SQL> var loc varchar2(30)SQL> exec :loc:='South San Francisco'PL/SQL procedure successfully completed.SQL> SELECT   2  emp.last_name,emp.first_name,j.job_title,d.department_name,l.city,l.state_province,l.postal_code,l.street_address,  3  emp.email,emp.phone_number,emp.hire_date,emp.salary,mgr.last_name  4  from hr.employees emp,hr.employees mgr,hr.departments d,hr.locations  5  l,hr.jobs j  6  where l.city =:loc  7  and emp.manager_id=mgr.employee_id  8  and emp.department_id=d.department_id  9  and d.location_id=l.location_id 10  and emp.job_id=j.job_id;--每次都是以嵌套循环来完毕整个的查询流程，这就是一个最好的运行计划45 rows selected.Execution Plan----------------------------------------------------------Plan hash value: 4121168346-----------------------------------------------------------------------------------------------------| Id  | Operation                       | Name              | Rows  | Bytes | Cost (%CPU)| Time     |-----------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT                |                   |    15 |  2580 |     8   (0)| 00:00:01 ||   1 |  NESTED LOOPS                   |                   |    15 |  2580 |     8   (0)| 00:00:01 ||   2 |   NESTED LOOPS                  |                   |    15 |  2400 |     6   (0)| 00:00:01 ||   3 |    NESTED LOOPS                 |                   |    15 |  1995 |     5   (0)| 00:00:01 ||   4 |     NESTED LOOPS                |                   |     4 |   268 |     3   (0)| 00:00:01 ||   5 |      TABLE ACCESS BY INDEX ROWID| LOCATIONS         |     1 |    48 |     2   (0)| 00:00:01 ||*  6 |       INDEX RANGE SCAN          | LOC_CITY_IX       |     1 |       |     1   (0)| 00:00:01 ||   7 |      TABLE ACCESS BY INDEX ROWID| DEPARTMENTS       |     4 |    76 |     1   (0)| 00:00:01 ||*  8 |       INDEX RANGE SCAN          | DEPT_LOCATION_IX  |     4 |       |     0   (0)| 00:00:01 ||   9 |     TABLE ACCESS BY INDEX ROWID | EMPLOYEES         |     4 |   264 |     1   (0)| 00:00:01 ||* 10 |      INDEX RANGE SCAN           | EMP_DEPARTMENT_IX |    10 |       |     0   (0)| 00:00:01 ||  11 |    TABLE ACCESS BY INDEX ROWID  | JOBS              |     1 |    27 |     1   (0)| 00:00:01 ||* 12 |     INDEX UNIQUE SCAN           | JOB_ID_PK         |     1 |       |     0   (0)| 00:00:01 ||  13 |   TABLE ACCESS BY INDEX ROWID   | EMPLOYEES         |     1 |    12 |     1   (0)| 00:00:01 ||* 14 |    INDEX UNIQUE SCAN            | EMP_EMP_ID_PK     |     1 |       |     0   (0)| 00:00:01 |-----------------------------------------------------------------------------------------------------Predicate Information (identified by operation id):---------------------------------------------------   6 - access("L"."CITY"=:LOC)   8 - access("D"."LOCATION_ID"="L"."LOCATION_ID")  10 - access("EMP"."DEPARTMENT_ID"="D"."DEPARTMENT_ID")  12 - access("EMP"."JOB_ID"="J"."JOB_ID")  14 - access("EMP"."MANAGER_ID"="MGR"."EMPLOYEE_ID")Statistics----------------------------------------------------------       2035  recursive calls          0  db block gets        669  consistent gets         19  physical reads          0  redo size       3923  bytes sent via SQL*Net to client        407  bytes received via SQL*Net from client          4  SQL*Net roundtrips to/from client         24  sorts (memory)          0  sorts (disk)         45  rows processed