如何查看执行计划
1、设置信息收集等级(设置all有额外负载，默认值： ALTER SESSION SET STATISTICS_LEVEL=TYPICAL;)
SQL> alter session set statistics_level=ALL;
Session altered.2、执行一个sql
SQL> select e.employee_id,e.job_id,j.min_salary,j.max_salary
from hr.employees e, hr.jobs j
where e.job_id = j.job_id;3.1、查看刚才执行sql的执行计划(dbms_xplan.DISPLAY_CURSOR的三个参数：SQL_ID Child Number Format)
SQL> select * from table(dbms_xplan.DISPLAY_CURSOR(null, null, 'ALLSTATS'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 1hrgu1ksy8xhs, child number 0
-------------------------------------
select e.employee_id,e.job_id,j.min_salary,j.max_salary from
hr.employees e, hr.jobs j where e.job_id = j.job_id
Plan hash value: 2650640655
--------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | O/1/M |
--------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | | 321 |00:00:00.01 | 60 | | | |
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | MERGE JOIN | | 3 | 107 | 321 |00:00:00.01 | 60 | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| JOBS | 3 | 19 | 57 |00:00:00.01 | 42 | | | |
| 3 | INDEX FULL SCAN | JOB_ID_PK | 3 | 19 | 57 |00:00:00.01 | 21 | | | |
|* 4 | SORT JOIN | | 57 | 107 | 321 |00:00:00.01 | 18 | 9216 | 9216 | 3/0/0|
| 5 | VIEW | index$_join$_001 | 3 | 107 | 321 |00:00:00.01 | 18 | | | |
|* 6 | HASH JOIN | | 3 | | 321 |00:00:00.01 | 18 | 1519K| 1519K| 3/0/0|
| 7 | INDEX FAST FULL SCAN | EMP_EMP_ID_PK | 3 | 107 | 321 |00:00:00.01 | 9 | | | |
| 8 | INDEX FAST FULL SCAN | EMP_JOB_IX | 3 | 107 | 321 |00:00:00.01 | 9 | | | |
--------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------
4 - access("E"."JOB_ID"="J"."JOB_ID")
filter("E"."JOB_ID"="J"."JOB_ID")
6 - access(ROWID=ROWID)
28 rows selected.3.2 dbms_xplan.DISPLAY_CURSOR的三个参数
(1)SQL_ID :查询如下
SQL> select sql_id,sql_text
2 from v$sql
3 where sql_text not like '%like%'
4 and sql_text like '%hr.employee%';
SQL_ID SQL_TEXT
---------------------------------------------------------------------------------------------------------------------------------------------
1hrgu1ksy8xhs select e.employee_id,e.job_id,j.min_salary,j.max_salary from hr.employees e, hr.jobs j where e.job_id = j.job_id----(2)Child Number：查询如下
SQL> select child_number from v$sql where sql_id = '1hrgu1ksy8xhs';
CHILD_NUMBER
------------
0----(3)Format 如下
ALLSTATS = IOSTATS + MEMSTATSIOSTATS 显示该游标累计执行的IO统计信息(Buffers, Reads)
MEMSTATS 累计执行的PGA使用信息(Omem 1Mem Used-Mem)
LAST 仅显示最后一次执行的统计信息
Advanced 显示outline、Query Block Name、 Column Projection等信息
PEEKED_BINDS 打印解析时使用的绑定变量
Typical 不打印PROJECTION, ALIAS
3.3使用dbms_xplan.DISPLAY_CURSOR的三个参数，再次精确的查询执行计划
SQL> select * from table(dbms_xplan.DISPLAY_CURSOR('1hrgu1ksy8xhs', 0, 'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 1hrgu1ksy8xhs, child number 0
-------------------------------------
select e.employee_id,e.job_id,j.min_salary,j.max_salary from
hr.employees e, hr.jobs j where e.job_id = j.job_id
Plan hash value: 2650640655
--------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 107 |00:00:00.01 | 20 | | | |
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
| 1 | MERGE JOIN | | 1 | 107 | 107 |00:00:00.01 | 20 | | | |
| 2 | TABLE ACCESS BY INDEX ROWID| JOBS | 1 | 19 | 19 |00:00:00.01 | 14 | | | |
| 3 | INDEX FULL SCAN | JOB_ID_PK | 1 | 19 | 19 |00:00:00.01 | 7 | | | |
|* 4 | SORT JOIN | | 19 | 107 | 107 |00:00:00.01 | 6 | 9216 | 9216 | 8192 (0)|
| 5 | VIEW | index$_join$_001 | 1 | 107 | 107 |00:00:00.01 | 6 | | | |
|* 6 | HASH JOIN | | 1 | | 107 |00:00:00.01 | 6 | 1519K| 1519K| 1528K (0)|
| 7 | INDEX FAST FULL SCAN | EMP_EMP_ID_PK | 1 | 107 | 107 |00:00:00.01 | 3 | | | |
| 8 | INDEX FAST FULL SCAN | EMP_JOB_IX | 1 | 107 | 107 |00:00:00.01 | 3 | | | |
--------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
4 - access("E"."JOB_ID"="J"."JOB_ID")
filter("E"."JOB_ID"="J"."JOB_ID")
6 - access(ROWID=ROWID)
28 rows selected.