OPTIMIZER HINTS

What are the hints and what are the different types of hints (INTERVIEW Question)


Ans: Hints(Optimizer hints) are used to alter the execution plan of the sql statments.

We should first get the explain plan of our SQL and determine what changes can be done to make the code operate without using hints if possible. However, Oracle hints such as ORDERED, LEADING, INDEX, FULL, and the various AJ and SJ Oracle hints can tame a wild optimizer and give you optimal performance.

With hints one can influence the optimizer. The usage of hints (with exception of the RULE-hint) causes Oracle to use the Cost Based optimizer.

The following syntax is used for hints:

select /*+ HINT */ name
from emp
where id =1;

Where HINT is replaced by the hint text.
When the syntax of the hint text is incorrect, the hint text is ignored and will not be used.
here you can find undocumented hints.

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)



Best regards,

Rafi.