oracle virtual index （oracle虚拟索引）

In 8.1.5 and above it is possible to create virtual indexes


Virtual indexes have a data dictionary definition, but no associated segment


Effectiveness of new indexes can be tested by generating theoretical execution plans


The CBO will consider virtual indexes if the hidden parameter _use_nosegment_indexes is set to true




做测试：


SQL> CREATE TABLE t1 AS


2 SELECT * FROM dba_objects


3 WHERE ROWNUM < 1000;





Table created.


SQL> ANALYZE TABLE t1 COMPUTE STATISTICS;





Table analyzed.





SQL> CREATE INDEX i1 ON t1 (owner, object_name) NOSEGMENT;





Index created.（创建虚拟索引使用NOSEGMENT clause)








SQL> EXECUTE DBMS_STATS.GENERATE_STATS (USER,'I1');





PL/SQL procedure successfully completed.





SQL> set autotrace traceonly


SQL> SELECT object_id FROM t1


2 WHERE owner = USER AND object_name = 'T1';





no rows selected








Execution Plan


----------------------------------------------------------


Plan hash value: 3617692013





--------------------------------------------------------------------------


| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |


--------------------------------------------------------------------------


| 0 | SELECT STATEMENT | | 1 | 21 | 5 (0)| 00:00:01 |


|* 1 | TABLE ACCESS FULL| T1 | 1 | 21 | 5 (0)| 00:00:01 |


--------------------------------------------------------------------------





Predicate Information (identified by operation id):


---------------------------------------------------





1 - filter("OBJECT_NAME"='T1' AND "OWNER"=USER@!)








Statistics


----------------------------------------------------------


1 recursive calls


0 db block gets


15 consistent gets


0 physical reads


0 redo size


278 bytes sent via SQL*Net to client


374 bytes received via SQL*Net from client


1 SQL*Net roundtrips to/from client


0 sorts (memory)


0 sorts (disk)


0 rows processed


(要使用虚拟索引，需要设置隐含参数_use_nosegment_indexes)





SQL> ALTER SESSION SET "_use_nosegment_indexes" = TRUE;








Session altered.


SQL> SELECT object_id FROM t1


2 WHERE owner = USER AND object_name = 'T1';





no rows selected








Execution Plan


----------------------------------------------------------


Plan hash value: 1704772559





------------------------------------------------------------------------------------


| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |


------------------------------------------------------------------------------------


| 0 | SELECT STATEMENT | | 1 | 21 | 2 (0)| 00:00:01 |


| 1 | TABLE ACCESS BY INDEX ROWID| T1 | 1 | 21 | 2 (0)| 00:00:01 |


|* 2 | INDEX RANGE SCAN | I1 | 1 | | 1 (0)| 00:00:01 |


------------------------------------------------------------------------------------





Predicate Information (identified by operation id):


---------------------------------------------------





2 - access("OWNER"=USER@! AND "OBJECT_NAME"='T1')








Statistics


----------------------------------------------------------


1 recursive calls


0 db block gets


15 consistent gets


0 physical reads


0 redo size


278 bytes sent via SQL*Net to client


374 bytes received via SQL*Net from client


1 SQL*Net roundtrips to/from client


0 sorts (memory)


0 sorts (disk)


0 rows processed