use_nl,use_hash,use_merge的3种连接方式驱动与被驱动关系
2014-05-31 21:41
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SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
PL/SQL Release 11.2.0.3.0 - Production
CORE 11.2.0.3.0 Production
TNS for Linux: Version 11.2.0.3.0 - Production
NLSRTL Version 11.2.0.3.0 - Production
--use_nl,use_hash,use_merge的3种连接方式驱动与被驱动关系
一.HASH连接方式
--建一个小表和一个大一些的表
SQL> create table t1 as select * from dba_objects where rownum<11;
Table created.
SQL> create table t2 as select * from dba_objects;
Table created.
SQL> select count(1) from t1;
COUNT(1)
----------
10
SQL> select count(1) from t2;
COUNT(1)
----------
75211
--试验HASH，可以看出hash是存在驱动与被驱动关系，驱动表适合于两表间较小的一个，耗用更少的排序开销；
SQL> select /*+ leading(t2) use_hash(t2,t1) */t1.status,t2.status
2 from t1,t2
3 where t1.object_id=t2.object_id;
STATUS STATUS
------- -------
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
10 rows selected.
--看到排序区中排序的开销为3439K；
SQL> @allstat
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 315bd4pgdyt86, child number 1
-------------------------------------
select /*+ leading(t2) use_hash(t2,t1) */t1.status,t2.status from t1,t2
where t1.object_id=t2.object_id
Plan hash value: 2959412835
-------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.08 | 1078 | 1071 | | | |
|* 1 | HASH JOIN | | 1 | 10 | 10 |00:00:00.08 | 1078 | 1071 | 3439K| 1573K| 5617K (0)|
| 2 | TABLE ACCESS FULL| T2 | 1 | 79628 | 75211 |00:00:00.02 | 1074 | 1071 | | | |
| 3 | TABLE ACCESS FULL| T1 | 1 | 10 | 10 |00:00:00.01 | 4 | 0 | | | |
-------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
25 rows selected.
--而T1表为驱动表时，排序的开销为1206K比上面的3439K小得多；
SQL> select /*+ leading(t1) use_hash(t2,t1) */t1.status,t2.status
2 from t1,t2
3 where t1.object_id=t2.object_id;
STATUS STATUS
------- -------
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
VALID VALID
10 rows selected.
SQL> @allstat
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 3zc11bg9gdq31, child number 1
-------------------------------------
select /*+ leading(t1) use_hash(t2,t1) */t1.status,t2.status from t1,t2
where t1.object_id=t2.object_id
Plan hash value: 1838229974
-------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
-------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.08 | 1078 | 1071 | | | |
|* 1 | HASH JOIN | | 1 | 10 | 10 |00:00:00.08 | 1078 | 1071 | 1206K| 1206K| 1126K (0)|
| 2 | TABLE ACCESS FULL| T1 | 1 | 10 | 10 |00:00:00.01 | 3 | 0 | | | |
| 3 | TABLE ACCESS FULL| T2 | 1 | 79628 | 75211 |00:00:00.03 | 1075 | 1071 | | | |
-------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
25 rows selected.
--而ORACLE在预估COST时第一类的COST也是大于第2类，也就是默认O是会采用第2种方式；
--第1类的COST为419，而第2类的COST预估为305；
SQL> set auto traceonly
SP2-0158: unknown SET autocommit option "traceonly"
Usage: SET AUTO[COMMIT] { OFF | ON | IMM[EDIATE] | n }
SQL> set autot traceonly exp
SQL>
SQL> select /*+ leading(t2) use_hash(t2,t1) */t1.status,t2.status
2 from t1,t2
3 where t1.object_id=t2.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 2959412835
-----------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 360 | | 419 (1)| 00:00:06 |
|* 1 | HASH JOIN | | 10 | 360 | 2336K| 419 (1)| 00:00:06 |
| 2 | TABLE ACCESS FULL| T2 | 79628 | 1399K| | 301 (1)| 00:00:04 |
| 3 | TABLE ACCESS FULL| T1 | 10 | 180 | | 3 (0)| 00:00:01 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
SQL> select /*+ leading(t1) use_hash(t2,t1) */t1.status,t2.status
2 from t1,t2
3 where t1.object_id=t2.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 1838229974
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 360 | 305 (1)| 00:00:04 |
|* 1 | HASH JOIN | | 10 | 360 | 305 (1)| 00:00:04 |
| 2 | TABLE ACCESS FULL| T1 | 10 | 180 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| T2 | 79628 | 1399K| 301 (1)| 00:00:04 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
--HASH连接方式默认当然是采用后一种COST较小那类；
SQL> select t1.status,t2.status
2 from t1,t2
3 where t1.object_id=t2.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 1838229974
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 10 | 360 | 305 (1)| 00:00:04 |
|* 1 | HASH JOIN | | 10 | 360 | 305 (1)| 00:00:04 |
| 2 | TABLE ACCESS FULL| T1 | 10 | 180 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| T2 | 79628 | 1399K| 301 (1)| 00:00:04 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
二.排序合并连接方式
--可以看到无论是哪种表排在前面先执行，其开销都是一样的；也就是说明merge方式是不存在驱动与被驱动的区别关系 ；
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 2yw072zjb9h5b, child number 1
-------------------------------------
select /*+ leading(t1) use_merge(t2,t1) */t1.status,t2.status from
t1,t2 where t1.object_id=t2.object_id
Plan hash value: 412793182
--------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.06 | 1077 | 1071 | | | |
| 1 | MERGE JOIN | | 1 | 10 | 10 |00:00:00.06 | 1077 | 1071 | | | |
| 2 | SORT JOIN | | 1 | 10 | 10 |00:00:00.01 | 3 | 0 | 2048 | 2048 | 2048 (0)|
| 3 | TABLE ACCESS FULL| T1 | 1 | 10 | 10 |00:00:00.01 | 3 | 0 | | | |
|* 4 | SORT JOIN | | 10 | 79628 | 10 |00:00:00.06 | 1074 | 1071 | 2675K| 740K| 2377K (0)|
| 5 | TABLE ACCESS FULL| T2 | 1 | 79628 | 75211 |00:00:00.02 | 1074 | 1071 | | | |
--------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
28 rows selected.
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID csjsgn0hh4dr6, child number 1
-------------------------------------
select /*+ leading(t2) use_merge(t2,t1) */t1.status,t2.status from
t1,t2 where t1.object_id=t2.object_id
Plan hash value: 1792967693
--------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.09 | 1077 | 1071 | | | |
| 1 | MERGE JOIN | | 1 | 10 | 10 |00:00:00.09 | 1077 | 1071 | | | |
| 2 | SORT JOIN | | 1 | 79628 | 54 |00:00:00.09 | 1074 | 1071 | 2675K| 740K| 2377K (0)|
| 3 | TABLE ACCESS FULL| T2 | 1 | 79628 | 75211 |00:00:00.02 | 1074 | 1071 | | | |
|* 4 | SORT JOIN | | 54 | 10 | 10 |00:00:00.01 | 3 | 0 | 2048 | 2048 | 2048 (0)|
| 5 | TABLE ACCESS FULL| T1 | 1 | 10 | 10 |00:00:00.01 | 3 | 0 | | | |
--------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")
filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
三.嵌套循环连接方式是影响最为明显的，buffers数相差了10倍之多；
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 35g7vtpc63s04, child number 0
-------------------------------------
select /*+ leading(t1) use_nl(t2,t1) */t1.status,t2.status from t1,t2
where t1.object_id=t2.object_id
Plan hash value: 1967407726
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.11 | 10745 | 10710 |
| 1 | NESTED LOOPS | | 1 | 10 | 10 |00:00:00.11 | 10745 | 10710 |
| 2 | TABLE ACCESS FULL| T1 | 1 | 10 | 10 |00:00:00.01 | 4 | 0 |
|* 3 | TABLE ACCESS FULL| T2 | 10 | 1 | 10 |00:00:00.11 | 10741 | 10710 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 1a5dw45kqph9p, child number 0
-------------------------------------
select /*+ leading(t2) use_nl(t2,t1) */t1.status,t2.status from t1,t2
where t1.object_id=t2.object_id
Plan hash value: 4016936828
----------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | Reads |
----------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 10 |00:00:00.93 | 226K| 1071 |
| 1 | NESTED LOOPS | | 1 | 10 | 10 |00:00:00.93 | 226K| 1071 |
| 2 | TABLE ACCESS FULL| T2 | 1 | 79628 | 75211 |00:00:00.06 | 1075 | 1071 |
|* 3 | TABLE ACCESS FULL| T1 | 75211 | 1 | 10 |00:00:00.74 | 225K| 0 |
----------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("T1"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
25 rows selected.
小结：从以上可看出，影响大到小的顺序为nl->hash->merge(不影响);
应用：日常工作环境中经常可看到，由于nl方式不当而应使用hash的案例，毕竟NL对数据量的大小影响是最为敏感；
而ORACLE选择NL方式并没有错，错就错在预估值那里，所以当用explain plan for....,autot等方式看并不能看出问题；
当通过running time的统计信息收集然后select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
就会发现真正的原因是预估值与实际值的问题相差过大导致的；
在这种情况下执行计划的绑定是比较实用和有效的；
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