实验环境：Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

1、创建表插入数据

sql>createtabletxtx(idint,namechar(2),txchar(3),id1int,primarykey(id,name,tx));
表已创建。
sql>insertintotxtxvalues(1,'tx',1);
已创建1行。
sql>insertintotxtxvalues(2,2);
已创建1行。
sql>insertintotxtxvalues(3,3);
已创建1行。
sql>commit;

sql>select*fromtxtx;
IDNATXID1
-------------------------
1txtx1
2txtx2
3txtx3

2、执行计划

sql>explainplanforselect*fromtxtxwhereid=1andid1=1andtx='tx';
已解释。
sql>setlinesize200
sql>select*fromtable(DBMS_XPLAN.DISPLAY);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Planhashvalue:4191381592
--------------------------------------------------------------------------
|Id|Operation|Name|Rows|Bytes|Cost(%cpu)|Time|
--------------------------------------------------------------------------
|0|SELECTSTATEMENT||1|35|3(0)|00:00:01|
|*1|TABLEACCESSFULL|TXTX|1|35|3(0)|00:00:01|
--------------------------------------------------------------------------
PredicateInformation(identifiedbyoperationid):
---------------------------------------------------
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1-filter("ID"=1AND"ID1"=1AND"TX"='tx')
Note
-----
-dynamicsamplingusedforthisstatement(level=2)
已选择17行。

通过以上执行计划，可以看出，不含前导列，进行了全表扫描，以下使用了前导列，查询速度就上来了

sql>explainplanforselect*fromtxtxwhereid=1andname='tx'andtx='tx';
已解释。
sql>select*fromtable(DBMS_XPLAN.DISPLAY);
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Planhashvalue:913771524
--------------------------------------------------------------------------------------------
|Id|Operation|Name|Rows|Bytes|Cost(%cpu)|Time|
--------------------------------------------------------------------------------------------
|0|SELECTSTATEMENT||1|35|1(0)|00:00:01|
|1|TABLEACCESSBYINDEXROWID|TXTX|1|35|1(0)|00:00:01|
|*2|INDEXUNIQUESCAN|SYS_C0024000|1||1(0)|00:00:01|
--------------------------------------------------------------------------------------------
PredicateInformation(identifiedbyoperationid):
PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
---------------------------------------------------
2-access("ID"=1AND"NAME"='tx'AND"TX"='tx')
已选择14行。