    3076239  bytes received via SQL*Net from client
     279622  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
    4194304  rows processed



对比逻辑读:
走TEST_IDX1 索引:

616418  consistent gets
56597  physical reads

走TEST_IDX2 索引:
617237  consistent gets
58752  physical reads


继续测试:

SQL> select /*+index(test test_idx1)*/ * from test where object_id=10 and owner='SCOTT';

100000 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2624864549

-----------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|   452 | 93564 |   202K  (1)| 00:40:30 |
|*  1 |  TABLE ACCESS BY INDEX ROWID| TEST	|   452 | 93564 |   202K  (1)| 00:40:30 |
|*  2 |   INDEX RANGE SCAN	    | TEST_IDX1 |  4412K|	|  8701   (1)| 00:01:45 |
-----------------------------------------------------------------------------------------

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

   1 - filter("OWNER"='SCOTT')
   2 - access("OBJECT_ID"=10)

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
      71101  consistent gets
      56599  physical reads
	  0  redo size
    1448081  bytes sent via SQL*Net to client
      73745  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
     100000  rows processed
select /*+index(test test_idx2)*/ * from test where object_id=10 and owner='SCOTT';SQL> 

100000 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2533302371

-----------------------------------------------------------------------------------------
| Id  | Operation		    | Name	| Rows	| Bytes | Cost (%CPU)| Time	|
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	    |		|   452 | 93564 |     7   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| TEST	|   452 | 93564 |     7   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN	    | TEST_IDX2 |    72 |	|     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------

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

   2 - access("OBJECT_ID"=10 AND "OWNER"='SCOTT')

Note
-----
   - dynamic sampling used for this statement (level=2)


Statistics
----------------------------------------------------------
	  1  recursive calls
	  0  db block gets
      14774  consistent gets
	  0  physical reads
	  0  redo size
    9367927  bytes sent via SQL*Net to client
      73745  bytes received via SQL*Net from client
       6668  SQL*Net roundtrips to/from client
	  0  sorts (memory)
	  0  sorts (disk)
     100000  rows processed

此时TABLE ACCESS BY INDEX ROWID过滤了4194304条记录,走组合索引效率更高:



对比逻辑读:
走TEST_IDX1 索引:

71101  consistent gets
56599  physical reads

走TEST_IDX2 索引:
14774  consistent gets
0  physical reads

结论:如果TABLE ACCESS BY INDEX ROWID不能过滤大量数据,那么走组合索引效率未必高。