NESTED LOOPS 成本计算

SQL> select * from v$version where rownum=1;

BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production

SQL> create table t1 as select object_id id , object_name,owner from dba_objects;

Table created

SQL> create table t2 as select object_id id , status,temporary from dba_objects;

Table created

SQL> BEGIN
  2  DBMS_STATS.GATHER_TABLE_STATS(ownname => 'TEST',
  3  tabname => 'T1',
  4  estimate_percent => 100,
  5  method_opt => 'for all columns size AUTO',
  6  degree => DBMS_STATS.AUTO_DEGREE,
  7  cascade=>TRUE
  8  );
  9  END;
 10  /

PL/SQL procedure successfully completed

SQL> BEGIN
  2  DBMS_STATS.GATHER_TABLE_STATS(ownname => 'TEST',
  3  tabname => 'T2',
  4  estimate_percent => 100,
  5  method_opt => 'for all columns size AUTO',
  6  degree => DBMS_STATS.AUTO_DEGREE,
  7  cascade=>TRUE
  8  );
  9  END;
 10  /

PL/SQL procedure successfully completed

SQL> explain plan for select /*+ use_nl(t1,t2) */ t1.owner,t1.object_name,t2.status from t1,t2 where t1.id=t2.id and t2.status='VALID';

Explained

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------
Plan hash value: 1967407726
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      | 72474 |  3255K|  2876K  (2)| 09:35:18 |
|   1 |  NESTED LOOPS      |      | 72474 |  3255K|  2876K  (2)| 09:35:18 |
|   2 |   TABLE ACCESS FULL| T1   | 72474 |  2477K|    91   (2)| 00:00:02 |
|*  3 |   TABLE ACCESS FULL| T2   |     1 |    11 |    40   (3)| 00:00:01 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   3 - filter("T2"."STATUS"='VALID' AND "T1"."ID"="T2"."ID")

15 rows selected

SQL> explain plan for select /*+ use_nl(t1,t2) */ t1.owner,t1.object_name,t2.status from t1,t2 where t1.id=t2.id and t2.status='VALID';

Explained

SQL> select OPERATION,OPTIONS,IO_COST,CPU_COST FROM PLAN_TABLE;

OPERATION                      OPTIONS                                     IO_COST                                CPU_COST
------------------------------ -------------------- ------------------------------ ---------------------------------------
SELECT STATEMENT                                                           2837902                           1247963677419
NESTED LOOPS                                                               2837902                           1247963677419
TABLE ACCESS                   FULL                                             90                                16640000
TABLE ACCESS                   FULL                                             39                                17219238

SQL> select pname, pval1 from sys.aux_stats$ where sname='SYSSTATS_MAIN';

PNAME                               PVAL1
------------------------------ ----------
CPUSPEED                                0
CPUSPEEDNW                     2696.05568
IOSEEKTIM                              10
IOTFRSPEED                           4096
MAXTHR                         
MBRC                                    0
MREADTIM                                0
SLAVETHR                       
SREADTIM                                0

9 rows selected

由于MBRC=0 所以这里采用的是非工作量统计

mreadtim=ioseektim+db_file_multiblock_count*db_block_size/iotftspeed

SQL> select (select pval1 from sys.aux_stats$ where pname = 'IOSEEKTIM') +
  2         (select value
          from v$parameter
         where name = 'db_file_multiblock_read_count') *
       (select value from v$parameter where name = 'db_block_size') /
       (select pval1 from sys.aux_stats$ where pname = 'IOTFRSPEED') "mreadtim"
  3    4    5    6    7    from dual;

  mreadtim
----------
        42

sreadtim=ioseektim+db_block_size/iotfrspeed 

SQL> select (select pval1 from sys.aux_stats$ where pname = 'IOSEEKTIM') +
       (select value from v$parameter where name = 'db_block_size') /
       (select pval1 from sys.aux_stats$ where pname = 'IOTFRSPEED') "sreadtim"
  from dual;  2    3    4

  sreadtim
----------
        12    

CPU COST = ceil(PLAN_TABLE.cpu_cost/cpuspeed/1000/sreadtim)

SQL> select ceil(1247963677419/2696.05568/1000/12) from dual; ---得到总的CPU COST

CEIL(1247963677419/2696.05568/
------------------------------
                         38574

SQL> select 2837902+38574 from dual; --- 总的IO COST + 总的CPU COST 

2837902+38574
-------------
      2876476
      
COST 基本上等于2876K    

IO COST 等于 2837902，它是怎么得到的呢？
           
select owner,blocks from dba_tables where owner='TEST' and table_name in ('T1','T2');

SQL>  select owner,table_name,blocks from dba_tables where owner='TEST' and table_name in ('T1','T2');

OWNER                          TABLE_NAME                         BLOCKS
------------------------------ ------------------------------ ----------
TEST                           T2                                    179
TEST                           T1                                    403

NESTED LOOPS 成本计算方法如下：

COST = (OUTER TABLE IO COST) + (OUTER TABLE CARDINALITY) * (INNER TABLE IO COST) + CPU COST

所以 IO COST = CEIL(outer table blocks*mreadtim/db_file_multiblock_read_count/sreadtim)+ 
               CEIL((outer table cardinality)*inner table blocks*mreadtim/db_file_multiblock_read_count/sreadtim)+
               _tablescan_cost_plus_one*2 ---- 这里 有2次全表扫描 所以加2次，
               
注意：不管全表扫描会发生多少次,_tablescan_cost_plus_one 始终在计算COST末尾添加,有多少全表扫描就加几               
  
SQL> SELECT CEIL(403*42/16/12)+ceil(72474*(179*42/16/12))+1+1 FROM DUAL;

CEIL(403*42/16/12)+CEIL(72474*(179*42/16/12))+1+1
-------------------------------------------------
                                          2837902

再来一个例子

SQL> explain plan for select /*+ use_nl(t1,t2) */ t1.owner,t1.object_name,t2.status from t1,t2 where t1.id=t2.id and t1.owner='ROBINSON';

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 1967407726

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |  2416 |   108K| 95709   (2)| 00:19:09 |
|   1 |  NESTED LOOPS      |      |  2416 |   108K| 95709   (2)| 00:19:09 |
|*  2 |   TABLE ACCESS FULL| T1   |  2416 | 84560 |    91   (2)| 00:00:02 |
|*  3 |   TABLE ACCESS FULL| T2   |     1 |    11 |    40   (3)| 00:00:01 |
---------------------------------------------------------------------------

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

   2 - filter("T1"."OWNER"='ROBINSON')
   3 - filter("T1"."ID"="T2"."ID")

16 rows selected.

SQL> select OPERATION,OPTIONS,IO_COST,CPU_COST FROM PLAN_TABLE;

OPERATION            OPTIONS                     IO_COST   CPU_COST
-------------------- -------------------- -------------- ----------
SELECT STATEMENT                                   94693 3.2867E+10
NESTED LOOPS                                       94693 3.2867E+10
TABLE ACCESS         FULL                             90   20263700
TABLE ACCESS         FULL                             39   13595488

现在来计算一下 IO_COST = 94693 是怎么计算出来的

SQL> SELECT CEIL(403*42/16/12)+ceil(2416*(179*42/16/12))+1+1 FROM DUAL;

CEIL(403*42/16/12)+CEIL(2416*(179*42/16/12))+1+1
------------------------------------------------
                                           94693

                           
由此可见Oracle11g 成本算法和9i 一样，没有变化，再次记录一下嵌套循环成本算法

NESTED LOOPS COST = (OUTER TABLE IO COST) + 
                    (OUTER TABLE CARDINALITY) * (INNER TABLE IO COST) + TOTAL CPU COST