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主题： [转帖]oracle hints的那点事

UrkrassKEXT

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发表于：

2019-9-12 16:36:44 | [全部帖] [楼主帖]

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引言：hints翻译成中文就是提示，暗示的意思，它在数据库中作用就是更改SQL语句的执行方式，你可以使用hints强制sql按照你所设置的方式执行sql，一般用来做性能诊断和调优，不建议在开发中使用。

1.写一条SQL，使它通过全表扫描方式的效率优于索引访问，分别给出各自的执行计划。

LEO1@LEO1> create table leo1 as select * from dba_objects; 创建leo1表
Table created.
LEO1@LEO1> create index idx_leo1 on leo1(object_id); 在这个object_id列上创建索引
Index created.
LEO1@LEO1> execute dbms_stats.gather_table_stats('LEO1','LEO1',cascade=>true); 分析表和索引
PL/SQL procedure successfully completed.
LEO1@LEO1> select count(*) from leo1; 表上有71958行记录
COUNT(*)
---------------
71958
LEO1@LEO1> select /*+ full(leo1) */ * from leo1 where object_id>100;
71859 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2716644435
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      | 71862 |  6807K|   287   (1)| 00:00:04 |
|*  1 |  TABLE ACCESS FULL| LEO1  | 71862 |  6807K|   287   (1)| 00:00:04 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID">100)
Statistics
----------------------------------------------------------
1  recursive calls
0  db block gets

5762 consistent gets 5762次一致性读

0  physical reads
0  redo size
3715777  bytes sent via SQL*Net to client
53214  bytes received via SQL*Net from client
4792  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
71859  rows processed
LEO1@LEO1> select /*+ index(leo1 idx_leo1) */ * from leo1 where object_id>100;
71859 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1434365503
----------------------------------------------------------------------------------------
| Id  | Operation                   | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |          | 71862 |  6807K|  1232   (1)| 00:00:15 |
|   1 |  TABLE ACCESS BY INDEX ROWID| LEO1     | 71862 |  6807K|  1232   (1)| 00:00:15 |
|*  2 |   INDEX RANGE SCAN         | IDX_LEO1 | 71862 |       |   160   (0)| 00:00:02 |
----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID">100)
Statistics
----------------------------------------------------------
1  recursive calls
0  db block gets

10735 consistent gets 10735次一致性读

0  physical reads
0  redo size
8241805  bytes sent via SQL*Net to client
53214  bytes received via SQL*Net from client
4792  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
71859  rows processed

小结：上面的比较来看访问相同记录行，全表扫描并不总是性能最差的。为什么会这样呢，这要看提取的记录数占总记录数的比例是大还是小。一般来讲小于总体20%时走索引的效率高（并不绝对），如果你检索的记录数很大，其实不用先扫描索引块在访问数据块，直接全扫描数据块反而效率更高。因为走索引访问一个数据块需要2次IO，走全表扫描访问一个数据块需要1次IO，代价显而易见了！

2.自己构造三条关联查询的SQL，分别适用于nested loop join，hash join,merge join 关联,对于每条sql语句，分别通过hint产生其它两种关联方式的执行计划，并比较性能差异。

表关联-Nested Loop Join 嵌套循环关联

LEO1@LEO1> create table a as select * from dba_objects; a是一张大表
Table created.
LEO1@LEO1> create table b as select * from dba_objects where rownum<99; b是一张小表（外部表）
Table created.
LEO1@LEO1> create index idx_a on a(object_id); 在a上建一个索引，键值重复率较低
Index created.
LEO1@LEO1> execute dbms_stats.gather_table_stats('LEO1','a',cascade=>true); a表和索引都分析一下
PL/SQL procedure successfully completed.
LEO1@LEO1> execute dbms_stats.gather_table_stats('LEO1','b',cascade=>true); b表也分析一下
PL/SQL procedure successfully completed.
LEO1@LEO1> set autotrace trace explain;
LEO1@LEO1> select a.* from a,b where a.object_id=b.object_id;
Execution Plan
----------------------------------------------------------
Plan hash value: 3337251606
--------------------------------------------------------------------------------------
| Id  | Operation                    | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |       |    98 |  9800 |   199   (0)| 00:00:03 |
|   1 |  NESTED LOOPS               |       |       |       |            |          |
|   2 |   NESTED LOOPS              |       |    98 |  9800 |   199   (0)| 00:00:03 |
|   3 |    TABLE ACCESS FULL          | B     |    98 |   294 |     3   (0)| 00:00:01 |
|*  4 |    INDEX RANGE SCAN          | IDX_A |     1 |       |     1   (0)| 00:00:01 |
|   5 |   TABLE ACCESS BY INDEX ROWID| A     |     1 |    97 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

4 - access("A"."OBJECT_ID"="B"."OBJECT_ID") 谓词条件

数据访问：全表扫描小表b拿出一条记录，去大表a中匹配（索引扫描a表），嵌套循环遍历a，如果找到匹配记录，就去a表rowid所在的数据块上取出，最后需要的就是a表里面整个数据。

使用场景：1.外部表是一张小表例 b表

2.关联的表是一张大表，并在关联字段上创建索引，最好是主键例 a 表

3.索引键值重复率低

Hash Join
LEO1@LEO1> select /*+ use_hash(a,b) */ a.* from a,b where a.object_id=b.object_id;
98 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 4090908061
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |    98 |  9800 |   291   (1)| 00:00:04 |
|*  1 |  HASH JOIN         |      |    98 |  9800 |   291   (1)| 00:00:04 |
|   2 |   TABLE ACCESS FULL| B    |    98 |   294 |     3   (0)| 00:00:01 |
|   3 |   TABLE ACCESS FULL| A    | 71955 |  6816K|   287   (1)| 00:00:04 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."OBJECT_ID"="B"."OBJECT_ID")
Statistics
----------------------------------------------------------
176  recursive calls
0  db block gets
1060  consistent gets
2  physical reads
0  redo size
5504  bytes sent via SQL*Net to client
590  bytes received via SQL*Net from client
8  SQL*Net roundtrips to/from client
5  sorts (memory)
0  sorts (disk)
98  rows processed
MERGE Join
LEO1@LEO1> select /*+ use_merge(a,b) */ a.* from a,b where a.object_id=b.object_id;
98 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3307526271
--------------------------------------------------------------------------------------
| Id  | Operation                    | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |       |    98 |  9800 | 1238   (1)| 00:00:15 |
|   1 | MERGE JOIN                  |       |    98 |  9800 |  1238   (1) | 00:00:15 |
|   2 |   TABLE ACCESS BY INDEX ROWID| A     | 71955 |  6816K|  1234   (1) | 00:00:15 |
|   3 |    INDEX FULL SCAN           | IDX_A | 71955 |       |   160   (0) | 00:00:02 |
|*  4 |   SORT JOIN                  |       |    98 |   294 |     4  (25)| 00:00:01 |
|   5 |    TABLE ACCESS FULL         | B     |    98 |   294 |    3   (0) | 00:00:01 |
--------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("A"."OBJECT_ID"="B"."OBJECT_ID")
filter("A"."OBJECT_ID"="B"."OBJECT_ID")
Statistics
----------------------------------------------------------
1  recursive calls
0  db block gets
22  consistent gets
0  physical reads
0  redo size
5388  bytes sent via SQL*Net to client
590  bytes received via SQL*Net from client
8  SQL*Net roundtrips to/from client
1  sorts (memory)
0  sorts (disk)
98  rows processed

比较小结：大家从cost值上看到走nested loops要比后2个关联方式代价小，说明CBO优化器的选择还是正确的。

表关联-Hash Join 哈希关联

应用场景：1.一个是大表，一个是小表，两个表进行关联操作

2.当两个表没有索引时进行关联，使用hash方式匹配效率较高

3.如果两个表有索引又进行了hash关联，那么哈希完后，结果只受哈希列表影响，不受索引影响了

LEO1@LEO1> drop table a purge; 删除a表重新建
Table dropped.
LEO1@LEO1> drop table b purge; 删除b表重新建
Table dropped.
LEO1@LEO1> create table a as select * from dba_objects; a是一张大表，无索引
Table created.
LEO1@LEO1> create table b as select * from dba_objects where rownum<1000; b是一张小表，无索引
Table created.
LEO1@LEO1> select a.* from a,b where a.object_id=b.object_id; a表b表进行关联操作

999 rows selected. 返回999行

Execution Plan
----------------------------------------------------------
Plan hash value: 4090908061
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |   999 |   214K|   294   (1)| 00:00:04 |
|*  1 | HASH JOIN        |      |   999 |   214K|   294   (1)| 00:00:04 |

| 2 | TABLE ACCESS FULL| B | 999 | 12987 | 6 (0)| 00:00:01 | b表小代价也小

| 3 | TABLE ACCESS FULL| A | 83813 | 16M| 287 (1)| 00:00:04 | a表大代价相对也大

---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."OBJECT_ID"="B"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
532  recursive calls
0  db block gets
1261  consistent gets
1038  physical reads
0  redo size
51276  bytes sent via SQL*Net to client
1250  bytes received via SQL*Net from client
68  SQL*Net roundtrips to/from client
5  sorts (memory)
0  sorts (disk)
999  rows processed

数据访问：全表扫描a、b表，先把小表b做哈希后build到内存中，在对大表a做哈希，然后从大表a中取数据到小表b中比较，最后把匹配的数据返回给用户，这种哈希匹配效率高。（我们也可以叫做2个数据集的比较，哈希完后oracle会把数据分布到一个个哈希区，然后是大数据集哈希区与小数据集哈希区比较，也就是n对n比较，不像nested loops 是1：n比较，因此性能好）

Nested loops
LEO1@LEO1> select /*+ use_nl(a,b) */ a.* from a,b where a.object_id=b.object_id;
999 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 4193326952
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |   999 |   214K|   285K  (1) |00:57:09 |
|   1 |  NESTED LOOPS      |      |   999 |   214K|  285K  (1) | 00:57:09 |
|   2 |   TABLE ACCESS FULL| B    |   999 | 12987 |     6   (0) | 00:00:01 |
|*  3 |   TABLE ACCESS FULL| A    |     1 |   207 |   286   (1) | 00:00:04 |
---------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

3 - filter("A"."OBJECT_ID"="B"."OBJECT_ID") 谓词条件filter（过滤）就代表是全表扫描

Note
-----

- dynamic sampling used for this statement (level=2) 动态采样，级别越高，采集数据越多，结果越精确，但消耗资源也越多

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets

1029120 consistent gets 嵌套循环比较，比hash多出了816倍一致性读

0  physical reads
0  redo size
51276  bytes sent via SQL*Net to client
1250  bytes received via SQL*Net from client
68  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
999  rows processed
Merge Join
LEO1@LEO1> select /*+ use_merge(a,b) */ a.* from a,b where a.object_id=b.object_id;
999 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 3028542103
------------------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |   999 |   214K|       |  4066   (1)| 00:00:49 |
|   1 |  MERGE JOIN         |     |   999 |   214K|       |  4066   (1)| 00:00:49 |
|   2 |   SORT JOIN         |     |   999 | 12987 |         |  7  (15)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| B    |   999 | 12987 |         |  6   (0)| 00:00:01 |
|*  4 |   SORT JOIN         |     | 83813 |    16M|    39M|  4059   (1)| 00:00:49 |
|   5 |    TABLE ACCESS FULL| A    | 83813 |    16M|        |  287   (1)| 00:00:04 |
------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("A"."OBJECT_ID"="B"."OBJECT_ID")
filter("A"."OBJECT_ID"="B"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
7  recursive calls
0  db block gets
1141  consistent gets
0  physical reads
0  redo size
51142  bytes sent via SQL*Net to client
1250  bytes received via SQL*Net from client
68  SQL*Net roundtrips to/from client
2  sorts (memory)
0  sorts (disk)
999  rows processed

数据访问：先对a、b表进行整体排序，在逐条进行比较，cost值比hash join大了13倍，Rows列比hash join多返回了两行，这些都证明了merge Join 没有hash join性能好。

表关联-Merge Join 合并关联

Merge join场合：如果2个表都是经过整体排序后的，那么它们在关联的时候就会走Merge join。

我们还用如上的a、b表做测试比较

LEO1@LEO1> select * from (select * from a order by object_id) a,(select * from b order by object_id) b where a.object_id=b.object_id;
999 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2924767385
-------------------------------------------------------------------------------------
| Id  | Operation            | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |      | 83813 |    33M|       |  4066   (1)| 00:00:49 |
|   1 | MERGE JOIN          |      | 83813 |    33M|       |  4066   (1)| 00:00:49 |
|   2 |   VIEW               |      | 83813 |    16M|       |  4059   (1)| 00:00:49 |
|   3 |    SORT ORDER BY     |      | 83813 |    16M|    19M|  4059   (1)| 00:00:49 |
|   4 |     TABLE ACCESS FULL| A    | 83813 |    16M|       |   287   (1)| 00:00:04 |
|*  5 |   SORT JOIN          |      |   999 |   201K|       |     7  (15)| 00:00:01 |
|   6 |    TABLE ACCESS FULL | B    |   999 |   201K|       |     6   (0)| 00:00:01 |
-------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
5 - access("A"."OBJECT_ID"="B"."OBJECT_ID")
filter("A"."OBJECT_ID"="B"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)
Statistics
----------------------------------------------------------
291  recursive calls
0  db block gets
1169  consistent gets
0  physical reads
0  redo size
85714  bytes sent via SQL*Net to client
1250  bytes received via SQL*Net from client
68  SQL*Net roundtrips to/from client
2  sorts (memory)
0  sorts (disk)
999  rows processed

小结：结合上面的结果由于需要先排序，则返回的行数又多了，从而增加了等待时间和代价，通常merge join的效果并不是很好因为代价太大了。

表关联-leading( ) 指定表访问的顺序

LEO1@LEO1> create table c as select * from dba_objects where rownum<100; 创建c表
Table created.
LEO1@LEO1> select /*+ leading(c b a) */ * from a,b,c where a.object_id=b.object_id and b.object_id=c.object_id; 利用上面a、b表做三表关联
99 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 455705007
----------------------------------------------------------------------------
| Id  | Operation           | Name | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |    99 | 61479 |   298   (2)| 00:00:04 |
|*  1 |  HASH JOIN          |      |    99 | 61479 |   298   (2)| 00:00:04 |
|*  2 |   HASH JOIN         |      |    99 | 40986 |    10  (10)| 00:00:01 |
|   3 |    TABLE ACCESS FULL| C    |    99 | 20493 |     3   (0)| 00:00:01 |
|   4 |    TABLE ACCESS FULL| B    |   999 |   201K|     6   (0)| 00:00:01 |
|   5 |   TABLE ACCESS FULL | A    | 83813 |    16M|   287   (1)| 00:00:04 |
----------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - access("A"."OBJECT_ID"="B"."OBJECT_ID")
2 - access("B"."OBJECT_ID"="C"."OBJECT_ID")
Note
-----
- dynamic sampling used for this statement (level=2)

小结：我们看到执行计划中访问表的顺序（C->B->A）就是我们指定好的顺序leading(c b a)，说明hints生效。

3.通过append hint来插入数据，演示它和普通插入数据的性能比较。

LEO1@LEO1> set timing on 显示执行时间
LEO1@LEO1> insert into leo1 select * from leo1; 普通加载数据，会扫描空闲空间加以利用
71958 rows created.

Elapsed: 00:00:00.89 执行了00.89秒

Execution Plan
----------------------------------------------------------
Plan hash value: 2716644435
---------------------------------------------------------------------------------
| Id  | Operation                | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------
|   0 | INSERT STATEMENT         |      | 71958 |  6816K|   287   (1)| 00:00:04 |
|   1 |  LOAD TABLE CONVENTIONAL | LEO1 |       |       |            |          |
|   2 |   TABLE ACCESS FULL      | LEO1 | 71958 |  6816K|   287   (1)| 00:00:04 |
---------------------------------------------------------------------------------
Statistics
----------------------------------------------------------
518  recursive calls
15560  db block gets

3693 consistent gets 产生了3693次一致性读

4 physical reads

13892928 redo size 产生了13892928大小redo日志

843  bytes sent via SQL*Net to client
792  bytes received via SQL*Net from client
3  SQL*Net roundtrips to/from client
2  sorts (memory)
0  sorts (disk)
71958  rows processed
LEO1@LEO1> rollback; 回滚
Rollback complete.
Elapsed: 00:00:00.10
LEO1@LEO1> insert /*+ append */ into leo1 select * from leo1; 直接加载数据，不扫描空闲空间，直接定位HWM加载数据，效率高
71958 rows created.

Elapsed: 00:00:00.36 执行了00.36秒

Execution Plan
----------------------------------------------------------
ERROR:
ORA-12838: cannot read/modify an object after modifying it in parallel（无法在并行模式下修改之后读写对象）

SP2-0612: Error generating AUTOTRACE EXPLAIN report 生成执行计划报告时出错

Statistics
----------------------------------------------------------
340 recursive calls
2441 db block gets

2253 consistent gets 产生了2253次一致性读

0 physical reads

2268672 redo size 产生了2268672大小redo日志

829  bytes sent via SQL*Net to client
806  bytes received via SQL*Net from client
3  SQL*Net roundtrips to/from client
2  sorts (memory)
0  sorts (disk)
71958  rows processed
LEO1@LEO1> rollback; 回滚
Rollback complete.
Elapsed: 00:00:00.09

小结：从比较结果一眼看出，直接加载的效率要比普通加载高很多，时间上差不多快了一倍。原因有以下2点：

第一点：普通加载会扫描空闲空间，利用这些空闲空间插入数据，直接加载不扫描空闲空间直接定位到HWM直接加载数据，从而效率较高

第二点：可以看出普通加载的一致性读和redo量都要大于直接加载，产生这些数据量也是要消耗资源的，所以普通加载没有直接加载性能好。

4.用cardinality hint来模拟表中的数据，写一条SQL语句并给出它的执行计划。

名词解释：cardinality这个关键字在10g执行计划里被rows代替，实际上两个词指的是一个东西。

Cardinality（基数）在执行计划中表示每一步操作返回的记录数，这个数是oracle估算出来的并不是真实返回的记录数，CBO根据这个值计算权重，来选择使用哪种方式来访问数据。

作用：1.我们一般使用“cardinality”hints来比较不同数量返回值在执行计划中效率。

2.当有特殊场景不容易模拟出来的时候，我们可以使用“cardinality”hints方式来轻松解决

LEO1@LEO1> select count(*) from leo1; leo1表有71958条记录
COUNT(*)
----------------
71958
LEO1@LEO1> create table leo2 as select * from dba_objects; 创建leo2表
Table created.
LEO1@LEO1> insert into leo2 select * from leo2; 在插入一次，为了比leo1表记录数多一倍，好做比较
71960 rows created.
LEO1@LEO1> create index idx_leo2 on leo2(object_id); 在object_id字段上创建索引
Index created.
LEO1@LEO1> select count(*) from leo2; 现在有143920条记录
COUNT(*)
----------------
143920
LEO1@LEO1> execute dbms_stats.gather_table_stats('LEO1','LEO2',cascade=>true); leo2和索引都做分析
PL/SQL procedure successfully completed.
LEO1@LEO1> set autotrace traceonly;
LEO1@LEO1> select * from leo1,leo2 where leo1.object_id=leo2.object_id;

143916 rows selected. 返回143916行

Execution Plan
----------------------------------------------------------
Plan hash value: 2436308224
-----------------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |   141K|    26M|       |  2291   (1) | 00:00:28 |
|*  1 |  HASH JOIN        |      |   141K|    26M|  7664K|  2291   (1) | 00:00:28 |
|   2 |   TABLE ACCESS FULL| LEO1  | 71958 |  6816K|       |   587   (1) | 00:00:08 |
|   3 |   TABLE ACCESS FULL| LEO2  |   143K|    13M|       |   588   (1) | 00:00:08 |
-----------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------

1 - access("LEO1"."OBJECT_ID"="LEO2"."OBJECT_ID") 谓词条件2个索引字段相等

Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets

13672 consistent gets 全表扫描产生了13672个一致性读

2134  physical reads
0  redo size
12630296  bytes sent via SQL*Net to client
106058  bytes received via SQL*Net from client
9596  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
143916  rows processed

我们强制指定leo1表返回100行，来看执行计划如何选择访问数据的方式

LEO1@LEO1> select /*+ cardinality(leo1 100) */ * from leo1,leo2 where leo1.object_id=leo2.object_id;

143916 rows selected. 也返回143916行，返回值没有按执行计划走

Execution Plan
----------------------------------------------------------
Plan hash value: 2751515442
-----------------------------------------------------------------------------------------
| Id  | Operation                    | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT             |          |   197 | 38218 |   887   (1)| 00:00:11 |
|   1 |  NESTED LOOPS              |          |       |       |            |          |
|   2 |   NESTED LOOPS             |          |   197 | 38218 |   887   (1)| 00:00:11 |
|   3 |    TABLE ACCESS FULL         | LEO1     |   100 |  9700 |   587   (1)| 00:00:08 |
|*  4 |    INDEX RANGE SCAN         | IDX_LEO2 |     2 |       |    1   (0)| 00:00:01 |
|   5 |   TABLE ACCESS BY INDEX ROWID| LEO2     |     2 |   194 |     3   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
4 - access("LEO1"."OBJECT_ID"="LEO2"."OBJECT_ID")
Statistics
----------------------------------------------------------
1  recursive calls
0  db block gets

177213 consistent gets 因为有索引扫描所以有177213个一致性读

2134 physical reads 物理读都是一样的，说明只有内存IO增加了

0  redo size
7727088  bytes sent via SQL*Net to client
106058  bytes received via SQL*Net from client
9596  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
143916  rows processed

数据访问：全表扫描小表leo1（因为强制指定返回100行就认为是小表）拿出一条记录，去大表leo2中匹配（索引扫描leo2表，因为当检索范围较大时扫描索引的速度较快），嵌套循环遍历leo2，如果找到匹配记录，就去leo2表rowid所在的数据块上取出，最后需要的就是leo2表里面整个数据。

使用场景：1.外部表是一张小表例 leo1表因为记录少会执行全表扫描

2.内部表是一张大表，并在关联字段上创建索引，当检索范围较大时扫描索引的速度较快

3. 当有特殊场景不容易模拟出来的时候，我们可以使用“cardinality”hints方式来轻松解决.

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发表于：

2019-10-9 0:30:00 | [全部帖] [楼主帖]

2 楼

为了方便大家阅读，我对文章中错误号来解释一下吧！

Error Id: ORA-12838

Title: cannot read/modify an object after modifying it in parallel

Description:

cannot read/modify an object after modifying it in parallel

Action:

Rewrite the transaction, or break it up into two transactions: one containing the initial modification and the second containing the parallel modification operation.

Cause:

Within the same transaction, an attempt was made to add read or modification statements on a table after it had been modified in parallel or with direct load. This is not permitted.

也许你已明白，但对一个人有用也是我存在的理由！^_^ By:持之以恒的大白

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