概述：
NESTED LOOP:
对于被连接的数据子集较小的情况，嵌套循环连接是个较好的选择。在嵌套循环中，内表被外表驱动，外表返回的每一行都要在内表中检索找到与它匹配的行，因此整个查询返回的结果集不能太大（大于1 万不适合），要把返回子集较小表的作为外表（CBO 默认外表是驱动表），而且在内表的连接字段上一定要有索引。当然也可以用ORDERED 提示来改变CBO默认的驱动表，使用USE_NL(table_name1 table_name2)可是强制CBO 执行嵌套循环连接。
HASH JOIN :
散列连接是CBO 做大数据集连接时的常用方式，优化器使用两个表中较小的表（或数据源）利用连接键在内存中建立散列表，然后扫描较大的表并探测散列表，找出与散列表匹配的行。
这种方式适用于较小的表完全可以放于内存中的情况，这样总成本就是访问两个表的成本之和。但是在表很大的情况下并不能完全放入内存，这时优化器会将它分割成若干不同的分区，不能放入内存的部分就把该分区写入磁盘的临时段，此时要有较大的临时段从而尽量提高I/O 的性能。
也可以用USE_HASH(table_name1 table_name2)提示来强制使用散列连接。如果使用散列连接HASH_AREA_SIZE 初始化参数必须足够的大，如果是9i，Oracle建议使用SQL工作区自动管理，设置WORKAREA_SIZE_POLICY 为AUTO，然后调整PGA_AGGREGATE_TARGET 即可。
MERGE JOIN排序合并连接
通常情况下散列连接的效果都比排序合并连接要好，然而如果行源已经被排过序，在执行排序合并连接时不需要再排序了，这时排序合并连接的性能会优于散列连接。可以使用USE_MERGE(table_name1 table_name2)来强制使用排序合并连接
几种方式的操作方式
merge join
merge join的操作通常分三步：
1、对连接的每个表做table access full;
2、对table access full的结果进行排序。
3、进行merge join对排序结果进行合并。
在全表扫描比索引范围扫描再通过rowid进行表访问更可取的情况下，merge join会比nested loops性能更佳。当表特别小或特别巨大的时候，实行全表访问可能会比索引范围扫描更有效。mrege join的性能开销几乎都在前两步。
hash join
对两个表进行全表扫描，然后oracle读取涉及连接的其中一个表，并且在内存里创建来自表的连接列的唯一关键字的位图。当读取和处理第二个表的行时，创建值的位图被用做过滤器。如果一个行成功的通过位图过滤，则hash算法用于数据查找和后来的连接。（这里涉及数学问题，我也弄的不是很清楚）。
以下条件下hash join可能有优势：
两个巨大的表之间的连接。
在一个巨大的表和一个小表之间的连接。
Nested Loops
会循环外表（驱动表），逐个比对和内表的连接是否符合条件。在驱动表比较小，内表比较大，而且内外表的连接列有索引的时候比较好。当SORT_AREA空间不足的时候，Oracle也会选择使用NL。基于Cost的Oracle优化器(CBO)会自动选择较小的表做外表。
连接方式总结：
1)）嵌套循环(nest loop):
对于被连接的数据子集较小的情况，嵌套循环连接是较好的选择。在嵌套循环中，外表驱动内表，外表返回的每一行都要在内表中检索找到它匹配的行，因此整个查询返回的结果集不能太大（大于10000不合适），要把返回子集较小的表作为外表（驱动表），而且在内表的连接字段上一定要有索引。
2）哈希连接(hash join):
哈希连接是大数据集连接时常用的方式，优化器使用两个表中较小的表，利用连接键在内存中建立散列表，然后扫描较大的表并探测散列表，找出与散列表匹配的行。
这种方式适用于较小的表完全可以放入内存的情况，这样成本就是访问两个表的成本之和。但是在表很大的情况下并不能完全放入内存，这时优化器将它分割成若干不同的分区，不能放入内存的部分就把该分区写入磁盘的临时段。
哈希连接只能应用于等值连接(如WHERE A.COL3 = B.COL4)、非等值连接(WHERE A.COL3 > B.COL4)、外连接(WHERE A.COL3 = B.COL4(+))。
3）排序合并连接（Sort Merge Join ）
通常情况下哈希连接的效果都比排序合并连接要好。然而如果行源已经被排过序，在执行排序合并连接时不需要再排序了，这时排序归并连接的性能会忧于哈希连接。
测试实验：
--创建测试表
SQL> Create Table t As Select * From dba_objects;
SQL> Insert /*+ append*/ t Select * From t;
SQL> Insert /*+ append*/ t Select * From t;
......
SQL> commit;
SQL> Create Table t1 As Select * From user_objects;
SQL> Create Table t2 As Select * From dba_objects;
SQL> Insert /*+ append*/ t2 Select * From t2;
......
SQL> commit;
SQL> select count(1) from t;
COUNT(1)
----------
3199936
SQL> select count(1) from t2;
COUNT(1)
----------
101220
SQL> select count(1) from t1;
COUNT(1)
----------
13
SQL> Create Index ind_id On t(object_id);
SQL> Create Index ind1_id On t1(object_id);
SQL> Create Index ind2_id On t2(object_id);
1.默认使用Nested loops的SQL语句的情况
--不加hints会默认使用Nested loops join
SQL> Select * From t,t1 Where t.object_id=t1.object_id;
576 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1308275697
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 833 | 208K| 862 (1)| 00:00:11 |
| 1 | TABLE ACCESS BY INDEX ROWID| T | 64 | 6144 | 66 (0)| 00:00:01 |
| 2 | NESTED LOOPS | | 833 | 208K| 862 (1)| 00:00:11 |
| 3 | TABLE ACCESS FULL | T1 | 13 | 2080 | 3 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IND_ID | 64 | | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T"."OBJECT_ID"="T1"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
5 recursive calls
0 db block gets
687 consistent gets
0 physical reads
1484 redo size
14877 bytes sent via SQL*Net to client
910 bytes received via SQL*Net from client
40 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
576 rows processed
--强制使用Hash Jion
SQL> Select /*+ use_hash(t1,t)*/ * From t1,t Where t.object_id=t1.object_id;
576 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1444793974
---------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 833 | 208K| 9931 (3)| 00:02:00 |
|* 1 | HASH JOIN | | 833 | 208K| 9931 (3)| 00:02:00 |
| 2 | TABLE ACCESS FULL| T1 | 13 | 2080 | 3 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL| T | 3204K| 293M| 9890 (2)| 00:01:59 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T"."OBJECT_ID"="T1"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
44124 consistent gets
0 physical reads
524 redo size
71981 bytes sent via SQL*Net to client
910 bytes received via SQL*Net from client
40 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
576 rows processed
--强制使用Merge join
SQL> Select /*+ use_merge(t1,t)*/ * From t1,t Where t.object_id=t1.object_id;
576 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2036048135
------------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 833 | 208K| | 80548 (1)| 00:16:07 |
| 1 | MERGE JOIN | | 833 | 208K| | 80548 (1)| 00:16:07 |
| 2 | TABLE ACCESS BY INDEX ROWID| T1 | 13 | 2080 | | 2 (0)| 00:00:01 |
| 3 | INDEX FULL SCAN | IND1_ID | 13 | | | 1 (0)| 00:00:01 |
|* 4 | SORT JOIN | | 3204K| 293M| 794M| 80546 (1)| 00:16:07 |
| 5 | TABLE ACCESS FULL | T | 3204K| 293M| | 9890 (2)| 00:01:59 |
------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T"."OBJECT_ID"="T1"."OBJECT_ID")
filter("T"."OBJECT_ID"="T1"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
44097 consistent gets
0 physical reads
524 redo size
14899 bytes sent via SQL*Net to client
910 bytes received via SQL*Net from client
40 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
576 rows processed
结论：
逻辑读分别为：
Nested loops join 687
Hash Jion 44124
Merge join 44097
可以看到，大表t和小表t1关联使用Nested loops join关联性能会比较好。
2.默认使用Hash Jion的SQL语句的情况
--不加hints会默认使用Hash Jion
SQL> set timing on
SQL> Select t.object_id From t,t2 Where t.object_id=t2.object_id;
6364032 rows selected.
Elapsed: 00:00:31.58
Execution Plan
----------------------------------------------------------
Plan hash value: 3237705646
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5713K| 98M| | 4442 (4)| 00:00:54 |
|* 1 | HASH JOIN | | 5713K| 98M| 2224K| 4442 (4)| 00:00:54 |
| 2 | INDEX FAST FULL SCAN| IND2_ID | 90908 | 1154K| | 55 (4)| 00:00:01 |
| 3 | INDEX FAST FULL SCAN| IND_ID | 3204K| 15M| | 1596 (4)| 00:00:20 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("T"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
430719 consistent gets
0 physical reads
0 redo size
109700403 bytes sent via SQL*Net to client
4667440 bytes received via SQL*Net from client
424270 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
6364032 rows processed
--强制使用Nested loops join
SQL> Select /*+ use_nl(t,t2)*/ t.object_id From t,t2 Where t.object_id=t2.object_id;
6364032 rows selected.
Elapsed: 00:00:31.45
Execution Plan
----------------------------------------------------------
Plan hash value: 3551542035
---------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5713K| 98M| 98323 (1)| 00:19:40 |
| 1 | NESTED LOOPS | | 5713K| 98M| 98323 (1)| 00:19:40 |
| 2 | INDEX FAST FULL SCAN| IND2_ID | 90908 | 1154K| 55 (4)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | IND_ID | 63 | 315 | 2 (0)| 00:00:01 |
---------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
838760 consistent gets
0 physical reads
0 redo size
109700403 bytes sent via SQL*Net to client
4667440 bytes received via SQL*Net from client
424270 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
6364032 rows processed
--强制使用merge join
SQL> Select /*+ use_merge(t,t2)*/ t.object_id From t,t2 Where t.object_id=t2.object_id;
6364032 rows selected.
Elapsed: 00:00:31.78
Execution Plan
----------------------------------------------------------
Plan hash value: 1712472574
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 5713K| 98M| | 7661 (2)| 00:01:32 |
| 1 | MERGE JOIN | | 5713K| 98M| | 7661 (2)| 00:01:32 |
| 2 | INDEX FULL SCAN | IND_ID | 3204K| 15M| | 7137 (2)| 00:01:26 |
|* 3 | SORT JOIN | | 90908 | 1154K| 3576K| 495 (3)| 00:00:06 |
| 4 | INDEX FAST FULL SCAN| IND2_ID | 90908 | 1154K| | 55 (4)| 00:00:01 |
------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T"."OBJECT_ID"="T2"."OBJECT_ID")
filter("T"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
430689 consistent gets
0 physical reads
0 redo size
109700403 bytes sent via SQL*Net to client
4667440 bytes received via SQL*Net from client
424270 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
6364032 rows processed
结论：
逻辑读分别为：
Nested loops join 838760
Hash Jion 430719
Merge join 430689
可以看到，大表t和大表t2关联使用Hash Jion和Merge join关联性能差不多，Nested loops join性能最差。
3.默认使用Merge Jion的SQL语句的情况
--不加hints会默认使用Merge Jion
SQL> Select t.object_id From t2,(Select * From t Where owner='SYS' Order By object_id) t Where t.object_id=t2.object_id;
2910592 rows selected.
Elapsed: 00:00:15.57
Execution Plan
----------------------------------------------------------
Plan hash value: 2780935651
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2589K| 59M| | 16553 (2)| 00:03:19 |
| 1 | MERGE JOIN | | 2589K| 59M| | 16553 (2)| 00:03:19 |
| 2 | INDEX FULL SCAN | IND2_ID | 90908 | 1154K| | 240 (2)| 00:00:03 |
|* 3 | SORT JOIN | | 1452K| 15M| 55M| 16300 (2)| 00:03:16 |
|* 4 | TABLE ACCESS FULL| T | 1452K| 15M| | 9822 (2)| 00:01:58 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - access("T"."OBJECT_ID"="T2"."OBJECT_ID")
filter("T"."OBJECT_ID"="T2"."OBJECT_ID")
4 - filter("OWNER"='SYS')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
89745 consistent gets
0 physical reads
124 redo size
50169586 bytes sent via SQL*Net to client
2134921 bytes received via SQL*Net from client
194041 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
2910592 rows processed
--强制使用Nested join
SQL> Select /*+ use_nl(t2,t)*/ t.object_id From t2,(Select * From t Where owner='SYS' Order By object_id) t Where t.object_id=t2.object_id;
2910592 rows selected.
Elapsed: 00:00:17.18
Execution Plan
----------------------------------------------------------
Plan hash value: 1858129701
---------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
---------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2589K| 59M| | 1482K (1)| 04:56:28 |
| 1 | SORT ORDER BY | | 2589K| 59M| 79M| 1482K (1)| 04:56:28 |
| 2 | NESTED LOOPS | | 2589K| 59M| | 1463K (1)| 04:52:47 |
|* 3 | TABLE ACCESS FULL| T | 1452K| 15M| | 9822 (2)| 00:01:58 |
|* 4 | INDEX RANGE SCAN | IND2_ID | 2 | 26 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
3 - filter("OWNER"='SYS')
4 - access("T"."OBJECT_ID"="T2"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
1523249 consistent gets
0 physical reads
568 redo size
50169586 bytes sent via SQL*Net to client
2134921 bytes received via SQL*Net from client
194041 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
2910592 rows processed
--强制使用Hash join
SQL> Select /*+ use_hash(t2,t)*/ t.object_id From t2,(Select * From t Where owner='SYS' Order By object_id) t Where t.object_id=t2.object_id;
2910592 rows selected.
Elapsed: 00:00:16.34
Execution Plan
----------------------------------------------------------
Plan hash value: 2900034259
------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes |TempSpc| Cost (%CPU)| Time |
------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 2589K| 59M| | 30029 (2)| 00:06:01 |
| 1 | SORT ORDER BY | | 2589K| 59M| 79M| 30029 (2)| 00:06:01 |
|* 2 | HASH JOIN | | 2589K| 59M| 2224K| 11615 (2)| 00:02:20 |
| 3 | INDEX FAST FULL SCAN| IND2_ID | 90908 | 1154K| | 55 (4)| 00:00:01 |
|* 4 | TABLE ACCESS FULL | T | 1452K| 15M| | 9822 (2)| 00:01:58 |
------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T"."OBJECT_ID"="T2"."OBJECT_ID")
4 - filter("OWNER"='SYS')
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
4 recursive calls
0 db block gets
44373 consistent gets
0 physical reads
524 redo size
50169586 bytes sent via SQL*Net to client
2134921 bytes received via SQL*Net from client
194041 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
2910592 rows processed
结论：
逻辑读分别为：
Nested loops join 1523249
Hash Jion 44373
Merge join 89745
可以看到，T2在和t的排序的限定结果集关联时，执行计划使用了merge join，但此时执行效率最的还是hash join，因为此时同样是大表之间关联，采用hash join效果会更好一些。