oracle hint

Hint概述
基于代价的优化器是很聪明的,在绝大多数情况下它会选择正确的优化器,减轻了DBA的负担。但有时它也聪明反被聪明误,选择了很差的执行计划,使某个语句的执行变得奇慢无比。

此时就需要DBA进行人为的干预,告诉优化器使用我们指定的存取路径或连接类型生成执行计划,从 而使语句高效的运行。例如,如果我们认为对于一个特定的语句,执行全表扫描要比执行索引扫描更有效,则我们就可以指示优化器使用全表扫描。在Oracle中,是通过为语句添加 Hints(提示)来实现干预优化器优化的目的。
不建议在代码中使用hint,在代码使用hint使得CBO无法根据实际的数据状态选择正确的执行计划。毕竟数据是不断变化的,10g以后的CBO也越来越完善,大多数情况下我们该让Oracle自行决定采用什么执行计划。

Oracle Hints是一种机制,用来告诉优化器按照我们的告诉它的方式生成执行计划。我们可以用Oracle Hints来实现:
1) 使用的优化器的类型
2) 基于代价的优化器的优化目标,是all_rows还是first_rows。
3) 表的访问路径,是全表扫描,还是索引扫描,还是直接利用rowid。
4) 表之间的连接类型
5) 表之间的连接顺序
6) 语句的并行程度

除了”RULE”提示外,一旦使用的别的提示,语句就会自动的改为使用CBO优化器,此时如果你的数据字典中没有统计数据,就会使用缺省的统计数据。所以建议大家如果使用CBO或Hints提示,则最好对表和索引进行定期的分析。

如何使用Hints:

Hints只应用在它们所在sql语句块(statement  block,由select、update、delete关键字标识)上,对其它SQL语句或语句的其它部分没有影响。如:对于使用union操作的2个sql语句,如果只在一个sql语句上有Hints,则该Hints不会影响另一个sql语句。

我们可以使用注释(comment)来为一个语句添加Hints,一个语句块只能有一个注释,而且注释只能放在SELECT, UPDATE, or DELETE关键字的后面

使用Oracle Hints的语法:

{DELETE|INSERT|SELECT|UPDATE} /*+ hint [text] [hint[text]]... */

or

{DELETE|INSERT|SELECT|UPDATE} --+ hint [text] [hint[text]]...

注解:
1) DELETE、INSERT、SELECT和UPDATE是标识一个语句块开始的关键字,包含提示的注释只能出现在这些关键字的后面,否则提示无效。
2) “+”号表示该注释是一个Hints,该加号必须立即跟在”/*”的后面,中间不能有空格。
3) hint是下面介绍的具体提示之一,如果包含多个提示,则每个提示之间需要用一个或多个空格隔开。
4) text 是其它说明hint的注释性文本

5)使用表别名。如果在查询中指定了表别名,那么提示必须也使用表别名。例如:select /*+ index(e,dept_idx) */ * from emp e;
6)不要在提示中使用模式名称:如果在提示中指定了模式的所有者,那么提示将被忽略。例如:
select /*+ index(scott.emp,dept_idx) */ * from emp

注意:如果你没有正确的指定Hints,Oracle将忽略该Hints,并且不会给出任何错误。
hint被忽略

如果CBO认为使用hint会导致错误的结果时,hint将被忽略,详见下例
SQL> select /*+ index(t t_ind) */ count(*) from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 2966233522
-------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Cost (%CPU)| Time     |
-------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |    57   (2)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |            |          |
|   2 |   TABLE ACCESS FULL| T    | 50366 |    57   (2)| 00:00:01 |
-------------------------------------------------------------------

因为我们是对记录求总数,且我们并没有在建立索引时指定不能为空,索引如果CBO选择在索引上进行count时,但索引字段上的值为空时,结果将不准确,故CBO没有选择索引。
SQL>  select /*+ index(t t_ind) */ count(id) from t;
Execution Plan
----------------------------------------------------------
Plan hash value: 646498162
--------------------------------------------------------------------------
| Id  | Operation        | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT |       |     1 |     5 |   285   (1)| 00:00:04 |
|   1 |  SORT AGGREGATE  |       |     1 |     5 |            |          |
|   2 |   INDEX FULL SCAN| T_IND | 50366 |   245K|   285   (1)| 00:00:04 |
--------------------------------------------------------------------------

因为我们只对id进行count,这个动作相当于count索引上的所有id值,这个操作和对表上的id字段进行count是一样的(组函数会忽略null值)

Hint的具体用法

和优化器相关的hint

1. /*+ALL_ROWS*/
表明对语句块选择基于开销的优化方法,并获得最佳吞吐量,使资源消耗最小化.
例如:
SELECT /*+ALL+_ROWS*/ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO=’SCOTT’;
2. /*+FIRST_ROWS*/
表明对语句块选择基于开销的优化方法,并获得最佳响应时间,使资源消耗最小化.
例如:
SELECT /*+FIRST_ROWS*/ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO=’SCOTT’;


3. /*+CHOOSE*/
表明如果数据字典中有访问表的统计信息,将基于开销的优化方法,并获得最佳的吞吐量;
表明如果数据字典中没有访问表的统计信息,将基于规则开销的优化方法;
例如:
SELECT /*+CHOOSE*/ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO=’SCOTT’;

4. /*+RULE*/
表明对语句块选择基于规则的优化方法.
例如:
SELECT /*+ RULE */ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO=’SCOTT’;

5. /*+FULL(TABLE)*/
表明对表选择全局扫描的方法.
例如:
SELECT /*+FULL(A)*/ EMP_NO,EMP_NAM FROM BSEMPMS A WHERE EMP_NO=’SCOTT’;

6. /*+ROWID(TABLE)*/
提示明确表明对指定表根据ROWID进行访问.
例如:
SELECT /*+ROWID(BSEMPMS)*/ * FROM BSEMPMS WHERE ROWID>=’AAAAAAAAAAAAAA’
AND EMP_NO=’SCOTT’;

7. /*+CLUSTER(TABLE)*/
提示明确表明对指定表选择簇扫描的访问方法,它只对簇对象有效.
例如:
SELECT /*+CLUSTER */ BSEMPMS.EMP_NO,DPT_NO FROM BSEMPMS,BSDPTMS
WHERE DPT_NO=’TEC304′ AND BSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

8. /*+INDEX(TABLE INDEX_NAME)*/
表明对表选择索引的扫描方法.
例如:
SELECT /*+INDEX(BSEMPMS SEX_INDEX) USE SEX_INDEX BECAUSE THERE ARE FEWMALE BSEMPMS */ FROM BSEMPMS WHERE SEX=’M';

9. /*+INDEX_ASC(TABLE INDEX_NAME)*/
表明对表选择索引升序的扫描方法.
例如:
SELECT /*+INDEX_ASC(BSEMPMS PK_BSEMPMS) */ FROM BSEMPMS WHERE DPT_NO=’SCOTT’;

10. /*+INDEX_COMBINE*/
为指定表选择位图访问路经,如果INDEX_COMBINE中没有提供作为参数的索引,将选择出位图索引的布尔组合方式.
例如:
SELECT /*+INDEX_COMBINE(BSEMPMS SAL_BMI HIREDATE_BMI)*/ * FROM BSEMPMS
WHERE SAL<5000000 AND HIREDATE

11. /*+INDEX_JOIN(TABLE INDEX_NAME)*/
提示明确命令优化器使用索引作为访问路径.
例如:
SELECT /*+INDEX_JOIN(BSEMPMS SAL_HMI HIREDATE_BMI)*/ SAL,HIREDATE
FROM BSEMPMS WHERE SAL<60000;

12. /*+INDEX_DESC(TABLE INDEX_NAME)*/
表明对表选择索引降序的扫描方法.
例如:
SELECT /*+INDEX_DESC(BSEMPMS PK_BSEMPMS) */ FROM BSEMPMS WHERE DPT_NO='SCOTT';

13. /*+INDEX_FFS(TABLE INDEX_NAME)*/
对指定的表执行快速全索引扫描,而不是全表扫描的办法.
例如:
SELECT /*+INDEX_FFS(BSEMPMS IN_EMPNAM)*/ * FROM BSEMPMS WHERE DPT_NO='TEC305';

14. /*+ADD_EQUAL TABLE INDEX_NAM1,INDEX_NAM2,...*/
提示明确进行执行规划的选择,将几个单列索引的扫描合起来.
例如:
SELECT /*+INDEX_FFS(BSEMPMS IN_DPTNO,IN_EMPNO,IN_SEX)*/ * FROM BSEMPMS WHERE EMP_NO='SCOTT' AND DPT_NO='TDC306';

15. /*+USE_CONCAT*/
对查询中的WHERE后面的OR条件进行转换为UNION ALL的组合查询.
例如:
SELECT /*+USE_CONCAT*/ * FROM BSEMPMS WHERE DPT_NO='TDC506' AND SEX='M';

16. /*+NO_EXPAND*/
对于WHERE后面的OR 或者IN-LIST的查询语句,NO_EXPAND将阻止其基于优化器对其进行扩展.
例如:
SELECT /*+NO_EXPAND*/ * FROM BSEMPMS WHERE DPT_NO='TDC506' AND SEX='M';

17. /*+NOWRITE*/
禁止对查询块的查询重写操作.

18. /*+REWRITE*/
可以将视图作为参数.

19. /*+MERGE(TABLE)*/
能够对视图的各个查询进行相应的合并.
例如:
SELECT /*+MERGE(V) */ A.EMP_NO,A.EMP_NAM,B.DPT_NO FROM BSEMPMS A (SELET DPT_NO
,AVG(SAL) AS AVG_SAL FROM BSEMPMS B GROUP BY DPT_NO) V WHERE A.DPT_NO=V.DPT_NO
AND A.SAL>V.AVG_SAL;

20. /*+NO_MERGE(TABLE)*/
对于有可合并的视图不再合并.
例如:
SELECT /*+NO_MERGE(V) */ A.EMP_NO,A.EMP_NAM,B.DPT_NO FROM BSEMPMS A (SELECT DPT_NO,AVG(SAL) AS AVG_SAL FROM BSEMPMS B GROUP BY DPT_NO) V WHERE A.DPT_NO=V.DPT_NO AND A.SAL>V.AVG_SAL;

21. /*+ORDERED*/
根据表出现在FROM中的顺序,ORDERED使ORACLE依此顺序对其连接.
例如:
SELECT /*+ORDERED*/ A.COL1,B.COL2,C.COL3 FROM TABLE1 A,TABLE2 B,TABLE3 C WHERE A.COL1=B.COL1 AND B.COL1=C.COL1;

22. /*+USE_NL(TABLE)*/
将指定表与嵌套的连接的行源进行连接,并把指定表作为内部表.
例如:
SELECT /*+ORDERED USE_NL(BSEMPMS)*/ BSDPTMS.DPT_NO,BSEMPMS.EMP_NO,BSEMPMS.EMP_NAM FROM BSEMPMS,BSDPTMS WHERE BSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

23. /*+USE_MERGE(TABLE)*/
将指定的表与其他行源通过合并排序连接方式连接起来.
例如:
SELECT /*+USE_MERGE(BSEMPMS,BSDPTMS)*/ * FROM BSEMPMS,BSDPTMS WHERE BSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

24. /*+USE_HASH(TABLE)*/
将指定的表与其他行源通过哈希连接方式连接起来.
例如:
SELECT /*+USE_HASH(BSEMPMS,BSDPTMS)*/ * FROM BSEMPMS,BSDPTMS WHERE BSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

25. /*+DRIVING_SITE(TABLE)*/
强制与ORACLE所选择的位置不同的表进行查询执行.
例如:
SELECT /*+DRIVING_SITE(DEPT)*/ * FROM BSEMPMS,DEPT@BSDPTMS WHERE BSEMPMS.DPT_NO=DEPT.DPT_NO;

26. /*+LEADING(TABLE)*/
将指定的表作为连接次序中的首表.

27. /*+CACHE(TABLE)*/
当进行全表扫描时,CACHE提示能够将表的检索块放置在缓冲区缓存中最近最少列表LRU的最近使用端
例如:
SELECT /*+FULL(BSEMPMS) CAHE(BSEMPMS) */ EMP_NAM FROM BSEMPMS;

28. /*+NOCACHE(TABLE)*/
当进行全表扫描时,CACHE提示能够将表的检索块放置在缓冲区缓存中最近最少列表LRU的最近使用端
例如:
SELECT /*+FULL(BSEMPMS) NOCAHE(BSEMPMS) */ EMP_NAM FROM BSEMPMS;

29. /*+APPEND*/
直接插入到表的最后,可以提高速度.
insert /*+append*/ into test1 select * from test4 ;

30. /*+NOAPPEND*/
通过在插入语句生存期内停止并行模式来启动常规插入.
insert /*+noappend*/ into test1 select * from test4 ;

 

10、/*+ INDEX_SS(T T_IND) */
从9i开始,oracle引入了这种索引访问方式。当在一个联合索引中,某些谓词条件并不在联合索引的第一列时,可以通过Index Skip Scan来访问索引获得数据。当联合索引第一列的唯一值个数很少时,使用这种方式比全表扫描效率高。

SQL> create table t as select 1 id,object_name from dba_objects;
Table created.
SQL> insert into t select 2,object_name from dba_objects;      
50366 rows created.
SQL> insert into t select 3,object_name from dba_objects;      
50366 rows created.
SQL> insert into t select 4,object_name from dba_objects;      
50366 rows created.
SQL> commit;
Commit complete.
SQL> create index t_ind on t(id,object_name);
Index created.
SQL> exec dbms_stats.gather_table_stats('HR','T',cascade=>true);
PL/SQL procedure successfully completed.
执行全表扫描
SQL> select /*+ full(t) */ * from t where object_name='EMPLOYEES';
6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     5 |   135 |   215   (3)| 00:00:03 |
|*  1 |  TABLE ACCESS FULL| T    |     5 |   135 |   215   (3)| 00:00:03 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - filter("OBJECT_NAME"='EMPLOYEES')
Statistics
----------------------------------------------------------
          0  recursive calls
          0  db block gets
        942  consistent gets
          0  physical reads
          0  redo size
        538  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          6  rows processed
不采用hint
SQL>  select * from t where object_name='EMPLOYEES';
6 rows selected.
Execution Plan
----------------------------------------------------------
Plan hash value: 2869677071
--------------------------------------------------------------------------
| Id  | Operation        | Name  | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT |       |     5 |   135 |     5   (0)| 00:00:01 |
|*  1 |  INDEX SKIP SCAN | T_IND |     5 |   135 |     5   (0)| 00:00:01 |
--------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   1 - access("OBJECT_NAME"='EMPLOYEES')
       filter("OBJECT_NAME"='EMPLOYEES')
Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
         17  consistent gets
          1  physical reads
          0  redo size
        538  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          6  rows processed
当全表扫描扫描了942个块,联合索引只扫描了17个数据块。可以看到联合索引的第一个字段的值重复率很高时,即使谓词中没有联合索引的第一个字段,依然会使用index_ss方式,效率远远高于全表扫描效率。但当第一个字段的值重复率很低时,使用index_ss的效率要低于全表扫描,读者可以自行实验
和表的关联相关的hint

/*+ leading(table_1,table_2) */

在多表关联查询中,指定哪个表作为驱动表,即告诉优化器首先要访问哪个表上的数据。
select /*+ leading(t,t1) */ t.* from t,t1 where t.id=t1.id;
/*+ order */

让Oracle根据from后面表的顺序来选择驱动表,oracle建议使用leading,他更为灵活
select /*+ order */ t.* from t,t1 where t.id=t1.id;
/*+ use_nl(table_1,table_2) */
在多表关联查询中,指定使用nest loops方式进行多表关联。

select /*+ use_nl(t,t1) */ t.* from t,t1 where t.id=t1.id;

/*+ use_hash(table_1,table_2) */
在多表关联查询中,指定使用hash join方式进行多表关联。

select /*+ use_hash(t,t1) */ t.* from t,t1 where t.id=t1.id;

/*+ use_merge(table_1,table_2) */
在多表关联查询中,指定使用merge join方式进行多表关联。

select /*+ use_merge(t,t1) */ t.* from t,t1 where t.id=t1.id;

/*+ no_use_nl(table_1,table_2) */
在多表关联查询中,指定不使用nest loops方式进行多表关联。

select /*+ no_use_nl(t,t1) */ t.* from t,t1 where t.id=t1.id;

/*+ no_use_hash(table_1,table_2) */
在多表关联查询中,指定不使用hash join方式进行多表关联。

select /*+ no_use_hash(t,t1) */ t.* from t,t1 where t.id=t1.id;

/*+ no_use_merge(table_1,table_2) */
在多表关联查询中,指定不使用merge join方式进行多表关联。

select /*+ no_use_merge(t,t1) */ t.* from t,t1 where t.id=t1.id;

其他常用的hint

/*+ parallel(table_name n) */

在sql中指定执行的并行度,这个值将会覆盖自身的并行度

select /*+ parallel(t 4) */ count(*)  from t;

/*+ no_parallel(table_name) */

在sql中指定执行的不使用并行

select /*+ no_parallel(t) */ count(*)  from t;

/*+ append */以直接加载的方式将数据加载入库

insert into t /*+ append */ select * from t;

/*+ dynamic_sampling(table_name n) */

设置sql执行时动态采用的级别,这个级别为0~10
select /*+ dynamic_sampling(t 4) */ * from t where id > 1234

/*+ cache(table_name) */
进行全表扫描时将table置于LRU列表的最活跃端,类似于table的cache属性

select /*+ full(employees) cache(employees) */ last_name from employees
附录hint表格

 

Hints for Optimization Approaches and Goals

ALL_ROWS The ALL_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal of best throughput (that is, minimum total resource consumption).
FIRST_ROWS The FIRST_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal  of best response time (minimum resource usage to return first row). In newer Oracle version you should give a parameter with this hint: FIRST_ROWS(n) means that the optimizer will determine an executionplan to give a fast response for returning the first n rows.
CHOOSE The CHOOSE hint causes the optimizer to choose between the rule-based  approach and the cost-based approach for a SQL statement based on the  presence of statistics for the tables accessed by the statement
RULE The RULE hint explicitly chooses rule-based optimization for a statement block. This hint also causes the optimizer to ignore any other hints  specified for the statement block. The RULE hint does not work any more in Oracle 10g.

Hints for Access Paths

FULL The FULL hint explicitly chooses a full table scan for the specified table. The syntax of the FULL hint is FULL(table) where table specifies the alias of the table (or table name if alias does not exist) on which the full table  scan is to be performed.
ROWID The ROWID hint explicitly chooses a table scan by ROWID for the specified table. The syntax of the ROWID hint is ROWID(table) where table specifies the name or alias of the table on which the table access by ROWID is to be performed.  (This hint depricated in Oracle 10g)
CLUSTER The CLUSTER hint explicitly chooses a cluster scan to access the specified table. The syntax of the CLUSTER hint is CLUSTER(table) where table specifies the name or alias of the table to be accessed by a cluster scan.
HASH The HASH hint explicitly chooses a hash scan to access the specified table. The syntax of the HASH hint is HASH(table) where table specifies the name or alias of the table to be accessed by a hash scan.
HASH_AJ The HASH_AJ hint transforms a NOT IN subquery into a hash anti-join to  access the specified table. The syntax of the HASH_AJ hint is HASH_AJ(table) where table specifies the name or alias of the table to be  accessed.(depricated in Oracle 10g)
INDEX The INDEX hint explicitly chooses an index scan for the specified table. The syntax of the INDEX hint is INDEX(table index) where:table specifies the name or alias of the table associated with the index to be scanned and index specifies an index on which an index scan is to be performed. This hint may optionally specify one or more indexes:
NO_INDEX The NO_INDEX hint explicitly disallows a set of indexes for the specified table. The syntax of the NO_INDEX hint is NO_INDEX(table index)
INDEX_ASC The INDEX_ASC hint explicitly chooses an index scan for the specified table. If the statement uses an index range scan, Oracle scans the index entries in ascending order of their indexed values.
INDEX_COMBINE If no indexes are given as arguments for the INDEX_COMBINE hint, the optimizer will use on the table whatever boolean combination of bitmap indexes has the best cost estimate. If certain indexes are given as arguments, the optimizer will try to use some boolean combination of those particular bitmap indexes. The syntax of INDEX_COMBINE is INDEX_COMBINE(table index).
INDEX_JOIN Explicitly instructs the optimizer to use an index join as an access  path. For the hint to have a positive effect, a sufficiently small  number of indexes must exist that contain all the columns required to  resolve the query. 
INDEX_DESC The INDEX_DESC hint explicitly chooses an index scan for the specified table. If the statement uses an index range scan, Oracle scans the index entries in descending order of their indexed values.
INDEX_FFS This hint causes a fast full index scan to be performed rather than a full table.
NO_INDEX_FFS Do not use fast full index scan (from Oracle 10g)
INDEX_SS Exclude range scan from query plan (from Oracle 10g)
INDEX_SS_ASC Exclude range scan from query plan (from Oracle 10g)
INDEX_SS_DESC Exclude range scan from query plan (from Oracle 10g)
NO_INDEX_SS The NO_INDEX_SS hint causes the optimizer to exclude a skip scan of the specified indexes on the  specified table. (from Oracle 10g)

Hints for Query Transformations

NO_QUERY_TRANSFORMATION Prevents the optimizer performing query transformations. (from Oracle 10g)
USE_CONCAT The USE_CONCAT hint forces combined OR conditions in the WHERE clause of a query to be transformed into a compound query using the UNION ALL set operator. Normally, this transformation occurs only if the cost of the query using the concatenations is cheaper than the cost without them.
NO_EXPAND The NO_EXPAND hint prevents the optimizer from considering OR-expansion  for queries having OR conditions or IN-lists in the WHERE clause.  Usually, the optimizer considers using OR expansion and uses this method if it decides that the cost is lower than not using it.
REWRITE The REWRITE hint forces the optimizer to rewrite a query in terms of  materialized views, when possible, without cost consideration. Use the  REWRITE hint with or without a view list. If you use REWRITE with a view list and the list contains an eligible materialized view, then Oracle  uses that view regardless of its cost.
NOREWRITE / NO_REWRITE In Oracle 10g renamed to NO_REWRITE. The NOREWRITE/NO_REWRITE hint disables query rewrite for the query block, overriding the setting of  the parameter QUERY_REWRITE_ENABLED.
MERGE The MERGE hint lets you merge views in a query.
NO_MERGE The NO_MERGE hint causes Oracle not to merge mergeable views. This hint is most often used to reduce the number of possible permutations for a query and make optimization faster.
FACT The FACT hint indicated that the table should be considered as a fact table.  This is used in the context of the star transformation.
NO_FACT The NO_FACT hint is used in the context of the star transformation to  indicate to the transformation that the hinted table should not be  considered as a fact table.
STAR_TRANSFORMATION The STAR_TRANSFORMATION hint makes the optimizer use the best plan in  which the transformation has been used. Without the hint, the optimizer  could make a query optimization decision to use the best plan generated  without the transformation, instead of the best plan for the transformed query.
NO_STAR_TRANSFORMATION Do not use star transformation (from Oracle 10g)
UNNEST The UNNEST hint specifies subquery unnesting.
NO_UNNEST Use of the NO_UNNEST hint turns off unnesting for specific subquery blocks.

Hints for Join Orders

LEADING Give this hint to indicate the leading table in a join. This will indicate only 1 table. If you want to specify the whole order of tables, you can use the ORDERED hint. Syntax: LEADING(table)
ORDERED The ORDERED hint causes Oracle to join tables in the order in which they appear in the FROM clause. If you omit the ORDERED hint from a SQL statement performing a join , the optimizer chooses the order in which to join the tables. You may want to use the ORDERED hint to specify a join order if you know something about the number of rows selected from each table that the optimizer does not. Such information would allow you to choose an inner and outer table better than the optimizer could.

Hints for Join Operations

USE_NL The USE_NL hint causes Oracle to join each specified table to another row source with a nested loops join using the specified table as the inner table. The syntax of the USE_NL hint is USE_NL(table table) where table is the name or alias of a table to be used as the inner table of a nested loops join.
NO_USE_NL Do not use nested loop (from Oracle 10g)
USE_NL_WITH_INDEX Specifies a nested loops join. (from Oracle 10g)
USE_MERGE The USE_MERGE hint causes Oracle to join each specified table with another row source with a sort-merge join. The syntax of the USE_MERGE hint is USE_MERGE(table table) where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a sort-merge join.
NO_USE_MERGE Do not use merge (from Oracle 10g)
USE_HASH The USE_HASH hint causes Oracle to join each specified table with another row source with a hash join. The syntax of the USE_HASH hint is USE_HASH(table table) where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a hash join.
NO_USE_HASH Do not use hash (from Oracle 10g)
Hints for Parallel Execution
PARALLEL The PARALLEL hint allows you to specify the desired number of concurrent query servers that can be used for the query. The syntax is PARALLEL(table number number). The PARALLEL hint must use the table alias if an alias is specified in the query. The PARALLEL hint can then take two values separated by commas after the table name. The first value specifies the degree of parallelism for the given table, the second value specifies how the table is to be split among the instances of a parallel server. Specifying DEFAULT or no value signifies the query coordinator should examine the settings of the initialization parameters (described in a later section) to determine the default degree of parallelism.
NOPARALLEL / NO_PARALLEL The NOPARALLEL hint allows you to disable parallel scanning of a table, even if the table was created with a PARALLEL clause. In Oracle 10g this hint was renamed to NO_PARALLEL.
PQ_DISTRIBUTE The PQ_DISTRIBUTE hint improves the performance of parallel join  operations. Do this by specifying how rows of joined tables should be  distributed among producer and consumer query servers. Using this hint  overrides decisions the optimizer would normally make.
NO_PARALLEL_INDEX The NO_PARALLEL_INDEX hint overrides a PARALLEL attribute setting on an index to avoid a parallel index scan operation.
Additional Hints
APPEND When the APPEND hint is used with the INSERT statement, data is appended to the table. Existing free space in the block is not used. If a table or an index is specified with nologging, this hint applied  with an insert statement produces  a direct path insert which reduces generation of redo.
NOAPPEND Overrides the append mode.
CACHE The CACHE hint specifies that the blocks retrieved for the table in the hint are placed at the most recently used end of the LRU list in the buffer cache when a full table scan is performed. This option is useful for small lookup tables. In the following example, the CACHE hint overrides the table default caching specification.
NOCACHE The NOCACHE hint specifies that the blocks retrieved for this table are placed at the least recently used end of the LRU list in the buffer cache when a full table scan is performed. This is the normal behavior of blocks in the buffer cache.
PUSH_PRED The PUSH_PRED hint forces pushing of a join predicate into the view.
NO_PUSH_PRED The NO_PUSH_PRED hint prevents pushing of a join predicate into the view.
PUSH_SUBQ The PUSH_SUBQ hint causes nonmerged subqueries to be evaluated at the earliest possible place in the execution plan.
NO_PUSH_SUBQ The NO_PUSH_SUBQ hint causes non-merged subqueries to be evaluated as the last step in the execution plan.
QB_NAME Specifies a name for a query block. (from Oracle 10g)
CURSOR_SHARING_EXACT Oracle can replace literals in SQL statements with bind variables, if it is safe to do so. This is controlled with the CURSOR_SHARING startup  parameter. The CURSOR_SHARING_EXACT hint causes this behavior to be  switched off. In other words, Oracle executes the SQL statement without  any attempt to replace literals by bind variables.
DRIVING_SITE The DRIVING_SITE hint forces query execution to be done for the table at a different site than that selected by Oracle
DYNAMIC_SAMPLING The DYNAMIC_SAMPLING hint lets you control dynamic sampling to improve  server performance by determining more accurate predicate selectivity  and statistics for tables and indexes. You can set the value of  DYNAMIC_SAMPLING to a value from 0 to 10. The higher the level, the more effort the compiler puts into dynamic sampling and the more broadly it  is applied. Sampling defaults to cursor level unless you specify a  table.
SPREAD_MIN_ANALYSIS This hint omits some of the compile time optimizations of the rules,  mainly detailed dependency graph analysis, on spreadsheets. Some  optimizations such as creating filters to selectively populate  spreadsheet access structures and limited rule pruning are still used. (from Oracle 10g)

Hints with unknown status

MERGE_AJ The MERGE_AJ hint transforms a NOT IN subquery into a merge anti-join to access the specified table. The syntax of the MERGE_AJ hint is MERGE_AJ(table) where table specifies the name or alias of the table to  be accessed.(depricated in Oracle 10g)
AND_EQUAL The AND_EQUAL hint explicitly chooses an execution plan that uses an access path that merges the scans on several single-column indexes. The syntax of the AND_EQUAL hint is AND_EQUAL(table index index) where table specifies the name or alias of the table associated with the indexes to be merged. and index specifies an index on which an index scan is to be performed. You must specify at least two indexes. You cannot specify more than five. (depricated in Oracle 10g)
STAR The STAR hint forces the large table to be joined last using a nested loops join on the index. The optimizer will consider different permutations of the small tables. (depricated in Oracle 10g)
BITMAP Usage: BITMAP(table_name index_name) Uses a bitmap index to access the table. (depricated ?)
HASH_SJ Use a Hash Anti-Join to evaluate a NOT IN sub-query. Use this hint in the sub-query, not in the main query. Use this when your high volume NOT IN sub-query is using a FILTER or NESTED LOOPS join.  Try MERGE_AJ if HASH_AJ refuses to work.(depricated in Oracle 10g)
NL_SJ Use a Nested Loop in a sub-query. (depricated in Oracle 10g)
NL_AJ Use an anti-join in a sub-query. (depricated in Oracle 10g)
ORDERED_PREDICATES (depricated in Oracle 10g)
EXPAND_GSET_TO_UNION (depricated in Oracle 10g)

 

 

Hint 是Oracle 提供的一种SQL语法,它允许用户在SQL语句中插入相关的语法,从而影响SQL的执行方式。

因为Hint的特殊作用,所以对于开发人员不应该在代码中使用它,Hint 更像是Oracle提供给DBA用来分析问题的工具 。在SQL代码中使用Hint,可能导致非常严重的后果,因为数据库的数据是变化的,在某一时刻使用这个执行计划是最优的,在另一个时刻,却可能很差,这也是CBO 取代RBO的原因之一,规则是死的,而数据是时刻变化的,为了获得最正确的执行计划,只有知道表中数据的实际情况,通过计算各种执行计划的成本,则其最优,才是最科学的,这也是CBO的工作机制。 在SQL代码中加入Hint,特别是性能相关的Hint是很危险的做法。

Hints

Hints are comments in a SQL statement that pass instructions to the Oracle Database optimizer. The optimizer uses these hints to choose an execution plan for the statement, unless some condition exists that prevents the optimizer from doing so.

Hints were introduced in Oracle7, when users had little recourse if the optimizer generated suboptimal plans. Now Oracle provides a number of tools, including the SQL Tuning Advisor, SQL plan management, and SQL Performance Analyzer, to help you address performance problems that are not solved by the optimizer. Oracle strongly recommends that you use those tools rather than hints. The tools are far superior to hints, because when used on an ongoing basis, they provide fresh solutions as your data and database environment change.

Hints should be used sparingly, and only after you have collected statistics on the relevant tables and evaluated the optimizer plan without hints using the EXPLAIN PLAN statement. Changing database conditions as well as query performance enhancements in subsequent releases can have significant impact on how hints in your code affect performance.

The remainder of this section provides information on some commonly used hints. If you decide to use hints rather than the more advanced tuning tools, be aware that any short-term benefit resulting from the use of hints may not continue to result in improved performance over the long term.

Oracle 联机文档对Hint的说明:

http://download.oracle.com/docs/cd/E11882_01/server.112/e10592/sql_elements006.htm#SQLRF50705

之前整理的一篇文章:

常见Oracle HINT的用法

http://blog.csdn.net/tianlesoftware/archive/2009/12/13/4969702.aspx

在使用Hint时需要注意的一点是,并非任何时刻Hint都起作用。 导致HINT 失效的原因有如下2点:

(1) 如果CBO 认为使用Hint 会导致错误的结果时,Hint将被忽略。

如索引中的记录因为空值而和表的记录不一致时,结果就是错误的,会忽略hint。

(2) 如果表中指定了别名,那么Hint中也必须使用别名,否则Hint也会忽略。

Select /*+full(a)*/ * from t a; -- 使用hint

Select /*+full(t) */ * from t a; --不使用hint

根据hint的功能,可以分成如下几类:

 

Hint Hint 语法
优化器模式提示 ALL_ROWS Hint
  FIRST_ROWS Hint
  RULE Hint

 

访问路径提示 CLUSTER Hint
  FULL Hint
  HASH Hint
  INDEX Hint
  NO_INDEX Hint
  INDEX_ASC Hint
  INDEX_DESC Hint
  INDEX_COMBINE Hint
  INDEX_FFS Hint
  INDEX_SS Hint
  INDEX_SS_ASC Hint
  INDEX_SS_DESC Hint
  NO_INDEX_FFS Hint
  NO_INDEX_SS Hint
  ORDERED Hint
  LEADING Hint
  USE_HASH Hint
  NO_USE_HASH Hint
表连接顺序提示 USE_MERGE Hint
  NO_USE_MERGE Hint
  USE_NL Hint
  USE_NL_WITH_INDEX Hint
  NO_USE_NL Hint
表关联方式提示 PARALLEL Hint
  NO_PARALLEL Hint
  PARALLEL_INDEX Hint
  NO_PARALLEL_INDEX Hint
  PQ_DISTRIBUTE Hint
并行执行提示 FACT Hint
  NO_FACT Hint
  MERGE Hint
  NO_MERGE Hint
  NO_EXPAND Hint
  USE_CONCAT Hint
查询转换提示 REWRITE Hint
  NO_REWRITE Hint
  UNNEST Hint
  NO_UNNEST Hint
  STAR_TRANSFORMATION Hint
  NO_STAR_TRANSFORMATION Hint
  NO_QUERY_TRANSFORMATION Hint
  APPEND Hint
  NOAPPEND Hint
  CACHE Hint
  NOCACHE Hint
  CURSOR_SHARING_EXACT Hint
其他Hint DRIVING_SITE Hint
  DYNAMIC_SAMPLING Hint
  PUSH_PRED Hint
  NO_PUSH_PRED Hint
  PUSH_SUBQ Hint
  NO_PUSH_SUBQ Hint
  PX_JOIN_FILTER Hint
  NO_PX_JOIN_FILTER Hint
  NO_XML_QUERY_REWRITE Hint
  QB_NAME Hint
  MODEL_MIN_ANALYSIS Hint

一. 和优化器相关的Hint

Oracle 允许在系统级别,会话级别和SQL中(hint)优化器类型:

系统级别:

   1: SQL>alter system set optimizer_mode=all_rows;

会话级别:

SQL>alter system set optimizer_mode=all_rows;

关于优化器,参考:

Oracle Optimizer CBO RBO

http://blog.csdn.net/tianlesoftware/archive/2010/08/19/5824886.aspx

1.1 ALL_ROWS 和FIRST_ROWS(n) -- CBO 模式

对于OLAP系统,这种系统中通常都是运行一些大的查询操作,如统计,报表等任务。 这时优化器模式应该选择ALL_ROWS. 对于一些分页显示的业务,就应该用FIRST_ROWS(n)。 如果是一个系统上运行这两种业务,那么就需要在SQL 用hint指定优化器模式。

如:

SQL> select /* + all_rows*/ * from dave;

SQL> select /* + first_rows(20)*/ * from dave;

1.2 RULE Hint -- RBO 模式

尽管Oracle 10g已经弃用了RBO,但是仍然保留了这个hint。 它允许在CBO 模式下使用RBO 对SQL 进行解析。

如:

SQL> show parameter optimizer_mode

NAME TYPE VALUE

------------------------------------ ----------- ------------------------------

optimizer_mode string ALL_ROWS

SQL> set autot trace exp;

SQL> select /*+rule */ * from dave;

执行计划

----------------------------------------------------------

Plan hash value: 3458767806

----------------------------------

| Id | Operation | Name |

----------------------------------

| 0 | SELECT STATEMENT | |

| 1 | TABLE ACCESS FULL| DAVE |

----------------------------------

Note

-----

- rule based optimizer used (consider using cbo) -- 这里提示使用RBO

SQL>

二. 访问路径相关的Hint

这一部分hint 将直接影响SQL 的执行计划,所以在使用时需要特别小心。 该类Hint对DBA分析SQL性能非常有帮助,DBA 可以让SQL使用不同的Hint得到不同的执行计划,通过比较不同的执行计划来分析当前SQL性能。

2.1 FULL Hint

该Hint告诉优化器对指定的表通过全表扫描的方式访问数据。

示例:

SQL> select /*+full(dave) */ * from dave;

要注意,如果表有别名,在hint里也要用别名, 这点在前面已经说明。

2.2 INDEX Hint

Index hint 告诉优化器对指定的表通过索引的方式访问数据,当访问索引会导致结果集不完整时,优化器会忽略这个Hint。

示例:

SQL> select /*+index(dave index_dave) */ * from dave where id>1;

谓词里有索引字段,才会用索引。

2.3 NO_INDEX Hint

No_index hint 告诉优化器对指定的表不允许使用索引。

示例:

SQL> select /*+no_index(dave index_dave) */ * from dave where id>1;

2.4 INDEX_DESC Hint

该Hint 告诉优化器对指定的索引使用降序方式访问数据,当使用这个方式会导致结果集不完整时,优化器将忽略这个索引。

示例:

SQL> select /*+index_desc(dave index_dave) */ * from dave where id>1;

2.5 INDEX_COMBINE Hint

该Hint告诉优化器强制选择位图索引,当使用这个方式会导致结果集不完整时,优化器将忽略这个Hint。

示例:

SQL> select /*+ index_combine(dave index_bm) */ * from dave;

2.6 INDEX_FFS Hint

该hint告诉优化器以INDEX_FFS(INDEX Fast Full Scan)的方式访问数据。当使用这个方式会导致结果集不完整时,优化器将忽略这个Hint。

示例:

SQL> select /*+ index_ffs(dave index_dave) */ id from dave where id>0;

2.7 INDEX_JOIN Hint

索引关联,当谓词中引用的列上都有索引时,可以通过索引关联的方式来访问数据。

示例:

SQL> select /*+ index_join(dave index_dave index_bm) */ * from dave where id>0 and name='安徽安庆';

2.8 INDEX_SS Hint

该Hint强制使用index skip scan 的方式访问索引,从Oracle 9i开始引入这种索引访问方式,当在一个联合索引中,某些谓词条件并不在联合索引的第一列时(或者谓词并不在联合索引的第一列时),可以通过index skip scan 来访问索引获得数据。 当联合索引第一列的唯一值很小时,使用这种方式比全表扫描效率要高。当使用这个方式会导致结果集不完整时,优化器将忽略这个Hint。

示例:

SQL> select /*+ index_ss(dave index_union) */ * from dave where id>0;

三. 表关联顺序的Hint

表之间的连接方式有三种。 具体参考blog:

多表连接的三种方式详解 HASH JOIN MERGE JOIN NESTED LOOP

http://blog.csdn.net/tianlesoftware/archive/2010/08/20/5826546.aspx

3.1 LEADING hint

在一个多表关联的查询中,该Hint指定由哪个表作为驱动表,告诉优化器首先要访问哪个表上的数据。

示例:

SQL> select /*+leading(t1,t) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

SQL> select /*+leading(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

--------------------------------------------------------------------------------

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Ti

--------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 14 | 812 | 6 (17)| 00

| 1 | MERGE JOIN | | 14 | 812 | 6 (17)| 00

| 2 | TABLE ACCESS BY INDEX ROWID| DEPT | 4 | 80 | 2 (0)| 00

| 3 | INDEX FULL SCAN | PK_DEPT | 4 | | 1 (0)| 00

|* 4 | SORT JOIN | | 14 | 532 | 4 (25)| 00

| 5 | TABLE ACCESS FULL | EMP | 14 | 532 | 3 (0)| 00

--------------------------------------------------------------------------------

3.2 ORDERED Hint

该hint 告诉Oracle 按照From后面的表的顺序来选择驱动表,Oracle 建议在选择驱动表上使用Leading,它更灵活一些。

SQL> select /*+ordered */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

四. 表关联操作的Hint

4.1 USE_HASH,USE_NL,USE_MERGE hint

表之间的连接方式有三种。 具体参考blog:

多表连接的三种方式详解 HASH JOIN MERGE JOIN NESTED LOOP

http://blog.csdn.net/tianlesoftware/archive/2010/08/20/5826546.aspx

这三种关联方式是多表关联中主要使用的关联方式。 通常来说,当两个表都比较大时,Hash Join的效率要高于嵌套循环(nested loops)的关联方式。

Hash join的工作方式是将一个表(通常是小一点的那个表)做hash运算,将列数据存储到hash列表中,从另一个表中抽取记录,做hash运算,到hash 列表中找到相应的值,做匹配。

Nested loops 工作方式是从一张表中读取数据,访问另一张表(通常是索引)来做匹配,nested loops适用的场合是当一个关联表比较小的时候,效率会更高。

Merge Join 是先将关联表的关联列各自做排序,然后从各自的排序表中抽取数据,到另一个排序表中做匹配,因为merge join需要做更多的排序,所以消耗的资源更多。 通常来讲,能够使用merge join的地方,hash join都可以发挥更好的性能。

USE_HASH,USE_NL,USE_MERGE 这三种hint 就是告诉优化器使用哪种关联方式。

示例如下:

SQL> select /*+use_hash(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

SQL> select /*+use_nl(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

SQL> select /*+use_merge(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

4.2 NO_USE_HASH,NO_USE_NL,NO_USE_MERGE HINT

分别禁用对应的关联方式。

示例:

SQL> select /*+no_use_merge(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

SQL> select /*+no_use_nl(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

SQL> select /*+no_use_hash(t,t1) */ * from scott.dept t,scott.emp t1 where t.deptno=t1.deptno;

五. 并行执行相关的Hint

5.1 PARALLEL HINT

指定SQL 执行的并行度,这个值会覆盖表自身设定的并行度,如果这个值为default,CBO使用系统参数值。

示例:

SQL> select /*+parallel(t 4) */ * from scott.dept t;

关于表的并行度,我们在创建表的时候可以指定,如:

SQL> CREATE TABLE Anqing

2 (

3 name VARCHAR2 (10)

4 )

5 PARALLEL 2;

表已创建。

SQL> select degree from all_tables where table_name = 'ANQING'; -- 查看表的并行度

DEGREE

--------------------

2

SQL> alter table anqing parallel(degree 3); -- 修改表的并行度

表已更改。

SQL> select degree from all_tables where table_name = 'ANQING';

DEGREE

--------------------

3

SQL> alter table anqing noparallel; -- 取消表的并行度

表已更改。

SQL> select degree from all_tables where table_name = 'ANQING';

DEGREE

--------------------

1

5.2 NO_PARALLEL HINT

在SQL中禁止使用并行。

示例:

SQL> select /*+ no_parallel(t) */ * from scott.dept t;

六. 其他方面的一些Hint

6.1 APPEND HINT

提示数据库以直接加载的方式(direct load)将数据加载入库。

示例:

Insert /*+append */ into t as select * from all_objects;

这个hint 用的比较多。 尤其在插入大量的数据,一般都会用此hint。

Oracle 插入大量数据

http://blog.csdn.net/tianlesoftware/archive/2009/10/30/4745144.aspx

6.2 DYNAMIC_SAMPLING HINT

提示SQL 执行时动态采样的级别。 这个级别从0-10,它将覆盖系统默认的动态采样级别。

示例:

SQL> select /*+ dynamic_sampling(t 2) */ * from scott.emp t where t.empno>0;

6.3 DRIVING_SITE HINT

这个提示在分布式数据库操作中比较有用,比如我们需要关联本地的一张表和远程的表:

Select /* + driving_site(departmetns) */ * from employees,departments@dblink where

employees .department_id = departments.department_id;

如果没有这个提示,Oracle 会在远端机器上执行departments 表查询,将结果送回本地,再和employees表关联。 如果使用driving_site(departments), Oracle将查询本地表employees,将结果送到远端,在远端将数据库上的表与departments关联,然后将查询的结果返回本地。

如果departments查询结果很大,或者employees查询结果很小,并且两张表关联之后的结果集很小,那么就可以考虑把本地的结果集发送到远端。 在远端执行完后,在将较小的最终结果返回本地。

6.4 CACHE HINT

在全表扫描操作中,如果使用这个提示,Oracle 会将扫描的到的数据块放到LRU(least recently Used: 最近很少被使用列表,是Oracle 判断内存中数据块活跃程度的一个算法)列表的最被使用端(数据块最活跃端),这样数据块就可以更长时间地驻留在内存当中。 如果有一个经常被访问的小表,这个设置会提高查询的性能;同时CACHE也是表的一个属性,如果设置了表的cache属性,它的作用和hint一样,在一次全表扫描之后,数据块保留在LRU列表的最活跃端。

示例:

SQL> select /*+full(t) cache (t) */ * from scott.emp;

小结:

对于DBA来讲,掌握一些Hint操作,在实际性能优化中有很大的好处,比如我们发现一条SQL的执行效率很低,首先我们应当查看当前SQL的执行计划,然后通过hint的方式来改变SQL的执行计划,比较这两条SQL 的效率,作出哪种执行计划更优,如果当前执行计划不是最优的,那么就需要考虑为什么CBO 选择了错误的执行计划。当CBO 选择错误的执行计划,我们需要考虑表的分析是否是最新的,是否对相关的列做了直方图,是否对分区表做了全局或者分区分析等因素。

 

 

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其实Oracle的优化器有两种优化方式,

基于规则的优化方式(Rule-BasedOptimization,简称为RBO)10G以后将被废弃......

基于代价的优化方式(Cost-BasedOptimization,简称为CBO)

所以hint也不例外,除了/*+rule*/其他的都是CBO优化方式

RBO方式:

  优化器在分析SQL语句时,所遵循的是Oracle内部预定的一些规则。比如我们常见的,当一个where子句中的一列有索引时去走索引。

CBO方式:

   它是看语句的代价(Cost),这里的代价主要指Cpu和内存。优化器在判断是否用这种方式时,主要参照的是表及索引的统计信息。统计信息给出表的大小、有少行、每行的长度等信息。这些统计信息起初在库内是没有的,是做analyze后才出现的,很多的时侯过期统计信息会令优化器做出一个错误的执行计划,因此应及时更新这些信息。

 

优化模式包括Rule、Choose、First rows、All rows四种方式:

 

    Rule:基于规则的方式。

 

    Choolse:默认的情况下Oracle用的便是这种方式。指的是当一个表或或索引有统计信息,则走CBO的方式,如果表或索引没统计信息,表又不是特别的小,而且相应的列有索引时,那么就走索引,走RBO的方式。

 

    First Rows:它与Choose方式是类似的,所不同的是当一个表有统计信息时,它将是以最快的方式返回查询的最先的几行,从总体上减少了响应时间。

 

    All Rows:也就是我们所说的Cost的方式,当一个表有统计信息时,它将以最快的方式返回表的所有的行,从总体上提高查询的吞吐量。没有统计信息则走RBO的方式

 

Oracle在那配置默认的优化规则

    A、Instance级别我们可以通过在initSID.ora文件中设定OPTIMIZER_MODE=RULE/CHOOSE/FIRST_ROWS/ALL_ROWS如果没设定OPTIMIZER_MODE参数则默认用的是Choose方式。

    B、Sessions级别通过ALTER SESSION SETOPTIMIZER_MODE=RULE/CHOOSE/FIRST_ROWS/ALL_ROWS来设定。

    C、语句级别用Hint(/*+ ... */)来设定

为什么表的某个字段明明有索引,但执行计划却不走索引?

    1、优化模式是all_rows的方式

   2、表作过analyze,有统计信息

   3、表很小,Oracle的优化器认为不值得走索引。

提示

   不区分大小写,多个提示用空格分开

  如:select /*+ hint1(tab1) hint2(TAB1idx1) */ col1, col2 from tab1 where col1='xxx';

  如果表使用了别名,那么提示里也必须使用别名

如:select /*+hint1(t1) */ col1, col2 from tab1 t1 where col1='xxx';

如果使用同一个表的多个用,号分开

如: select /*+index(t1.A,t1.B) */ col1, col2

   from   tab1 t1

   where  col1='xxx';

 

oracle 10g hints知识,

    10g数据库可以使用更多新的optimizer hints来控制优化行为。现在让我们快速解析一下这些强大的新hints:

 

1、spread_min_analysis

 

   使用这一hint,你可以忽略一些关于如详细的关系依赖图分析等电子表格的编译时间优化规则。其他的一些优化,如创建过滤以有选择性的定位电子表格访问结构并限制修订规则等,得到了继续使用。

 

   由于在规则数非常大的情况下,电子表格分析会很长。这一提示可以帮助我们减少由此产生的数以百小时计的编译时间。

 

例:

    SELECT /*+ SPREAD_MIN_ANALYSIS */ ...

 

2、spread_no_analysis

 

   通过这一hint,可以使无电子表格分析成为可能。同样,使用这一hint可以忽略修订规则和过滤产生。如果存在一个电子表格分析,编译时间可以被减少到最低程度。

 

例:

    SELECT /*+ SPREAD_NO_ANALYSIS */ ...

 

3、use_nl_with_index

 

   这项hint使CBO通过嵌套循环把特定的表格加入到另一原始行。只有在以下情况中,它才使用特定表格作为内部表格:如果没有指定标签,CBO必须可以使用一些标签,且这些标签至少有一个作为索引键值加入判断;反之,CBO必须能够使用至少有一个作为索引键值加入判断的标签。

 

例:

 SELECT /*+ USE_NL_WITH_INDEX (polrecpolrind) */ ...

 

4、CARDINALITY

 

  此hint定义了对由查询或查询部分返回的基数的评价。注意如果没有定义表格,基数是由整个查询所返回的总行数。

 

例:

 SELECT /*+ CARDINALITY ( [tablespec] card ) */

 

5、SELECTIVITY

 

  此hint定义了对查询或查询部分选择性的评价。如果只定义了一个表格,选择性是在所定义表格里满足所有单一表格判断的行部分。如果定义了一系列表格,选择性是指在合并以任何顺序满足所有可用判断的全部表格后,所得结果中的行部分。

 

例:

  SELECT /*+ SELECTIVITY ( [tablespec] sel ) */

 

然而,注意如果hintsCARDINALITY和 SELECTIVITY都定义在同样的一批表格,二者都会被忽略。

 

6、no_use_nl

 

 Hint no_use_nl使CBO执行循环嵌套,通过把指定表格作为内部表格,把每个指定表格连接到另一原始行。通过这一hint,只有hash join和sort-merge joins会为指定表格所考虑。

 

例:

  SELECT /*+ NO_USE_NL ( employees ) */ ...

 

7、no_use_merge

 

  此hint使CBO通过把指定表格作为内部表格的方式,拒绝sort-merge把每个指定表格加入到另一原始行。

 

例:

 SELECT /*+ NO_USE_MERGE ( employees dept ) */ ...

 

8、no_use_hash

 

  此hint使CBO通过把指定表格作为内部表格的方式,拒绝hash joins把每个指定表格加入到另一原始行。

 

例:

 SELECT /*+ NO_USE_HASH ( employees dept ) */ ...

 

9、no_index_ffs

 

  此hint使CBO拒绝对指定表格的指定标签进行fastfull-index scan。

Syntax: /*+ NO_INDEX_FFS (tablespecindexspec ) */

 

 

在SQL优化过程中常见HINT的用法(前10个比较常用,前3个最常用):

 

1. /*+ INDEX */和 /*+INDEX(TABLE INDEX1, index2) */和 /*+ INDEX(tab1.col1 tab2.col2) */和 /*+ NO_INDEX */ 和 /*+ NO_INDEX(TABLE INDEX1, index2) */

 

表明对表选择索引的扫描方法.第一种不指定索引名是让oracle对表中可用索引比较并选择某个最佳索引;第二种是指定索引名且可指定多个索引;第三种是10g开始有的,指定列名,且表名可不用别名;第四种即全表扫描;第五种表示禁用某个索引,特别适合于准备删除某个索引前的评估操作.如果同时使用了INDEX和NO_INDEX则两个提示都会被忽略掉.

例如:SELECT /*+INDEX(BSEMPMS SEX_INDEX) USE SEX_INDEX BECAUSE THERE ARE FEWMALE BSEMPMS */FROM BSEMPMS WHERE SEX='M';

 

2. /*+ ORDERED*/

FROM子句中默认最后一个表是驱动表,ORDERED将from子句中第一个表作为驱动表.特别适合于多表连接非常慢时尝试.

例如:SELECT /*+ORDERED */ A.COL1,B.COL2,C.COL3 FROM TABLE1 A,TABLE2 B,TABLE3 CWHERE A.COL1=B.COL1 AND B.COL1=C.COL1;

 

3. /*+PARALLEL(table1,DEGREE) */和 /*+ NO_PARALLEL(table1) */

该提示会将需要执行全表扫描的查询分成多个部分(并行度)执行,然后在不同的操作系统进程中处理每个部分.该提示还可用于DML语句.如果SQL里还有排序操作,进程数会翻倍,此外还有一个一个负责组合这些部分的进程,如下面的例子会产生9个进程.如果在提示中没有指定DEGREE,那么就会使用创建表时的默认值.该提示在默认情况下会使用APPEND提示. NO_PARALLEL是禁止并行操作,否则语句会使用由于定义了并行对象而产生的并行处理.

例如:select /*+PARALLEL(tab_test,4) */ col1, col2 from tab_test order by col2;

 

4. /*+FIRST_ROWS */ 和 /*+FIRST_ROWS(n) */

表示用最快速度获得第1/n行,获得最佳响应时间,使资源消耗最小化.

在update和delete语句里会被忽略,使用分组语句如groupby/distinct/intersect/minus/union时也会被忽略.

例如:SELECT /*+FIRST_ROWS */ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO='SCOTT';

 

5. /*+ RULE */

表明对语句块选择基于规则的优化方法.

例如:SELECT /*+RULE */ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO='SCOTT';

 

6. /*+FULL(TABLE) */

表明对表选择全局扫描的方法.

例如:SELECT /*+FULL(A) */ EMP_NO,EMP_NAM FROM BSEMPMS A WHERE EMP_NO='SCOTT';

 

7. /*+LEADING(TABLE) */

类似于ORDERED提示,将指定的表作为连接次序中的驱动表.

 

8. /*+USE_NL(TABLE1,TABLE2) */

将指定表与嵌套的连接的行源进行连接,以最快速度返回第一行再连接,与USE_MERGE刚好相反.

例如:SELECT /*+ORDERED USE_NL(BSEMPMS) */ BSDPTMS.DPT_NO,BSEMPMS.EMP_NO,BSEMPMS.EMP_NAM FROMBSEMPMS,BSDPTMS WHERE BSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

 

9. /*+ APPEND*/ 和 /*+ NOAPPEND*/

直接插入到表的最后,该提示不会检查当前是否有插入操作所需的块空间而是直接添加到新块中,所以可以提高速度.当然也会浪费些空间,因为它不会使用那些做了delete操作的块空间. NOAPPEND提示则相反,所以会取消PARALLEL提示的默认APPEND提示.

例如:insert /*+append */ into test1 select * from test4;

insert /*+ parallel(test1) noappend */ intotest1 select * from test4;

 

10. /*+USE_HASH(TABLE1,table2) */

将指定的表与其它行源通过哈希连接方式连接起来.为较大的结果集提供最佳响应时间.类似于在连接表的结果中遍历每个表上每个结果的嵌套循环,指定的hash表将被放入内存,所以需要有足够的内存(hash_area_size或pga_aggregate_target)才能保证语句正确执行,否则将在磁盘里进行.

例如:SELECT /*+USE_HASH(BSEMPMS,BSDPTMS) */ * FROM BSEMPMS,BSDPTMS WHEREBSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

 

---------------------------------------------------------------------

 

11. /*+USE_MERGE(TABLE) */

将指定的表与其它行源通过合并排序连接方式连接起来.特别适合于那种在多个表大量行上进行集合操作的查询,它会将指定表检索到的的所有行排序后再被合并,与USE_NL刚好相反.

例如:SELECT /*+USE_MERGE(BSEMPMS,BSDPTMS) */ * FROM BSEMPMS,BSDPTMS WHEREBSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

 

12. /*+ALL_ROWS */

表明对语句块选择基于开销的优化方法,并获得最佳吞吐量,使资源消耗最小化.可能会限制某些索引的使用.

例如:SELECT /*+ALL+_ROWS */ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO='SCOTT';

 

13. /*+CLUSTER(TABLE) */

提示明确表明对指定表选择簇扫描的访问方法.如果经常访问连接表但很少修改它,那就使用集群提示.

例如:SELECT /*+CLUSTER */ BSEMPMS.EMP_NO,DPT_NO FROM BSEMPMS,BSDPTMS WHERE DPT_NO='TEC304' ANDBSEMPMS.DPT_NO=BSDPTMS.DPT_NO;

 

14. /*+INDEX_ASC(TABLE INDEX1, INDEX2) */

表明对表选择索引升序的扫描方法.从8i开始,这个提示和INDEX提示功能一样,因为默认oracle就是按照升序扫描索引的,除非未来oracle还推出降序扫描索引.

例如:SELECT /*+INDEX_ASC(BSEMPMS PK_BSEMPMS) */ FROM BSEMPMS WHERE DPT_NO='SCOTT';

 

15. /*+INDEX_COMBINE(TABLE INDEX1, INDEX2) */

指定多个位图索引,对于B树索引则使用INDEX这个提示,如果INDEX_COMBINE中没有提供作为参数的索引,将选择出位图索引的布尔组合方式.

例如:SELECT /*+INDEX_COMBINE(BSEMPMS SAL_BMI HIREDATE_BMI) */ * FROM BSEMPMS WHERESAL<5000000 AND HIREDATE<SYSDATE;

 

16. /*+INDEX_JOIN(TABLE INDEX1, INDEX2) */

合并索引, 所有数据都已经包含在这两个索引里, 不会再去访问表,比使用索引并通过rowid去扫描表要快5倍.

例如:SELECT /*+INDEX_JOIN(BSEMPMS SAL_HMI HIREDATE_BMI) */ SAL,HIREDATE FROM BSEMPMS WHERESAL<60000;

 

17. /*+INDEX_DESC(TABLE INDEX1, INDEX2) */

表明对表选择索引降序的扫描方法.

例如:SELECT /*+INDEX_DESC(BSEMPMS PK_BSEMPMS) */ FROM BSEMPMS WHERE DPT_NO='SCOTT';

 

18. /*+INDEX_FFS(TABLE INDEX_NAME) */

对指定的表执行快速全索引扫描,而不是全表扫描的办法.要求要检索的列都在索引里,如果表有很多列时特别适用该提示.

例如:SELECT /*+INDEX_FFS(BSEMPMS IN_EMPNAM) */ * FROM BSEMPMS WHERE DPT_NO='TEC305';

 

19. /*+NO_EXPAND */

对于WHERE后面的OR或者IN-LIST的查询语句,NO_EXPAND将阻止其基于优化器对其进行扩展,缩短解析时间.

例如:SELECT /*+NO_EXPAND */ * FROM BSEMPMS WHERE DPT_NO='TDC506' AND SEX='M';

 

20. /*+DRIVING_SITE(TABLE) */

强制与ORACLE所选择的位置不同的表进行查询执行.特别适用于通过dblink连接的远程表.

例如:SELECT /*+DRIVING_SITE(DEPT) */ * FROM BSEMPMS,DEPT@BSDPTMS DEPT WHEREBSEMPMS.DPT_NO=DEPT.DPT_NO;

 

21. /*+CACHE(TABLE) */和 /*+NOCACHE(TABLE) */

当进行全表扫描时,CACHE提示能够将表全部缓存到内存中,这样访问同一个表的用户可直接在内存中查找数据.比较适合数据量小但常被访问的表,也可以建表时指定cache选项这样在第一次访问时就可以对其缓存. NOCACHE则表示对已经指定了CACHE选项的表不进行缓存.

例如:SELECT /*+FULL(BSEMPMS) CAHE(BSEMPMS) */ EMP_NAM FROM BSEMPMS;

 

22. /*+PUSH_SUBQ */

当SQL里用到了子查询且返回相对少的行时,该提示可以尽可能早对子查询进行评估从而改善性能,不适用于合并连接或带远程表的连接.

例如:select /*+PUSH_SUBQ */ emp.empno, emp.ename, itemno from emp, orders where emp.empno =orders.empno and emp.deptno = (select deptno from dept where loc='XXX');

远程连接其他数据库,注意判断数据库是否启动,或者是否有需要的表,否则会出错

 

23. /*+INDEX_SS(TABLE INDEX1,INDEX2) */

指示对特定表的索引使用跳跃扫描,即当组合索引的第一列不在where子句中时,让其使用该索引

参考资料

Oracle SQL hints

  /*+hint */

/*+ hint(argument) */

/*+ hint(argument-1 argument-2) */

All hints except /*+ rule */ cause the CBOto be used. Therefore, it is good practise to analyze the underlying tables ifhints are used (or the query is fully hinted. There should be no schema namesin hints. Hints must use aliases if alias names are used for table names. Sothe following is wrong:

select /*+ index(scott.emp ix_emp) */ fromscott.emp emp_alias

better:

select /*+ index(emp_alias ix_emp) */ ...from scott.emp emp_alias

Why using hints

It is a perfect valid question to ask whyhints should be used. Oracle comes with an optimizer that promises to optimizea query's execution plan. When this optimizer is really doing a good job, nohints should be required at all. Sometimes, however, the characteristics of thedata in the database are changing rapidly, so that the optimizer (or moreaccuratly, its statistics) are out of date. In this case, a hint could help. Itmust also be noted, that Oracle allows to lock the statistics when they lookideal which should make the hints meaningless again.

Hint categories

Hints can be categorized as follows:

Hints for Optimization Approaches andGoals,

Hints for Access Paths, Hints for QueryTransformations,

Hints for Join Orders,

Hints for Join Operations,

Hints for Parallel Execution,

Additional Hints

 

DocumentedHints

Hints for Optimization Approaches and Goals

ALL_ROWS

One of the hints that 'invokes' the Costbased optimizer

ALL_ROWS is usually used for batchprocessing or data warehousing systems.

FIRST_ROWS

One of thehints that 'invokes' the Cost based optimizer

FIRST_ROWS is usually used for OLTPsystems.

CHOOSE

One of thehints that 'invokes' the Cost based optimizer

This hint lets the server choose (betweenALL_ROWS and FIRST_ROWS, based on statistics gathered.

RULE

The RULE hint should be considereddeprecated as it is dropped from Oracle9i2.

See also the following initializationparameters: optimizer_mode, optimizer_max_permutations, optimizer_index_cost_adj,optimizer_index_caching and

Hints for Access Paths

CLUSTER

Performs a nested loop by the cluster indexof one of the tables.

FULL

Performs full table scan.

HASH

Hashes one table (full scan) and creates ahash index for that table. Then hashes other table and uses hash index to findcorresponding records. Therefore not suitable for < or > join conditions.

ROWID

Retrieves therow by rowid

INDEX

Specifying that index index_name should beused on table tab_name: /*+ index (tab_name index_name) */

Specifying that the index should be usedthe the CBO thinks is most suitable. (Not always a good choice).

Starting with Oracle 10g, the index hint can be described: /*+index(my_tab my_tab(col_1, col_2)) */. Using the index on my_tab that startswith the columns col_1 and col_2.

INDEX_ASC

INDEX_COMBINE

INDEX_DESC

INDEX_FFS

INDEX_JOIN

NO_INDEX

AND_EQUAL

The AND_EQUAL hint explicitly chooses anexecution plan that uses an access path that merges the scans on severalsingle-column indexes

 

Hints for QueryTransformations

FACT

The FACT hint is used in the context of thestar transformation to indicate to the transformation that the hinted tableshould be considered as a fact table.

MERGE

NO_EXPAND

NO_EXPAND_GSET_TO_UNION

NO_FACT

NO_MERGE

NOREWRITE

REWRITE

STAR_TRANSFORMATION

USE_CONCAT

 

Hints for JoinOperations

DRIVING_SITE

HASH_AJ

HASH_SJ

LEADING

MERGE_AJ

MERGE_SJ

NL_AJ

NL_SJ

USE_HASH

USE_MERGE

USE_NL

 

Hints forParallel Execution

NOPARALLEL

PARALLEL

NOPARALLEL_INDEX

PARALLEL_INDEX

PQ_DISTRIBUTE

 

AdditionalHints

ANTIJOIN

APPEND

If a table or an index is specified withnologging, this hint applied with an insert statement produces a direct pathinsert which reduces generation of redo.

BITMAP

BUFFER

CACHE

CARDINALITY

CPU_COSTING

DYNAMIC_SAMPLING

INLINE

MATERIALIZE

NO_ACCESS

NO_BUFFER

NO_MONITORING

NO_PUSH_PRED

NO_PUSH_SUBQ

NO_QKN_BUFF

NO_SEMIJOIN

NOAPPEND

NOCACHE

OR_EXPAND

ORDERED

ORDERED_PREDICATES

PUSH_PRED

PUSH_SUBQ

QB_NAME

RESULT_CACHE (Oracle 11g)

SELECTIVITY

SEMIJOIN

SEMIJOIN_DRIVER

STAR

The STAR hint forces a star query plan tobe used, if possible. A star plan has the largest table in the query last inthe join order and joins it with a nested loops join on a concatenated index.The STAR hint applies when there are at least three tables, the large table'sconcatenated index has at least three columns, and there are no conflictingaccess or join method hints. The optimizer also considers differentpermutations of the small tables.

SWAP_JOIN_INPUTS

USE_ANTI

USE_SEMI

 

Undocumented hints:

BYPASS_RECURSIVE_CHECK

Workaraound forbug 1816154

BYPASS_UJVC

CACHE_CB

CACHE_TEMP_TABLE

CIV_GB

COLLECTIONS_GET_REFS

CUBE_GB

CURSOR_SHARING_EXACT

DEREF_NO_REWRITE

DML_UPDATE

DOMAIN_INDEX_NO_SORT

DOMAIN_INDEX_SORT

DYNAMIC_SAMPLING

DYNAMIC_SAMPLING_EST_CDN

EXPAND_GSET_TO_UNION

FORCE_SAMPLE_BLOCK

GBY_CONC_ROLLUP

GLOBAL_TABLE_HINTS

HWM_BROKERED

IGNORE_ON_CLAUSE

IGNORE_WHERE_CLAUSE

INDEX_RRS

INDEX_SS

INDEX_SS_ASC

INDEX_SS_DESC

LIKE_EXPAND

LOCAL_INDEXES

MV_MERGE

NESTED_TABLE_GET_REFS

NESTED_TABLE_SET_REFS

NESTED_TABLE_SET_SETID

NO_FILTERING

NO_ORDER_ROLLUPS

NO_PRUNE_GSETS

NO_STATS_GSETS

NO_UNNEST

NOCPU_COSTING

OVERFLOW_NOMOVE

PIV_GB

PIV_SSF

PQ_MAP

PQ_NOMAP

REMOTE_MAPPED

RESTORE_AS_INTERVALS

SAVE_AS_INTERVALS

SCN_ASCENDING

SKIP_EXT_OPTIMIZER

SQLLDR

SYS_DL_CURSOR

SYS_PARALLEL_TXN

SYS_RID_ORDER

TIV_GB

TIV_SSF

UNNEST

USE_TTT_FOR_GSETS

 

from:

https://blog.csdn.net/it_man/article/details/8185381

posted @ 2020-09-11 09:38  SolidMango  阅读(324)  评论(0编辑  收藏  举报