ORACLE Hints

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

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.
SELECT /*+ALL+_ROWS*/ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO='SCOTT'; 
2、/*+ FIRST_ROWS(n) */
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.
SELECT /*+FIRST_ROWS(20)*/ EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO='SCOTT'; 
3、/*+ RULE*/
表明对语句块选择基于规则的优化方法.
SELECT /*+ RULE */EMP_NO,EMP_NAM,DAT_IN FROM BSEMPMS WHERE EMP_NO='SCOTT'; 
和访问路径相关的hint

1、/*+ FULL(TABLE)*/
表明对表选择全局扫描的方法.
SELECT/*+FULL(A)*/ EMP_NO,EMP_NAM FROM BSEMPMS A WHERE EMP_NO='SCOTT'; 
2、/*+ INDEX(TABLE INDEX_NAME) */
表明对表选择索引的扫描方法.
SELECT/*+INDEX(BSEMPMS SEX_INDEX) */ * FROM BSEMPMS WHERE SEX='M'; 
5、/*+ INDEX_ASC(TABLE INDEX_NAME)*/
表明对表选择索引升序的扫描方法.
SELECT/*+INDEX_ASC(BSEMPMS PK_BSEMPMS) */ * FROM BSEMPMS WHERE DPT_NO='SCOTT'; 
6、/*+ 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.
SELECT/*+INDEX_COMBINE(BSEMPMS SAL_BMI HIREDATE_BMI) */  * FROM BSEMPMS
WHERE SAL<5000000 AND HIREDATE 
7、/*+ INDEX_JOIN(TABLE INDEX_NAME1 INDEX_NAME2)*/
当谓词中引用的列都有索引的时候，可以通过指定采用索引关联的方式，来访问数据
select /*+index_join(t t_ind t_bm) */ id from t where id=100 and object_name='EMPLOYEES' 
8、/*+ INDEX_DESC(TABLE INDEX_NAME)*/
表明对表选择索引降序的扫描方法.

SELECT /*+INDEX_DESC(BSEMPMS PK_BSEMPMS)*/ * FROM BSEMPMS WHEREDPT_NO='SCOTT'; 
9、/*+ 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.
SELECT /* +INDEX_FFS(BSEMPMS IN_EMPNAM)*/ * FROM BSEMPMS WHERE DPT_NO='TEC305';

10、/*+ INDEX_SS(TT_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。

 

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,t1where 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,t1where t.id=t1.id;

在多表关联查询中，指定使用hash join方式进行多表关联，并指定表t为驱动表。

select /*+use_hash(t,t1) leading(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.* fromt,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.* fromt,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.* fromt,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.* fromt,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 * fromt;

/*+ 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)