通常它会尽可能先过滤。从解释计划,你可以看到在过滤完成,并在那里完成,例如,创建一些表格和数据连接:
create table employees (id integer, dept_id integer, salary number);
create table dept (id integer, dept_name varchar2(10));
insert into dept values (1, 'IT');
insert into dept values (2, 'HR');
insert into employees
select level, mod(level, 2) + 1, level * 1000
from dual connect by level <= 100;
create index employee_uk1 on employees (id);
create index dept_uk1 on dept (id);
exec dbms_stats.gather_table_stats(user, 'DEPT');
现在,如果我解释这两个你提供,你会查询发现Oracle将每个查询转换为幕后的相同计划(它并不总是执行您认为它的操作 - Oracle拥有'重写'查询的许可,并且它执行了很多操作):
explain plan for
select e.*, d.*
from employees e, dept d
where e.dept_id = d.id
and e.salary > 5000;
select * from table(dbms_xplan.display());
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 96 | 1536 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN | | 96 | 1536 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 2 | 12 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_UK1 | 2 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 96 | 960 | 4 (25)| 00:00:01 |
|* 5 | TABLE ACCESS FULL | EMPLOYEES | 96 | 960 | 3 (0)| 00:00:01 |
4 - access("E"."DEPT_ID"="D"."ID")
filter("E"."DEPT_ID"="D"."ID")
5 - filter("E"."SALARY">5000)
注意应用于查询的过滤器操作。一旦你学会如何得到解释的计划以及如何阅读它们,你通常可以工作了甲骨文在做,因为它执行查询
explain plan for
select e.*, d.*
from (select * from employees where salary > 5000) e, dept d
where e.dept_id = d.id;
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 96 | 1536 | 6 (17)| 00:00:01 |
| 1 | MERGE JOIN | | 96 | 1536 | 6 (17)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 2 | 12 | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | DEPT_UK1 | 2 | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 96 | 960 | 4 (25)| 00:00:01 |
|* 5 | TABLE ACCESS FULL | EMPLOYEES | 96 | 960 | 3 (0)| 00:00:01 |
4 - access("EMPLOYEES"."DEPT_ID"="D"."ID")
filter("EMPLOYEES"."DEPT_ID"="D"."ID")
5 - filter("SALARY">5000)
:现在解释替代查询。