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