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表的连接顺序是否很重要

原创 Oracle 作者：abin1703 时间：2018-01-21 16:21:54 0 删除 编辑
SQL> create table tab_big  as select * from dba_objects where rownum<=30000;
create table tab_small  as select * from dba_objects where rownum<=10;
Table created.


Elapsed: 00:00:00.34
SQL> 


Table created.


Elapsed: 00:00:00.06
SQL> set autotrace traceonly
set linesize 1000
set timing on SQL> SQL> 
SQL> 
SQL> select count(*) from tab_big,tab_small   where tab_big.object_id=tab_small.object_id;


Elapsed: 00:00:00.23


Execution Plan
----------------------------------------------------------
Plan hash value: 3126178707


---------------------------------------------------------------------------------
| Id  | Operation     | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    | |     1 |    26 |   122   (0)| 00:00:02 |
|   1 |  SORT AGGREGATE     | |     1 |    26 |      | |
|*  2 |   HASH JOIN     | |    10 |   260 |   122   (0)| 00:00:02 |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |    10 |   130 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| TAB_BIG | 28743 |   364K|   119   (0)| 00:00:02 |
---------------------------------------------------------------------------------


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


   2 - access("TAB_BIG"."OBJECT_ID"="TAB_SMALL"."OBJECT_ID")


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




Statistics
----------------------------------------------------------
12  recursive calls
  0  db block gets
497  consistent gets
422  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> /


Elapsed: 00:00:00.03


Execution Plan
----------------------------------------------------------
Plan hash value: 3126178707


---------------------------------------------------------------------------------
| Id  | Operation     | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    | |     1 |    26 |   122   (0)| 00:00:02 |
|   1 |  SORT AGGREGATE     | |     1 |    26 |      | |
|*  2 |   HASH JOIN     | |    10 |   260 |   122   (0)| 00:00:02 |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |    10 |   130 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| TAB_BIG | 28743 |   364K|   119   (0)| 00:00:02 |
---------------------------------------------------------------------------------


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


   2 - access("TAB_BIG"."OBJECT_ID"="TAB_SMALL"."OBJECT_ID")


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




Statistics
----------------------------------------------------------
  0  recursive calls
  0  db block gets
424  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> select count(*) from tab_big,tab_small   where tab_small.object_id=tab_big.object_id;


Elapsed: 00:00:00.03


Execution Plan
----------------------------------------------------------
Plan hash value: 3126178707


---------------------------------------------------------------------------------
| Id  | Operation     | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    | |     1 |    26 |   122   (0)| 00:00:02 |
|   1 |  SORT AGGREGATE     | |     1 |    26 |      | |
|*  2 |   HASH JOIN     | |    10 |   260 |   122   (0)| 00:00:02 |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |    10 |   130 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| TAB_BIG | 28743 |   364K|   119   (0)| 00:00:02 |
---------------------------------------------------------------------------------


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


   2 - access("TAB_SMALL"."OBJECT_ID"="TAB_BIG"."OBJECT_ID")


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




Statistics
----------------------------------------------------------
  7  recursive calls
  0  db block gets
491  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> /


Elapsed: 00:00:00.02


Execution Plan
----------------------------------------------------------
Plan hash value: 3126178707


---------------------------------------------------------------------------------
| Id  | Operation     | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    | |     1 |    26 |   122   (0)| 00:00:02 |
|   1 |  SORT AGGREGATE     | |     1 |    26 |      | |
|*  2 |   HASH JOIN     | |    10 |   260 |   122   (0)| 00:00:02 |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |    10 |   130 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| TAB_BIG | 28743 |   364K|   119   (0)| 00:00:02 |
---------------------------------------------------------------------------------


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


   2 - access("TAB_SMALL"."OBJECT_ID"="TAB_BIG"."OBJECT_ID")


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




Statistics
----------------------------------------------------------
  0  recursive calls
  0  db block gets
424  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> select /*+rule*/ count(*) from tab_big,tab_small ;


Elapsed: 00:00:00.06


Execution Plan
----------------------------------------------------------
Plan hash value: 41936691


-----------------------------------------
| Id  | Operation     | Name |
-----------------------------------------
|   0 | SELECT STATEMENT    | |
|   1 |  SORT AGGREGATE     | |
|   2 |   NESTED LOOPS     | |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |
|   4 |    TABLE ACCESS FULL| TAB_BIG |
-----------------------------------------


Note
-----
   - rule based optimizer used (consider using cbo)




Statistics
----------------------------------------------------------
  1  recursive calls
  0  db block gets
       4213  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> /


Elapsed: 00:00:00.03


Execution Plan
----------------------------------------------------------
Plan hash value: 41936691


-----------------------------------------
| Id  | Operation     | Name |
-----------------------------------------
|   0 | SELECT STATEMENT    | |
|   1 |  SORT AGGREGATE     | |
|   2 |   NESTED LOOPS     | |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |
|   4 |    TABLE ACCESS FULL| TAB_BIG |
-----------------------------------------


Note
-----
   - rule based optimizer used (consider using cbo)




Statistics
----------------------------------------------------------
  0  recursive calls
  0  db block gets
       4213  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> select /*+rule*/ count(*) from tab_small,tab_big ;


Elapsed: 00:00:00.12


Execution Plan
----------------------------------------------------------
Plan hash value: 2127005654


-----------------------------------------
| Id  | Operation     | Name |
-----------------------------------------
|   0 | SELECT STATEMENT    | |
|   1 |  SORT AGGREGATE     | |
|   2 |   NESTED LOOPS     | |
|   3 |    TABLE ACCESS FULL| TAB_BIG |
|   4 |    TABLE ACCESS FULL| TAB_SMALL |
-----------------------------------------


Note
-----
   - rule based optimizer used (consider using cbo)




Statistics
----------------------------------------------------------
  1  recursive calls
  0  db block gets
      90421  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> /


Elapsed: 00:00:00.10


Execution Plan
----------------------------------------------------------
Plan hash value: 2127005654


-----------------------------------------
| Id  | Operation     | Name |
-----------------------------------------
|   0 | SELECT STATEMENT    | |
|   1 |  SORT AGGREGATE     | |
|   2 |   NESTED LOOPS     | |
|   3 |    TABLE ACCESS FULL| TAB_BIG |
|   4 |    TABLE ACCESS FULL| TAB_SMALL |
-----------------------------------------


Note
-----
   - rule based optimizer used (consider using cbo)




Statistics
----------------------------------------------------------
  0  recursive calls
  0  db block gets
      90421  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed

-----连接方式 嵌套循环连接（NL）
SQL> select /*+USE_NL(tab_big) */ count(*) from tab_big,tab_small   
  2  ;


Elapsed: 00:00:00.03


Execution Plan
----------------------------------------------------------
Plan hash value: 41936691


-------------------------------------------------------------------------
| Id  | Operation     | Name | Rows | Cost (%CPU)| Time |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT    | |     1 |  1179   (1)| 00:00:15 |
|   1 |  SORT AGGREGATE     | |     1 |      | |
|   2 |   NESTED LOOPS     | |   287K|  1179   (1)| 00:00:15 |
|   3 |    TABLE ACCESS FULL| TAB_SMALL |    10 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| TAB_BIG | 28743 |   118   (1)| 00:00:02 |
-------------------------------------------------------------------------


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




Statistics
----------------------------------------------------------
  9  recursive calls
  0  db block gets
       4280  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  0  sorts (memory)
  0  sorts (disk)
  1  rows processed


SQL> select /*+USE_NL(tab_small) */ count(*) from tab_big,tab_small   ;


Elapsed: 00:00:00.04


Execution Plan
----------------------------------------------------------
Plan hash value: 177389953


---------------------------------------------------------------------------
| Id  | Operation       | Name   | Rows  | Cost (%CPU)| Time   |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |   | 1 |  1179   (1)| 00:00:15 |
|   1 |  SORT AGGREGATE       |   | 1 |        |   |
|   2 |   MERGE JOIN CARTESIAN|   |   287K|  1179   (1)| 00:00:15 |
|   3 |    TABLE ACCESS FULL  | TAB_SMALL |    10 | 3   (0)| 00:00:01 |
|   4 |    BUFFER SORT       |   | 28743 |  1176   (1)| 00:00:15 |
|   5 |     TABLE ACCESS FULL | TAB_BIG   | 28743 |   118   (1)| 00:00:02 |
---------------------------------------------------------------------------


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




Statistics
----------------------------------------------------------
  7  recursive calls
  0  db block gets
491  consistent gets
  0  physical reads
  0  redo size
526  bytes sent via SQL*Net to client
523  bytes received via SQL*Net from client
  2  SQL*Net roundtrips to/from client
  1  sorts (memory)
  0  sorts (disk)
  1  rows processed


结论：原来表连接顺序的说法早就过时了，那是基于规则的时代，现在我们是基于代价的。hash 连接方式要比nl 、MERGE要好


来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/30345407/viewspace-2150361/ ，如需转载，请注明出处，否则将追究法律责任。

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