Oracle常见的表链接的方法
1 排序合并链接SMJ
Sort merge joinsql
排序合并总结：session
1 一般状况下，排序合并链接的效率远不如hash join，前者适用范围更广，hj只使用于等值链接，smj范围更广（<,>,>=,<=）oracle
2 一般状况下，smj并不适合OLTP系统，排序操做是很是昂贵的操做，ide
2 嵌套循环链接NL
优化器会根据必定的规则来肯定表T1，T2谁是驱动表，谁是被驱动表，驱动表用于外层循环，被驱动表用于内存循环，这里假设驱动表时T1，被驱动表时T2oop
目标sql中指定的谓词条件去访问T1，获得的结果集为1性能
而后遍历驱动结果集1同时遍历被驱动表T2,即先取出1中的第一条记录，接着遍历T2并按照条件去判断T2中是否存在配匹的记录，而后在取出1中的第二条记录。。。。优化
嵌套循环总结：this
1 若是t1对应的驱动结果集较少，同时t2的链接列上又有惟一性索引，则效率会很高spa
2 只要驱动结果集不多就具有嵌套循环的前提条件3d
3 嵌套循环能够实现快速响应，便可以第一时间返回通过链接且知足条件的记录，而没必要等待全部的链接操做所有作完才返回链接结果
若是使用了nl链接，而且t2的链接列上index，那么oracle访问该index是一般会使用单块读，则t1的返回n条结果，就会是t2访问该index n次，若是要回表，
则会回表n次，这就使得不在index 或者data buffer cache中的数据，发生物理I/O，
Oracle 11g使用了向量I/O，提升nl的链接效率
nested loop
outer table --驱动表
inner table
The second picture, shown in Figure 11-2, includes a representation of working through
an index on the second table, because an index is usually involved in this way when there is a
nested loop around.
例
create table t1 (col1 number, col2 varchar2(1));
create table t2 (col2 varchar2(1), col3 varchar2(2));
insert into t1 values(1,'A');
insert into t1 values(2,'B');
insert into t1 values(3,'C');
insert into t2 values('A','A1');
insert into t2 values('B','B1');
insert into t2 values('D','D1');
Connected to:
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bit Production
With the Partitioning, OLAP and Data Mining options
SQL> set linesize 1000
SQL> set pagesize 1000
SQL> set timing on
SQL> set autot trace only
SP2-0158: unknown SET option "only"
SQL> set autotrace traceonly;
SQL> select t1.col1,t1.col2,t2.col3
2 from t1,t2
3 where t1.col2=t2.col2;
Elapsed: 00:00:00.04
Execution Plan
----------------------------------------------------------
Plan hash value: 2253255382
--------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 3 | 60 | 4 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T2 | 1 | 5 | 1 (0)| 00:00:01 |
| 2 | NESTED LOOPS | | 3 | 60 | 4 (0)| 00:00:01 |
| 3 | TABLE ACCESS FULL | T1 | 3 | 45 | 3 (0)| 00:00:01 |
|* 4 | INDEX RANGE SCAN | IDX_T2 | 1 | | 0 (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("T1"."COL2"="T2"."COL2")
Note
-----
- dynamic sampling used for this statement
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
13 consistent gets
0 physical reads
0 redo size
469 bytes sent via SQL*Net to client
337 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
2 rows processed
3 哈希链接HJ
哈希链接是两个表在作链接时只要依靠哈希运算来获得结果集（仅适合CBO），在解析目标sql 时是否考虑哈希链接受限制与隐含参数（_HASH_JOIN_ENABLED），
默认值TRUE，若是值为false，强制使用hint，也是会走hj的
1 oracle会根据参数HASH_AREA_SIZE,DB_BLOCK_SIZE,_HASH_MULTIBLOCK_IO_COUNT来决定hash partition的数量，全部hash partition的集合称为Hash table，
2 表t1，t2在目标sql中的谓词条件后，获得结果集中的数据量较少的那个结果集会被oracle选为哈希链接的驱动结果集，假设t1的结果集1较少（驱动结果集），t2的结果2（被驱动结果集）
3 oracle会遍历结果集1，读取1中的每一条记录，并对每一条记录按照该记录t1中的链接列作哈希运算，
--小表在数据在指定谓词后作哈希运算放入pga中（超过放入temp），大表数据按照链接列作哈希运算，而后大表去配匹pga中的值，遍历完为止
哈希链接的优缺点：
1 哈希链接不必定会排序，大多数状况下不须要排序
2 哈希链接的驱动表所对应的链接列的可选择性尽量的好，会影响hash bucket中的记录数，哈希链接中，遍历hash bucket的动做发生在pga工做区中，不消耗逻辑读，
3 哈希链接适用于CBO，等值链接
4 哈希链接适合大表跟小表的链接，2个表作哈希链接，在指定了谓词后的sql中获得的数量较少的结果集所对应的hash table能彻底容纳在pga中，则效率会很高。
SQL> select /*+ leading (t1) use_hash(t2) */ 2 t1.col1,t1.col2,t2.col3 3 from t1,t2 4 where t1.col2=t2.col2; Elapsed: 00:00:00.25 Execution Plan ---------------------------------------------------------- Plan hash value: 1838229974 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 60 | 7 (15)| 00:00:01 | |* 1 | HASH JOIN | | 3 | 60 | 7 (15)| 00:00:01 | | 2 | TABLE ACCESS FULL| T1 | 3 | 45 | 3 (0)| 00:00:01 | | 3 | TABLE ACCESS FULL| T2 | 3 | 15 | 3 (0)| 00:00:01 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - access("T1"."COL2"="T2"."COL2") Note ----- - dynamic sampling used for this statement Statistics ---------------------------------------------------------- 7 recursive calls 0 db block gets 32 consistent gets 0 physical reads 0 redo size 469 bytes sent via SQL*Net to client 337 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 2 sorts (memory) 0 sorts (disk) 2 rows processed
4笛卡尔链接cross join
2个表在作链接是，没有指定任何链接条件的链接
SQL> select 2 t1.col1,t1.col2,t2.col3 3 from t1,t2; 9 rows selected. Elapsed: 00:00:00.03 Execution Plan ---------------------------------------------------------- Plan hash value: 787647388 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 9 | 162 | 8 (0)| 00:00:01 | | 1 | MERGE JOIN CARTESIAN| | 9 | 162 | 8 (0)| 00:00:01 | | 2 | TABLE ACCESS FULL | T1 | 3 | 45 | 3 (0)| 00:00:01 | | 3 | BUFFER SORT | | 3 | 9 | 5 (0)| 00:00:01 | | 4 | TABLE ACCESS FULL | T2 | 3 | 9 | 2 (0)| 00:00:01 | -----------------------------------------------------------------------------
5 反链接Anti join
作子查询展开时，oracle会常常把那些外部where条件为 no exists，not in ，<>all的子查询转换成对应的反链接
SQL> select * from t1 2 where t1.col2 not in (select col2 from t2); Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 895956251 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 15 | 5 (0)| 00:00:01 | |* 1 | FILTER | | | | | | | 2 | TABLE ACCESS FULL| T1 | 3 | 45 | 3 (0)| 00:00:01 | |* 3 | TABLE ACCESS FULL| T2 | 3 | 6 | 2 (0)| 00:00:01 | --------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter( NOT EXISTS (SELECT /*+ */ 0 FROM "T2" "T2" WHERE LNNVL("COL2"<>:B1))) 3 - filter(LNNVL("COL2"<>:B1)) SQL> select * from t1 2 where not exists (select 1 from t2 where t1.col2=t2.col2); Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 1534930707 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 17 | 3 (0)| 00:00:01 | | 1 | NESTED LOOPS ANTI | | 1 | 17 | 3 (0)| 00:00:01 | | 2 | TABLE ACCESS FULL| T1 | 3 | 45 | 3 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | IDX_T2 | 3 | 6 | 0 (0)| 00:00:01 | ----------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("T1"."COL2"="T2"."COL2") alter session set "_optimizer_null_aware_antijoin"=false
6 半链接semi join
半链接跟普通的链接不一样，半链接会去重？
对子查询展开，exists，in等
SQL> select * from t1 2 where t1.col2 in (select col2 from t2); Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 3783859632 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 51 | 3 (0)| 00:00:01 | | 1 | NESTED LOOPS SEMI | | 3 | 51 | 3 (0)| 00:00:01 | | 2 | TABLE ACCESS FULL| T1 | 3 | 45 | 3 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | IDX_T2 | 3 | 6 | 0 (0)| 00:00:01 | ----------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("T1"."COL2"="COL2") SQL> select * from t1 2 where exists (select 1 from t2 where t1.col2=t2.col2); Elapsed: 00:00:00.01 Execution Plan ---------------------------------------------------------- Plan hash value: 3783859632 ----------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 3 | 51 | 3 (0)| 00:00:01 | | 1 | NESTED LOOPS SEMI | | 3 | 51 | 3 (0)| 00:00:01 | | 2 | TABLE ACCESS FULL| T1 | 3 | 45 | 3 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | IDX_T2 | 3 | 6 | 0 (0)| 00:00:01 | ----------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("T1"."COL2"="T2"."COL2")
总结一下
- 在哪一种状况下用哪一种链接方法比较好：
A）排序合并链接(Sort Merge Join, SMJ)：
a) 对于非等值链接，这种链接方式的效率是比较高的。
b) 若是在关联的列上都有索引，效果更好。
c) 对于将2个较大的表源作链接，该链接方法比NL链接要好一些。
B）嵌套循环(Nested Loops, NL)：
a) 若是驱动表(外部表)比较小，而且在被驱动表(内部表)上有惟一索引，或有高选择性非惟一索引时，使用这种方法能够获得较好的效率。
b)嵌套循环链接有其它链接方法没有的的一个优势是：能够先返回已经链接的行，而没必要等待全部的链接操做处理完才返回数据，这能够实现快速的响应时间。
C）哈希链接(Hash Join, HJ)：
a) 这种方法是在oracle7后来引入的，使用了比较先进的链接理论，通常来讲，其效率应该好于其它2种链接，可是这种链接只能用在CBO优化器中，
并且须要设置合适的hash_area_size参数，才能取得较好的性能。
b) 在2个较大的表源之间链接时会取得相对较好的效率，在一个表源较小时则能取得更好的效率。
c) 只能用于等值链接中