在应用过程中，我们偶然会碰到想要在增加index前看看该index是否能带来预期的效果。为了达到这个目标，我们可以使用oem的virtual index的功能。为了能让大家比较简单地了解这个功能，我们为大家演示一下不在oem下virtual index的使用。
另外，即然叫virtual index， 那么当建立它时，应当不耗资源的。这对于很大的表，当我们想看建某个index是否能改善当前语句的执行计划时，显然很有帮助。我们来看例子：
有一news表，400多M。有语句：
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select count(*) from News
where Gmt_Origin >= trunc(sysdate + 0.5)
and Gmt_Origin < trunc(sysdate + 0.5) + 1
and news_type_general = 'y'
and ((News_Category_Id IN (100901,5002640,5002641,100902,5000042,5001060,5002041,9))
OR (News_Category_Id_2 IN (100901,5002640,5002641,100902,5000042,5001060,5002041,9))
OR (News_Category_Id_3 IN (100901,5002640,5002641,100902,5000042,5001060,5002041,9)));
它使用了index，然而效果并不理想：
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Operation Object Name Rows Bytes Cost Object Node In/Out PStart PStop
SELECT STATEMENT Hint=CHOOSE 1 267
SORT AGGREGATE 1 28
FILTER
TABLE Access BY INDEX ROWID NEWS 1 28 267
INDEX RANGE SCAN NEWS_RELATED3_IND 1 266
分析条件后，我们认为在gmt_origin上建index可能会有帮助，然而我们并不清楚有于其他indexes的影响，当我们建了基于gmt_origin的index时，是否的确会有帮助。
于是我们预备在gmt_origin上建个virtual index:
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SQL> alter session set sql_trace = true;
Session altered.
SQL> create index news_gorigin_ind on news(gmt_origin) nosegment ;
Index created.
trace是：
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create index news_gorigin_ind on news(gmt_origin) nosegment
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.06 0.08 2 4 0 0
Execute 1 0.02 0.42 0 2 2 0
Fetch 0 0.00 0.00 0 0 0 0
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 2 0.08 0.50 2 6 2 0
Misses in library cache during parse: 1
Optimizer goal: CHOOSE
我们可以从tracer看出，建立它的确是不耗多少资源。
为了能利用这个virtual index,我们还必须修改当前session参数：
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SQL> alter session set "_use_nosegment_indexes" = true;
Session altered.
现在我们来看看效果：
SQL> eXPlain plan for
2 select count(*) from News
3 where Gmt_Origin >= trunc(sysdate + 0.5)
4 and Gmt_Origin < trunc(sysdate + 0.5) + 1
5 and news_type_general = 'y'
6 and ((News_Category_Id IN (100901,5002640,5002641,100902,5000042,5001060,5002041,9))
7 OR (News_Category_Id_2 IN (100901,5002640,5002641,100902,5000042,5001060,5002041,9))
8 OR (News_Category_Id_3 IN (100901,5002640,5002641,100902,5000042,5001060,5002041,9)))
9 /
Explained.
SQL> @$Oracle_HOME/rdbms/admin/utlxpls
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------
-----------------------------------------------------------------------------------
Id Operation Name Rows Bytes Cost
-----------------------------------------------------------------------------------
0 SELECT STATEMENT 1 20 3
1 SORT AGGREGATE 1 20
2 FILTER
3 TABLE ACCESS BY INDEX ROWID NEWS 1 20 3
4 INDEX RANGE SCAN NEWS_GORIGIN_IND 3 2
-----------------------------------------------------------------------------------
Note: cpu costing is off, 'plan_table' is old version
12 rows selected.
执行计划显示它的确用到了我们预备建的index。
设置10053看看
-- Index stats
INDEX NAME: NEWS_GMODIFIED_IND COL#: 3
TOTAL :: LVLS: 2 #LB: 1295 #DK: 21318 LB/K: 1 DB/K: 8 CLUF: 186911
INDEX NAME: NEWS_GORIGIN_IND COL#: 7
TOTAL :: LVLS: 1 #LB: 25 #DK: 100 LB/K: 1 DB/K: 1 CLUF: 800
...
Access path: index (scan)
Index: NEWS_GORIGIN_IND
TABLE: NEWS
RSC_CPU: 0 RSC_IO: 3
IX_SEL: 4.5000e-03 TB_SEL: 1.1250e-05
...
Access path: index (index-only)
Index: NEWS_GORIGIN_IND
TABLE: NEWS
RSC_CPU: 0 RSC_IO: 2
IX_SEL: 4.5000e-03 TB_SEL: 4.5000e-03
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