SQL优化【基础01】-生成执行计划及计划中参数列的含义

基础01-生成执行计划
--对于11G平时最常用的是table函数+dbms_xplan包，其它方式如autot traceonly也偶尔用，下面就来介绍dbms_xplan
--explan plan for...,这个只是预估计划，除非明显连接方式错误，否则有时帮忙并不大，所以最直接还是用dbms_xplan
方法(1) alter session set statistics_level=all;
exec sql;
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

方法(2)
exec sql(加入hint /*+ gather_plan_statistics */)
select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

--最常用的几个参数,操作下便知
allstats last  --最后一次统计，不用漏了last，否则就是累加值了; 
all  --一般看计划，SQL_ID,PLAN_HASH_VALUE
advanced  --一般拿来看outline_data信息和绑定变量

--方法（1）示例
SQL> select * from v$version;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production

SQL> alter session set statistics_level=all;

Session altered.

SQL> select  t1.status,t2.status
  2  from t1,t2
  3  where t1.object_id=t2.object_id
  4  and t1.object_id=99;

no rows selected

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  581vna1k8tkk7, child number 0
-------------------------------------
select  t1.status,t2.status from t1,t2 where t1.object_id=t2.object_id
and t1.object_id=99

Plan hash value: 2959412835

----------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      0 |00:00:00.01 |       3 |       |       |          |
|*  1 |  HASH JOIN         |      |      1 |      1 |      0 |00:00:00.01 |       3 |   963K|   963K|  150K (0)|
|*  2 |   TABLE ACCESS FULL| T2   |      1 |      1 |      0 |00:00:00.01 |       3 |       |       |          |
|*  3 |   TABLE ACCESS FULL| T1   |      0 |     12 |      0 |00:00:00.01 |       0 |       |       |          |
----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
   2 - filter("T2"."OBJECT_ID"=99)
   3 - filter("T1"."OBJECT_ID"=99)

Note
-----
   - dynamic sampling used for this statement (level=2)

27 rows selected.

--方法(2)示例
SQL> select  /*+ gather_plan_statistics */t1.status,t2.status
  2  from t1,t2
  3  where t1.object_id=t2.object_id
  4  and t1.object_id=99;

no rows selected

SQL> @allstat--(放在文件中：select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));）

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID  27qqjb8bsq4vc, child number 0
-------------------------------------
select  /*+ gather_plan_statistics */t1.status,t2.status from t1,t2
where t1.object_id=t2.object_id and t1.object_id=99

Plan hash value: 2959412835

----------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |  OMem |  1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        |      0 |00:00:00.01 |       3 |       |       |          |
|*  1 |  HASH JOIN         |      |      1 |      1 |      0 |00:00:00.01 |       3 |   963K|   963K|  154K (0)|
|*  2 |   TABLE ACCESS FULL| T2   |      1 |      1 |      0 |00:00:00.01 |       3 |       |       |          |
|*  3 |   TABLE ACCESS FULL| T1   |      0 |     12 |      0 |00:00:00.01 |       0 |       |       |          |
----------------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
   2 - filter("T2"."OBJECT_ID"=99)
   3 - filter("T1"."OBJECT_ID"=99)

Note
-----
   - dynamic sampling used for this statement (level=2)

27 rows selected.

--接下来看下执行计划这些参数的含义，直接上图看了；