Oracle虛擬索引的運用
在做SQL調優的時候，有的時候需要加一個索引，測試下對性能提升有沒有幫組，如果此時這張表非常大，建索引將會非常之麻煩，這種場景虛擬索引就該登場了。下面來做個試驗：
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
PL/SQL Release 10.2.0.1.0 - Production
CORE 10.2.0.1.0 Production
TNS for 64-bit Windows: Version 10.2.0.1.0 - Production
NLSRTL Version 10.2.0.1.0 - Production
SQL> drop table test purge;
SQL> create table test as select * from user_objects;
SQL> select count(1),
count(distinct object_name) c_oname,
count(distinct object_type) s_otype
from test;
COUNT(1) C_ONAME S_OTYPE
---------- ---------- ----------
11500 11430 14
SQL> create index ind_test_name on test(object_name) nosegment;
SQL> create index ind_test_otype on test(object_type) nosegment;
SQL> exec dbms_stats.gather_table_stats(user,'test',cascade=>true);
SQL> --必須設置隱含參數”_use_nosegment_indexes”=true（默認為false)後，CBO才能使用虛擬索引
SQL> alter session set "_use_nosegment_indexes"=true;
SQL> set autotrace trace exp
SQL> select * from test where object_name='TEST';
執行計劃
----------------------------------------------------------
Plan hash value: 3675505035
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 86 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 1 | 86 | 2 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_TEST_NAME | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_NAME"='TEST')
SQL> select * from test where object_type='TEST';
執行計劃
----------------------------------------------------------
Plan hash value: 1357081020
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 821 | 70606 | 36 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL| TEST | 821 | 70606 | 36 (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_TYPE"='TEST')
SQL> set autotrace off
SQL> select index_name from user_indexes s where s.table_name ='TEST';
未選定行
SQL> col object_name format a15;
SQL> select object_name,object_id,object_type from user_objects s
where s.object_name in ('IND_TEST_NAME', 'IND_TEST_OTYPE');
OBJECT_NAME OBJECT_ID OBJECT_TYPE
--------------- ---------- -------------------
IND_TEST_NAME 670666 INDEX
IND_TEST_OTYPE 670667 INDEX
SQL> select segment_name,s.bytes from user_segments s where s.segment_name
in ('IND_TEST_NAME', 'IND_TEST_OTYPE');
未選定行
原理：加了虛擬索引後，同時開啟_use_nosegment_indexes，CBO在評估執行計劃的時候會把虛擬索引當做是一個真實的索引，有點欺騙CBO的意思。不能在user_indexes和user_segments中找到，但在user_objects可以找到。說明虛擬說只是加了數據字典，並沒有產生實際的數據，那它也應該不能像真實的索引哪有操作，再做一個試驗：
SQL> alter index ind_test_name rebuild;
alter index ind_test_name rebuild
*
第 1 行出現錯誤:
ORA-08114: 無法改變假索引
最後：虛擬索引只適合做性能調優，加了之後記得要刪除。
