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) |
