通常它會盡可能先過濾。從解釋計劃,你可以看到在過濾完成,並在那裏完成,例如,創建一些表格和數據連接:
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');
現在,如果我解釋這兩個你提供,你會查詢發現Oracle將每個查詢轉換爲幕後的相同計劃(它並不總是執行您認爲它的操作 - Oracle擁有'重寫'查詢的許可,並且它執行了很多操作):
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)
注意應用於查詢的過濾器操作。一旦你學會如何得到解釋的計劃以及如何閱讀它們,你通常可以工作了甲骨文在做,因爲它執行查詢
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)
:現在解釋替代查詢。