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本篇内容介绍了“数据库中外连接有OR关联条件只能走NL优化的方法是什么”的有关知识，在实际案例的操作过程中，不少人都会遇到这样的困境，接下来就让小编带领大家学习一下如何处理这些情况吧！希望大家仔细阅读，能够学有所成！
测试数据
drop table t1 purge; drop table t2 purge; create table t1 (id int,name varchar2(10),age int); insert into t1 values(1,'a',1); insert into t1 values(2,'b',2); insert into t1 values(3,'c',5); insert into t1 values(4,'d',1); insert into t1 values(5,'e',3); insert into t1 values(6,'f',6); create table t2 (id int,name varchar2(10)); insert into t2 values(1,'a'); insert into t2 values(2,'b'); insert into t2 values(3,'c'); insert into t2 values(1,'y');
外连接有OR关联条件只能走NL，若驱动表的结果集较大，则会产生大量的关联，会产生性能问题，需要进行优化。
在两个表做外连接时存在几种情况：
1、在进行外连接时，使用nl，此时主表被固定成驱动表，无法通过hint进行驱动表的调整
2、在进行外连接时，使用hash，可以通过hint调整驱动表和被驱动表
针对外连接的情况做以下的实验：
1、当执行计划是nl，t1是驱动表(主表)，t2是被驱动表，调整t2为驱动表，t1为被驱动表。
2、当执行计划是nl，t1是驱动表(主表)，t2是被驱动表，调整执行计划为hash。
3、当执行计划是hash，t1是驱动表(主表)，t2是被驱动表，调整t2为驱动表，t1为被驱动表。
4、当执行计划是hash，t1是驱动表(主表)，t2是被驱动表，调整执行计划为nl。
5、当外连接有OR关联条件(T1.ID = T2.ID OR T1.AGE = T2.ID)，进行等价改写
执行计划是nl的情况
有如下SQL：
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
T1_ID T1_NAME T1_AGE T2_ID T2_NAME
---------- ---------- ---------- ---------- ----------
1 a 1 1 a
2 b 2 2 b
3 c 5 3 c
4 d 1
5 e 3
6 f 6
执行计划：
Plan hash value: 3645848104
-----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 11 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 11 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 5 | INDEX RANGE SCAN | IDX_ID_T2_01 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
-----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")通过执行计划可以看到，走了nl，并且t1是驱动表。
1、当执行计划是nl，t1是驱动表(主表)，t2是被驱动表，调整t2为驱动表，t1为被驱动表。
在内连接中，可以实现驱动表和被驱动表的调整，但是在外连接中不能调整驱动表的顺序
SELECT /*+ leading(t2 t1) use_nl(t1)*/T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 109855138
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 11 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 11 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 5 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
--------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")通过执行计划可以看到，驱动表还是t1，并没有改变执行顺序，因此在执行计划是nl的外连接中无法进行驱动表和
被驱动表的调整。
不能调整的原因：
在进行外连接时，t1作为主表，左外连接t2，因此需要返回t1的全部数据。嵌套循环需要传值，主表传值给从表之后，
如果发现从表没有关联上，直接显示为 NULL 即可；
但是如果是从表传值给主表，没关联上的数据不能传值给主表，不可能传 NULL 给主表，所以两表关联是外连接的时候，
走嵌套循环驱动表只能固定为主表。
2、当执行计划是nl，t1是驱动表(主表)，t2是被驱动表，调整执行计划为hash。
想办法调整为hash
使用hint：use_hash()
驱动表：t1
被驱动表：t2
SELECT /*+ leading(t2 t1) use_hash(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 109855138
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 11 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 11 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 5 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
--------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")此时的hint未生效，走了最初的nl连接。
尝试使用其他hint
SWAP_JOIN_INPUTS ：说明连接当中谁做内建表（驱动表）
NO_SWAP_JOIN_INPUTS ：说明连接中谁做探测表（被驱动表）
SELECT /*+ leading(t2 t1) use_hash(t1) no_swap_join_inputs(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 109855138
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 11 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 11 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 5 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
--------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")此时的hint未生效，走了最初的nl连接。
原因和走nl，不能调整驱动表和被驱动表的原理一致，只可以改变表的连接方式，但是不能改变表的访问顺序。
3、当执行计划是hash，t1是驱动表(主表)，t2是被驱动表，调整t2为驱动表，t1为被驱动表。
想办法调整表的访问顺序
使用hint：use_hash()
驱动表：t2
被驱动表：t1
SELECT /*+ leading(t2 t1) use_hash(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 2391546071
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 14 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 14 | 2048 | 2048 | 2048 (0)|
|* 2 | HASH JOIN OUTER | | 1 | 6 | 6 |00:00:00.01 | 14 | 1753K| 1753K| 920K (0)|
| 3 | TABLE ACCESS FULL| T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS FULL| T2 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
-----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."ID")通过执行计划可以看到，驱动表还是t1，并没有改变执行顺序。
需要在加上一个hint
SWAP_JOIN_INPUTS ：说明连接当中谁做内建表（驱动表）
NO_SWAP_JOIN_INPUTS ：说明连接中谁做探测表（被驱动表）
SELECT /*+ leading(t2 t1) use_hash(t1) swap_join_inputs(t2) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 2146067096
--------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 14 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 14 | 2048 | 2048 | 2048 (0)|
|* 2 | HASH JOIN RIGHT OUTER| | 1 | 6 | 6 |00:00:00.01 | 14 | 2061K| 2061K| 872K (0)|
| 3 | TABLE ACCESS FULL | T2 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
--------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."ID")通过执行计划可以看到，此时驱动表已经变成了t2，被驱动表变成了t1，同时可以看到id=2的操作，
从原来的HASH JOIN OUTER 变成了HASH JOIN RIGHT OUTER，这部分是等价的，
相当于t1左外连接t2改写为t2右外连接t1。
SELECT /*+ leading(t2 t1) use_hash(t1) no_swap_join_inputs(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 2391546071
-----------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 14 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 14 | 2048 | 2048 | 2048 (0)|
|* 2 | HASH JOIN OUTER | | 1 | 6 | 6 |00:00:00.01 | 14 | 1753K| 1753K| 886K (0)|
| 3 | TABLE ACCESS FULL| T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS FULL| T2 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
-----------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."ID"="T2"."ID")此时hint为生效，还是原来的执行计划。
4、当执行计划是hash，t1是驱动表(主表)，t2是被驱动表，调整执行计划为nl。
把hash调整为nl
驱动表：t1
被驱动表：t2
t2的id创建索引
create index idx_id_t2 on t2(id);
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 109855138
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 11 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 11 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 5 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
--------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")执行计划中已经从hash变为nl，并且t1是驱动表，t2是被驱动表
把hash调整为nl
驱动表：t2
被驱动表：t1
t1的id创建索引
create index idx_id_t1 on t1(id);
SELECT /*+ leading(t2 t1) use_nl(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
ORDER BY 1;
Plan hash value: 109855138
--------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
--------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 11 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 11 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 5 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
--------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("T1"."ID"="T2"."ID")通过执行计划可以看到，驱动表还是t1，并没有改变执行顺序。
原因和走nl，不能调整驱动表和被驱动表的原理一致，只可以改变表的连接方式，但是不能改变表的访问顺序。
5、当外连接有OR关联条件，进行等价改写(2)
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
T1_ID T1_NAME T1_AGE T2_ID T2_NAME
---------- ---------- ---------- ---------- ----------
1 a 1 1 a
2 b 2 2 b
3 c 5 3 c
4 d 1 1 a
5 e 3 3 c
6 f 6
6 rows selected.
Plan hash value: 3004654521
------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 49 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 6 |00:00:00.01 | 49 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 49 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 1 | 5 |00:00:00.01 | 42 | | | |
|* 5 | TABLE ACCESS FULL| T2 | 6 | 1 | 5 |00:00:00.01 | 42 | | | |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - filter(("T1"."ID"="T2"."ID" OR "T1"."AGE"="T2"."ID"))T1作为主表和T2做外连接，需要返回T1的全部数据以及满足条件的T2记录，还有不满足条件的T2的结果null。
当使用了OR，则表示只要满足其中的一个条件即可返回T1和T2的记录。
假设T1和T2连接时是从第一行记录开始： 当T1拿出第一行记录的id和age的值传给T2表，和T2表的第一行记录进行匹配， 在这里有三种情况： 1、如果发现T1的id值和T2表的id的值相等，但是T1的age值和T2表的id的值不相等，那么返回T1的记录和T2的记录，第一行的记录； 2、如果发现T1的age值和T2表的id的值相等，但是T1的id值和T2表的id的值不相等，那么也返回T1的记录和T2的记录，第一行的记录； 3、如果发现T1的id值以及age值和T2表的id的值都相等，那么也返回T1的记录和T2的记录，第一行的记录； 这三种情况的结果就是要么返回一条记录，要么都不满足的情况下T2返回null 当第一行记录匹配完了，接下去该对T1的第二行记录和T2的第二行记录进行匹配，匹配的方法和情况还是和上述的方法一致。 直到把T1的所有记录都匹配一遍，才最终的得到满足条件的记录和不满足条件的T2的null。 因此在这种情况下，需要一行一行的去匹配数据，所以优化器选择了使用nl，需要嵌套循环的匹配数据。
这时候的执行计划肯定是有问题的：
1、被驱动表是全表扫描，连接列没有索引，t1传出一条数据，t2就需要全表扫描一次。
2、一般来说，走nl是小表在前，大表在后，但是在外连接中，走了nl，或者确定了主表，那么他就一定是驱动表，
这里的主表可以是一个表，也可以是一个过滤完的结果集，因此当主表的结果集很大的时候，驱动表就需要被驱动很多次，
做了大量的join操作，耗费很多的资源。
几种情况：
t1是小表，t2是大表，但是t2列没有索引，都是全表扫描；
t1是小表，t2是小表，但是t2列没有索引，都是全表扫描；
t1是大表，t2是大表，但是t2列没有索引，都是全表扫描；
t1是大表，t2是小表，但是t2列没有索引，都是全表扫描；
以上的操作都是有问题，走的是nl，但是被驱动表都是全表扫描。
还有其他情况，t2表的连接列有索引
t1是小表，t2是大表，但是t2列有索引；
t1是小表，t2是小表，但是t2列有索引；
t1是大表，t2是大表，但是t2列有索引；
t1是大表，t2是小表，但是t2列有索引；
以上的操作相比较全表扫描而言性能有所提高，但是也是存在大量的join。
当t2的id列有索引时
create index idx_id_t2 on t2(id);
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
Plan hash value: 2234182087
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 24 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 24 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 12 | 6 |00:00:00.01 | 24 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 2 | 5 |00:00:00.01 | 17 | | | |
| 5 | CONCATENATION | | 6 | | 5 |00:00:00.01 | 17 | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 4 |00:00:00.01 | 10 | | | |
|* 7 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 4 |00:00:00.01 | 6 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 1 |00:00:00.01 | 7 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 1 |00:00:00.01 | 6 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
7 - access("T1"."AGE"="T2"."ID")
9 - access("T1"."ID"="T2"."ID")
filter(LNNVL("T1"."AGE"="T2"."ID"))因为连接条件都是对t2的id进行关联，在t2的连接条件上有索引时，会使用索引，但是会进行两次索引扫描，然后回表，
然后把这个结果集作为一个视图。
t1给一条记录，则扫描一次视图，这样也是有问题的。
使用上述操作时存在以下问题：
1、访问方式被固定，只能使用nl，不管被驱动表的连接列是否有索引
2、当驱动表很大，被驱动表很小，使用nl的效率很低，被驱动表需要访问t1的行记录数(结果集)
优化思路：
1、调整驱动表和被驱动表的顺序
2、使用hash
1、调整驱动表和被驱动表的顺序
SELECT /*+ leading(t2 t1) use_nl(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
Plan hash value: 2234182087
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 24 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 24 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 12 | 6 |00:00:00.01 | 24 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 2 | 5 |00:00:00.01 | 17 | | | |
| 5 | CONCATENATION | | 6 | | 5 |00:00:00.01 | 17 | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 4 |00:00:00.01 | 10 | | | |
|* 7 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 4 |00:00:00.01 | 6 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 1 |00:00:00.01 | 7 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 1 |00:00:00.01 | 6 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
7 - access("T1"."AGE"="T2"."ID")
9 - access("T1"."ID"="T2"."ID")
filter(LNNVL("T1"."AGE"="T2"."ID"))原因：nl的外连接无法更改驱动表被驱动表。
2、使用hash
SELECT /*+ leading(t1 t2) use_hash(t2) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
Plan hash value: 2234182087
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 24 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 24 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 12 | 6 |00:00:00.01 | 24 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 2 | 5 |00:00:00.01 | 17 | | | |
| 5 | CONCATENATION | | 6 | | 5 |00:00:00.01 | 17 | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 4 |00:00:00.01 | 10 | | | |
|* 7 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 4 |00:00:00.01 | 6 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 1 |00:00:00.01 | 7 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 1 |00:00:00.01 | 6 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
7 - access("T1"."AGE"="T2"."ID")
9 - access("T1"."ID"="T2"."ID")
filter(LNNVL("T1"."AGE"="T2"."ID"))加hash的hint
SWAP_JOIN_INPUTS ：说明连接当中谁做内建表（驱动表）
NO_SWAP_JOIN_INPUTS ：说明连接中谁做探测表（被驱动表）
SELECT /*+ leading(t1 t2) use_hash(t2) swap_join_inputs(t1) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
Plan hash value: 2234182087
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 24 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 24 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 12 | 6 |00:00:00.01 | 24 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 2 | 5 |00:00:00.01 | 17 | | | |
| 5 | CONCATENATION | | 6 | | 5 |00:00:00.01 | 17 | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 4 |00:00:00.01 | 10 | | | |
|* 7 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 4 |00:00:00.01 | 6 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 1 |00:00:00.01 | 7 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 1 |00:00:00.01 | 6 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
7 - access("T1"."AGE"="T2"."ID")
9 - access("T1"."ID"="T2"."ID")
filter(LNNVL("T1"."AGE"="T2"."ID"))
SELECT /*+ leading(t1 t2) use_hash(t2) no_swap_join_inputs(t2) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
Plan hash value: 2234182087
----------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
----------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 24 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 24 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 12 | 6 |00:00:00.01 | 24 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 2 | 5 |00:00:00.01 | 17 | | | |
| 5 | CONCATENATION | | 6 | | 5 |00:00:00.01 | 17 | | | |
| 6 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 4 |00:00:00.01 | 10 | | | |
|* 7 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 4 |00:00:00.01 | 6 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 1 |00:00:00.01 | 7 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 2 | 1 |00:00:00.01 | 6 | | | |
----------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
7 - access("T1"."AGE"="T2"."ID")
9 - access("T1"."ID"="T2"."ID")
filter(LNNVL("T1"."AGE"="T2"."ID"))无法把执行计划调整为hash。
最终思路：
需要进行等价改写，使得这样的查询执行计划不走nl，或者可以更改驱动表(不可能，前面提过，nl的外连接无法更改驱动表)。
因此只虑等价改写，用来消除or的影响。
在进行等价改写时，又分为两种情况：
1、t2的id字段没有重复值
2、t2的id字段有重复值
当t2的id字段没有重复值，进行等价改写(感谢郭老师)：
SELECT *
FROM (SELECT T.*
,ROW_NUMBER() OVER(PARTITION BY T.T1_RID ORDER BY T.T2_ID) RN
FROM (SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
,T1.ROWID T1_RID
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
UNION ALL
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
,T1.ROWID T1_RID
FROM T1
LEFT JOIN T2
ON T1.AGE = T2.ID) T)
WHERE RN = 1
order by 1;
T1_ID T1_NAME T1_AGE T2_ID T2_NAME T1_RID RN
---------- ---------- ---------- ---------- ---------- ------------------ ----------
1 a 1 1 a AAAVuJAAEAAAByUAAA 1
2 b 2 2 b AAAVuJAAEAAAByUAAB 1
3 c 5 3 c AAAVuJAAEAAAByUAAC 1
4 d 1 1 a AAAVuJAAEAAAByUAAD 1
5 e 3 3 c AAAVuJAAEAAAByUAAE 1
6 f 6 AAAVuJAAEAAAByUAAF 1
6 rows selected.
Plan hash value: 3180408145
-----------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 28 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 28 | 2048 | 2048 | 2048 (0)|
|* 2 | VIEW | | 1 | 12 | 6 |00:00:00.01 | 28 | | | |
|* 3 | WINDOW SORT PUSHED RANK| | 1 | 12 | 12 |00:00:00.01 | 28 | 2048 | 2048 | 2048 (0)|
| 4 | VIEW | | 1 | 12 | 12 |00:00:00.01 | 28 | | | |
| 5 | UNION-ALL | | 1 | | 12 |00:00:00.01 | 28 | | | |
|* 6 | HASH JOIN OUTER | | 1 | 6 | 6 |00:00:00.01 | 14 | 1321K| 1321K| 939K (0)|
| 7 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 8 | TABLE ACCESS FULL | T2 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
|* 9 | HASH JOIN OUTER | | 1 | 6 | 6 |00:00:00.01 | 14 | 1321K| 1321K| 939K (0)|
| 10 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 11 | TABLE ACCESS FULL | T2 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
-----------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("RN"=1)
3 - filter(ROW_NUMBER() OVER ( PARTITION BY "T"."T1_RID" ORDER BY "T"."T2_ID")<=1)
6 - access("T1"."ID"="T2"."ID")
9 - access("T1"."AGE"="T2"."ID")
当t2的id列有索引时
create index idx_id_t2 on t2(id);
SELECT *
FROM (SELECT T.*
,ROW_NUMBER() OVER(PARTITION BY T.T1_RID ORDER BY T.T2_ID) RN
FROM (SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
,T1.ROWID T1_RID
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
UNION ALL
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
,T1.ROWID T1_RID
FROM T1
LEFT JOIN T2
ON T1.AGE = T2.ID) T)
WHERE RN = 1
order by 1;
Plan hash value: 1354803237
------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 25 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 25 | 2048 | 2048 | 2048 (0)|
|* 2 | VIEW | | 1 | 12 | 6 |00:00:00.01 | 25 | | | |
|* 3 | WINDOW SORT PUSHED RANK | | 1 | 12 | 12 |00:00:00.01 | 25 | 2048 | 2048 | 2048 (0)|
| 4 | VIEW | | 1 | 12 | 12 |00:00:00.01 | 25 | | | |
| 5 | UNION-ALL | | 1 | | 12 |00:00:00.01 | 25 | | | |
| 6 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 7 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
|* 10 | HASH JOIN OUTER | | 1 | 6 | 6 |00:00:00.01 | 14 | 1321K| 1321K| 897K (0)|
| 11 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 12 | TABLE ACCESS FULL | T2 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("RN"=1)
3 - filter(ROW_NUMBER() OVER ( PARTITION BY "T"."T1_RID" ORDER BY "T"."T2_ID")<=1)
9 - access("T1"."ID"="T2"."ID")
10 - access("T1"."AGE"="T2"."ID")
上面的查询使用了索引，但是下面的查询并未用到索引，可以使用hint指定使用索引
SELECT *
FROM (SELECT T.*
,ROW_NUMBER() OVER(PARTITION BY T.T1_RID ORDER BY T.T2_ID) RN
FROM (SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
,T1.ROWID T1_RID
FROM T1
LEFT JOIN T2
ON T1.ID = T2.ID
UNION ALL
SELECT /*+ leading(t1 t2) use_nl(t2) */T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
,T1.ROWID T1_RID
FROM T1
LEFT JOIN T2
ON T1.AGE = T2.ID) T)
WHERE RN = 1
order by 1;
Plan hash value: 4092066186
------------------------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 6 |00:00:00.01 | 22 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 6 |00:00:00.01 | 22 | 2048 | 2048 | 2048 (0)|
|* 2 | VIEW | | 1 | 12 | 6 |00:00:00.01 | 22 | | | |
|* 3 | WINDOW SORT PUSHED RANK | | 1 | 12 | 12 |00:00:00.01 | 22 | 2048 | 2048 | 2048 (0)|
| 4 | VIEW | | 1 | 12 | 12 |00:00:00.01 | 22 | | | |
| 5 | UNION-ALL | | 1 | | 12 |00:00:00.01 | 22 | | | |
| 6 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 7 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 8 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 3 |00:00:00.01 | 4 | | | |
|* 9 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 3 |00:00:00.01 | 3 | | | |
| 10 | NESTED LOOPS OUTER | | 1 | 6 | 6 |00:00:00.01 | 11 | | | |
| 11 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 12 | TABLE ACCESS BY INDEX ROWID| T2 | 6 | 1 | 4 |00:00:00.01 | 4 | | | |
|* 13 | INDEX RANGE SCAN | IDX_ID_T2 | 6 | 1 | 4 |00:00:00.01 | 3 | | | |
------------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter("RN"=1)
3 - filter(ROW_NUMBER() OVER ( PARTITION BY "T"."T1_RID" ORDER BY "T"."T2_ID")<=1)
9 - access("T1"."ID"="T2"."ID")
13 - access("T1"."AGE"="T2"."ID")通过执行计划可以看到，不走nl，都走了hash，并且通过Starts列可以看到，对每个表的访问次数都是1，
达到了通过改写SQL把nl调整为hash的效果。
最终的优化效果，逻辑读由49降到了22。
当t2的id字段有重复值，进行等价改写：
SQL> select * from t1;
ID NAME AGE
---------- ---------- ----------
1 a 1
2 b 2
3 c 5
4 d 1
5 e 3
6 f 6
6 rows selected.
Elapsed: 00:00:00.01
SQL> select * from t2;
ID NAME
---------- ----------
1 a
2 b
3 c
1 y
SELECT T1.ID T1_ID
,T1.NAME T1_NAME
,T1.AGE T1_AGE
,T2.ID T2_ID
,T2.NAME T2_NAME
FROM T1
LEFT JOIN T2
ON (T1.ID = T2.ID OR T1.AGE = T2.ID)
ORDER BY 1;
T1_ID T1_NAME T1_AGE T2_ID T2_NAME
---------- ---------- ---------- ---------- ----------
1 a 1 1 a
1 a 1 1 y
2 b 2 2 b
3 c 5 3 c
4 d 1 1 a
4 d 1 1 y
5 e 3 3 c
6 f 6
8 rows selected.
Plan hash value: 3004654521
------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 8 |00:00:00.01 | 49 | | | |
| 1 | SORT ORDER BY | | 1 | 6 | 8 |00:00:00.01 | 49 | 2048 | 2048 | 2048 (0)|
| 2 | NESTED LOOPS OUTER | | 1 | 6 | 8 |00:00:00.01 | 49 | | | |
| 3 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
| 4 | VIEW | | 6 | 1 | 7 |00:00:00.01 | 42 | | | |
|* 5 | TABLE ACCESS FULL| T2 | 6 | 1 | 7 |00:00:00.01 | 42 | | | |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - filter(("T1"."ID"="T2"."ID" OR "T1"."AGE"="T2"."ID"))
等价改写(感谢刘老师指导)
WITH TMP_A AS
(SELECT ID
,NAME
,AGE
,0 AS FLAG
FROM T1
UNION ALL
SELECT AGE
,NAME
,ID
,NULL
FROM T1
WHERE LNNVL(ID = AGE)),
TMP_B AS
(SELECT A.ID
,A.NAME
,A.AGE
,A.FLAG
,B.ID AS BID
,B.NAME AS BNAME
FROM TMP_A A
LEFT JOIN T2 B
ON A.ID = B.ID),
TMP_C AS
(SELECT NVL2(FLAG, ID, AGE) AS ID
,NAME
,NVL2(FLAG, AGE, ID) AS AGE
,BID
,BNAME
,FLAG
,DENSE_RANK() OVER(PARTITION BY NVL2(FLAG, ID, AGE), NAME, NVL2(FLAG, AGE, ID) ORDER BY NVL2(BID, 1, NULL) NULLS LAST) AS DRN
FROM TMP_B)
SELECT ID
,NAME
,AGE
,BID
,BNAME --,drn,flag
FROM TMP_C
WHERE DRN = 1
AND (FLAG IS NOT NULL OR BID IS NOT NULL)
ORDER BY 1
,2
,3
,4
,5;
Plan hash value: 1011965060
-----------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
-----------------------------------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | | 8 |00:00:00.01 | 21 | | | |
| 1 | SORT ORDER BY | | 1 | 12 | 8 |00:00:00.01 | 21 | 2048 | 2048 | 2048 (0)|
|* 2 | VIEW | | 1 | 12 | 8 |00:00:00.01 | 21 | | | |
|* 3 | WINDOW SORT PUSHED RANK| | 1 | 12 | 11 |00:00:00.01 | 21 | 2048 | 2048 | 2048 (0)|
|* 4 | HASH JOIN OUTER | | 1 | 12 | 11 |00:00:00.01 | 21 | 1645K| 1645K| 908K (0)|
| 5 | VIEW | | 1 | 9 | 9 |00:00:00.01 | 14 | | | |
| 6 | UNION-ALL | | 1 | | 9 |00:00:00.01 | 14 | | | |
| 7 | TABLE ACCESS FULL | T1 | 1 | 6 | 6 |00:00:00.01 | 7 | | | |
|* 8 | TABLE ACCESS FULL | T1 | 1 | 3 | 3 |00:00:00.01 | 7 | | | |
| 9 | TABLE ACCESS FULL | T2 | 1 | 4 | 4 |00:00:00.01 | 7 | | | |
-----------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - filter(("DRN"=1 AND ("FLAG" IS NOT NULL OR "BID" IS NOT NULL)))
3 - filter(DENSE_RANK() OVER ( PARTITION BY NVL2("A"."FLAG","A"."ID","A"."AGE"),"A"."NAME",NVL2("A"."FLAG","
A"."AGE","A"."ID") ORDER BY NVL2("B"."ID",1,NULL))<=1)
4 - access("A"."ID"="B"."ID")
8 - filter(LNNVL("ID"="AGE"))通过执行计划可以看到，不走nl，都走了hash，并且通过Starts列可以看到，对每个表的访问次数都是1，
达到了通过改写SQL把nl调整为hash的效果。
最终的优化效果，逻辑读由49降到了21。
“数据库中外连接有OR关联条件只能走NL优化的方法是什么”的内容就介绍到这里了，感谢大家的阅读。如果想了解更多行业相关的知识可以关注亿速云网站，小编将为大家输出更多高质量的实用文章！
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