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 200
SQL> 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。
未经允许不得转载:CORASQL » 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)
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