oracle在组合索引上，只使用部分列进行查询（查询时必须包含前导列，否则会走全表扫描）

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> create table txtx(id int,name char(2),tx char(3),id1 int,primary key(id,name,tx));表已创建。SQL> insert into txtx values(1,'tx','tx',1);已创建 1 行。SQL> insert into txtx values(2,'tx','tx',2);已创建 1 行。SQL> insert into txtx values(3,'tx','tx',3);已创建 1 行。SQL> commit;

SQL> select * from txtx;        ID NA TX         ID1---------- -- --- ----------         1 tx tx           1         2 tx tx           2         3 tx tx           3

2、执行计划

SQL> explain plan for select * from txtx  where id=1 and id1 =1 and tx='tx';已解释。SQL> set linesize 200SQL> select * from table(DBMS_XPLAN.DISPLAY);PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Plan hash value: 4191381592--------------------------------------------------------------------------| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------|   0 | SELECT STATEMENT  |      |     1 |    35 |     3   (0)| 00:00:01 ||*  1 |  TABLE ACCESS FULL| TXTX |     1 |    35 |     3   (0)| 00:00:01 |--------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------   1 - filter("ID"=1 AND "ID1"=1 AND "TX"='tx')Note-----   - dynamic sampling used for this statement (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> explain plan for select * from txtx  where id=1 and name ='tx' and tx='tx';已解释。SQL> select * from table(DBMS_XPLAN.DISPLAY);PLAN_TABLE_OUTPUT--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Plan hash value: 913771524--------------------------------------------------------------------------------------------| Id  | Operation                   | Name         | Rows  | Bytes | Cost (%CPU)| Time     |--------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT            |              |     1 |    35 |     1   (0)| 00:00:01 ||   1 |  TABLE ACCESS BY INDEX ROWID| TXTX         |     1 |    35 |     1   (0)| 00:00:01 ||*  2 |   INDEX UNIQUE SCAN         | SYS_C0024000 |     1 |       |     1   (0)| 00:00:01 |--------------------------------------------------------------------------------------------Predicate Information (identified by operation id):PLAN_TABLE_OUTPUT-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------   2 - access("ID"=1 AND "NAME"='tx' AND "TX"='tx')已选择14行。