Oracle® Database SQL Reference 10g Release 2 (10.2) Part Number B14200-02 |
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You can associate comments with SQL statements and schema objects.
Comments can make your application easier for you to read and maintain. For example, you can include a comment in a statement that describes the purpose of the statement within your application. With the exception of hints, comments within SQL statements do not affect the statement execution. Please refer to "Using Hints" on using this particular form of comment.
A comment can appear between any keywords, parameters, or punctuation marks in a statement. You can include a comment in a statement in two ways:
Begin the comment with a slash and an asterisk (/*). Proceed with the text of the comment. This text can span multiple lines. End the comment with an asterisk and a slash (*/). The opening and terminating characters need not be separated from the text by a space or a line break.
Begin the comment with -- (two hyphens). Proceed with the text of the comment. This text cannot extend to a new line. End the comment with a line break.
Some of the tools used to enter SQL have additional restrictions. For example, if you are using SQL*Plus, by default you cannot have a blank line inside a multiline comment. For more information, please refer to the documentation for the tool you use as an interface to the database.
A SQL statement can contain multiple comments of both styles. The text of a comment can contain any printable characters in your database character set.
Example These statements contain many comments:
SELECT last_name, salary + NVL(commission_pct, 0), job_id, e.department_id /* Select all employees whose compensation is greater than that of Pataballa.*/ FROM employees e, departments d /*The DEPARTMENTS table is used to get the department name.*/ WHERE e.department_id = d.department_id AND salary + NVL(commission_pct,0) > /* Subquery: */ (SELECT salary + NVL(commission_pct,0) /* total compensation is salar + commission_pct */ FROM employees WHERE last_name = 'Pataballa'); SELECT last_name, -- select the name salary + NVL(commission_pct, 0),-- total compensation job_id, -- job e.department_id -- and department FROM employees e, -- of all employees departments d WHERE e.department_id = d.department_id AND salary + NVL(commission_pct, 0) > -- whose compensation -- is greater than (SELECT salary + NVL(commission_pct,0) -- the compensation FROM employees WHERE last_name = 'Pataballa') -- of Pataballa. ;
You can associate a comment with a table, view, materialized view, or column using the COMMENT
command. Comments associated with schema objects are stored in the data dictionary. Please refer to COMMENT for a description of comments.
You can use comments in a SQL statement to pass instructions, or hints, 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.
Note:
Hints should be used sparingly, and only after you have collected statistics on the relevant tables and evaluated the optimizer plan without hints using theEXPLAIN 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.A statement block can have only one comment containing hints, and that comment must follow the SELECT
, UPDATE
, INSERT
, MERGE
, or DELETE
keyword. Only two hints are used with INSERT
statements: The APPEND
hint always follows the INSERT
keyword, and the PARALLEL
hint can follow the INSERT
keyword.
The following syntax diagram shows hints contained in both styles of comments that Oracle supports within a statement block. The hint syntax must follow immediately after an INSERT
, UPDATE
, DELETE
, SELECT
, or MERGE
keyword that begins the statement block.
hint::=
where:
The plus sign (+) causes Oracle to interpret the comment as a list of hints. The plus sign must follow immediately after the comment delimiter. No space is permitted.
hint
is one of the hints discussed in this section. The space between the plus sign and the hint is optional. If the comment contains multiple hints, then separate the hints by at least one space.
string
is other commenting text that can be interspersed with the hints.
The --+
syntax requires that the entire comment be on a single line.
Oracle Database ignores hints and does not return an error under the following circumstances:
The hint contains misspellings or syntax errors. However, the database does consider other correctly specified hints in the same comment.
The comment containing the hint does not follow a DELETE
, INSERT
, MERGE
, SELECT
, or UPDATE
keyword.
A combination of hints conflict with each other. However, the database does consider other hints in the same comment.
The database environment uses PL/SQL version 1, such as Forms version 3 triggers, Oracle Forms 4.5, and Oracle Reports 2.5.
Many hints can apply both to specific tables or indexes and more globally to tables within a view or to columns that are part of indexes. The syntactic elements tablespec
and indexspec
define these global hints.
tablespec::=
You must specify the table to be accessed exactly as it appears in the statement. If the statement uses an alias for the table, then use the alias rather than the table name in the hint. However, do not include the schema name with the table name within the hint, even if the schema name appears in the statement.
See Also:
Oracle Database Performance Tuning Guide for information on the following topics:When to use global hints and how Oracle interprets them
Using EXPLAIN PLAN
to learn how the optimizer is executing a query
indexspec::=
When tablespec
is followed by indexspec
in the specification of a hint, a comma separating the table name and index name is permitted but not required. Commas are also permitted, but not required, to separate multiple occurrences of indexspec
.
Specifying a Query Block in a Hint
You can specify an optional query block name in many hints to specify the query block to which the hint applies. This syntax lets you specify in the outer query a hint that applies to an inline view. When you specify a hint in the query block itself to which the hint applies, you omit the @queryblock
syntax.
The syntax of the query block argument is of the form @
queryblock
, where queryblock
is an identifier that specifies a query block in the query. The queryblock
identifier can either be system-generated or user-specified.
The system-generated identifier can be obtained by using EXPLAIN
PLAN
for the query. Pretransformation query block names can be determined by running EXPLAIN
PLAN
for the query using the NO_QUERY_TRANSFORMATION
hint. See "NO_QUERY_TRANSFORMATION Hint".
The user-specified name can be set with the QB_NAME
hint. See "QB_NAME Hint".
Table 2-21 lists the hints by functional category and contains cross-references to the syntax and semantics for each hint. An alphabetical listing of the hints follows the table.
See Also:
Oracle Database Performance Tuning Guide for information on:descriptions of hint categories and when to use them
Table 2-21 Hints by Functional Category
Hint | Link to Syntax and Semantics |
---|---|
Optimization Goals and Approaches |
|
-- |
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Access Path Hints |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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Join Order Hints |
|
-- |
|
Join Operation Hints |
|
-- |
|
-- |
|
Parallel Execution Hints |
|
-- |
|
-- |
|
Query Transformation Hints |
|
-- |
|
-- |
|
-- |
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-- |
|
-- |
|
-- |
|
Other Hints |
|
-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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-- |
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This section provides syntax and semantics for all hints in alphabetical order.
The ALL_ROWS
hint instructs the optimizer to optimize a statement block with a goal of best throughput—that is, minimum total resource consumption. For example, the optimizer uses the query optimization approach to optimize this statement for best throughput:
SELECT /*+ ALL_ROWS */ employee_id, last_name, salary, job_id FROM employees WHERE employee_id = 7566;
If you specify either the ALL_ROWS
or the FIRST_ROWS
hint in a SQL statement, and if the data dictionary does not have statistics about tables accessed by the statement, then the optimizer uses default statistical values, such as allocated storage for such tables, to estimate the missing statistics and to subsequently choose an execution plan. These estimates might not be as accurate as those gathered by the DBMS_STATS
package, so you should use the DBMS_STATS
package to gather statistics.
If you specify hints for access paths or join operations along with either the ALL_ROWS
or FIRST_ROWS
hint, then the optimizer gives precedence to the access paths and join operations specified by the hints.
The APPEND
hint instructs the optimizer to use direct-path INSERT
if your database is running in serial mode. Your database is in serial mode if you are not using Enterprise Edition. Conventional INSERT
is the default in serial mode, and direct-path INSERT
is the default in parallel mode.
In direct-path INSERT
, data is appended to the end of the table, rather than using existing space currently allocated to the table. As a result, direct-path INSERT
can be considerably faster than conventional INSERT
.
See Also:
Oracle Database Administrator's Guide for information on direct-path inserts(See "Specifying a Query Block in a Hint", tablespec::=)
The CACHE
hint instructs the optimizer to place the blocks retrieved for the table at the most recently used end of the LRU list in the buffer cache when a full table scan is performed. This hint is useful for small lookup tables.
In the following example, the CACHE
hint overrides the default caching specification of the table:
SELECT /*+ FULL (hr_emp) CACHE(hr_emp) */ last_name FROM employees hr_emp;
The CACHE
and NOCACHE
hints affect system statistics table scans (long tables)
and table scans (short tables)
, as shown in the V$SYSSTAT
data dictionary view.
(See "Specifying a Query Block in a Hint", tablespec::=)
The CLUSTER
hint instructs the optimizer to use a cluster scan to access the specified table. This hint applies only to clustered tables.
Oracle can replace literals in SQL statements with bind variables, when it is safe to do so. This replacement is controlled with the CURSOR_SHARING
initialization parameter. The CURSOR_SHARING_EXACT
hint instructs the optimizer to switch this behavior off. In other words, Oracle executes the SQL statement without any attempt to replace literals with bind variables.
(See "Specifying a Query Block in a Hint", tablespec::=)
The DRIVING_SITE
hint instructs the optimizer to execute the query at a different site than that selected by the database. This hint is useful if you are using distributed query optimization.
For example:
SELECT /*+ DRIVING_SITE(departments) */ * FROM employees, departments@rsite WHERE employees.department_id = departments.department_id;
If this query is executed without the hint, then rows from departments
are sent to the local site, and the join is executed there. With the hint, the rows from employees
are sent to the remote site, and the query is executed there and the result set is returned to the local site.
(See "Specifying a Query Block in a Hint", tablespec::=)
The DYNAMIC_SAMPLING
hint instructs the optimizer how to 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 tablespec
.
The integer
value is 0
to 10,
indicating the degree of sampling.
If a cardinality statistic already exists for the table, then the optimizer uses it. Otherwise, the optimizer enables dynamic sampling to estimate the cardinality statistic.
If you specify tablespec
and the cardinality statistic already exists, then:
If there is no single-table predicate (a WHERE
clause that evaluates only one table), then the optimizer trusts the existing statistics and ignores this hint. For example, the following query will not result in any dynamic sampling if employees
is analyzed:
SELECT /*+ dynamic_sampling(e 1) */ count(*) FROM employees e;
If there is a single-table predicate, then the optimizer uses the existing cardinality statistic and estimates the selectivity of the predicate using the existing statistics.
To apply dynamic sampling to a specific table, use the following form of the hint:
SELECT /*+ dynamic_sampling(employees 1) */ * FROM employees WHERE ..,
See Also:
Oracle Database Performance Tuning Guide for information about dynamic sampling and the sampling levels that you can set(See "Specifying a Query Block in a Hint", tablespec::=)
The FACT
hint is used in the context of the star transformation. It instructs the optimizer that the table specified in tablespec
should be considered as a fact table.
The FIRST_ROWS
hint instructs Oracle to optimize an individual SQL statement for fast response, choosing the plan that returns the first n
rows most efficiently. For integer
, specify the number of rows to return.
Note:
TheFIRST_ROWS
hint specified without an argument, which optimizes for the best plan to return the first single row, is retained for backward compatibility and plan stability only.For example, the optimizer uses the query optimization approach to optimize the following statement for best response time:
SELECT /*+ FIRST_ROWS(10) */ employee_id, last_name, salary, job_id FROM employees WHERE department_id = 20;
In this example each department contains many employees. The user wants the first 10 employees of department 20 to be displayed as quickly as possible.
The optimizer ignores this hint in DELETE
and UPDATE
statement blocks and in SELECT
statement blocks that include any blocking operations, such as sorts or groupings. Such statements cannot be optimized for best response time, because Oracle Database must retrieve all rows accessed by the statement before returning the first row. If you specify this hint in any such statement, then the database optimizes for best throughput.
(See "Specifying a Query Block in a Hint", tablespec::=)
The FULL
hint instructs the optimizer to perform a full table scan for the specified table. For example:
SELECT /*+ FULL(e) */ employee_id, last_name FROM hr.employees e WHERE last_name LIKE :b1;
Oracle Database performs a full table scan on the employees
table to execute this statement, even if there is an index on the last_name
column that is made available by the condition in the WHERE
clause.
The employees
table has alias e
in the FROM
clause, so the hint must refer to the table by its alias rather than by its name. Do not specify schema names in the hint even if they are specified in the FROM
clause.
(See "Specifying a Query Block in a Hint", tablespec::=)
The HASH
hint instructs the optimizer to use a hash scan to access the specified table. This hint applies only to tables stored in a table cluster.
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX
hint instructs the optimizer to use an index scan for the specified table. You can use the INDEX
hint for function-based, domain, B-tree, bitmap, and bitmap join indexes.
The behavior of the hint depends on the indexspec
specification:
If the INDEX
hint specifies a single available index, then the database performs a scan on this index. The optimizer does not consider a full table scan or a scan of another index on the table.
For a hint on a combination of multiple indexes, Oracle recommends using INDEX_COMBINE
rather than INDEX
, because it is a more versatile hint. If the INDEX
hint specifies a list of available indexes, then the optimizer considers the cost of a scan on each index in the list and then performs the index scan with the lowest cost. The database can also choose to scan multiple indexes from this list and merge the results, if such an access path has the lowest cost. The database does not consider a full table scan or a scan on an index not listed in the hint.
If the INDEX
hint specifies no indexes, then the optimizer considers the cost of a scan on each available index on the table and then performs the index scan with the lowest cost. The database can also choose to scan multiple indexes and merge the results, if such an access path has the lowest cost. The optimizer does not consider a full table scan.
For example:
SELECT /*+ INDEX (employees emp_department_ix)*/ employee_id, department_id FROM employees WHERE department_id > 50;
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_ASC
hint instructs the optimizer to use an index scan for the specified table. If the statement uses an index range scan, then Oracle Database scans the index entries in ascending order of their indexed values. Each parameter serves the same purpose as in "INDEX Hint".
The default behavior for a range scan is to scan index entries in ascending order of their indexed values, or in descending order for a descending index. This hint does not change the default order of the index, and therefore does not specify anything more than the INDEX
hint. However, you can use the INDEX_ASC
hint to specify ascending range scans explicitly should the default behavior change.
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_COMBINE
hint instructs the optimizer to use a bitmap access path for the table. If indexspec
is omitted from the INDEX_COMBINE
hint, then the optimizer uses whatever Boolean combination of indexes has the best cost estimate for the table. If you specify indexspec
, then the optimizer tries to use some Boolean combination of the specified indexes. Each parameter serves the same purpose as in "INDEX Hint". For example:
SELECT /*+ INDEX_COMBINE(e emp_manager_ix emp_department_ix) */ * FROM employees e WHERE manager_id = 108 OR department_id = 110;
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_DESC
hint instructs the optimizer to use a descending index scan for the specified table. If the statement uses an index range scan and the index is ascending, then Oracle scans the index entries in descending order of their indexed values. In a partitioned index, the results are in descending order within each partition. For a descending index, this hint effectively cancels out the descending order, resulting in a scan of the index entries in ascending order. Each parameter serves the same purpose as in "INDEX Hint". For example:
SELECT /*+ INDEX_DESC(e emp_name_ix) */ * FROM employees e;
See Also:
Oracle Database Performance Tuning Guide for information on full scans(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_FFS
hint instructs the optimizer to perform a fast full index scan rather than a full table scan.
Each parameter serves the same purpose as in "INDEX Hint". For example:
SELECT /*+ INDEX_FFS(e emp_name_ix) */ first_name FROM employees e;
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_JOIN
hint 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.
Each parameter serves the same purpose as in "INDEX Hint". For example, the following query uses an index join to access the manager_id
and department_id
columns, both of which are indexed in the employees
table.
SELECT /*+ INDEX_JOIN(e emp_manager_ix emp_department_ix) */ department_id FROM employees e WHERE manager_id < 110 AND department_id < 50;
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_SS
hint instructs the optimizer to perform an index skip scan for the specified table. If the statement uses an index range scan, then Oracle scans the index entries in ascending order of their indexed values. In a partitioned index, the results are in ascending order within each partition.
Each parameter serves the same purpose as in "INDEX Hint". For example:
SELECT /*+ INDEX_SS(e emp_name_ix) */ last_name FROM employees e WHERE first_name = 'Steven';
See Also:
Oracle Database Performance Tuning Guide for information on index skip scans(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_SS_ASC
hint instructs the optimizer to perform an index skip scan for the specified table. If the statement uses an index range scan, then Oracle Database scans the index entries in ascending order of their indexed values. In a partitioned index, the results are in ascending order within each partition. Each parameter serves the same purpose as in "INDEX Hint".
The default behavior for a range scan is to scan index entries in ascending order of their indexed values, or in descending order for a descending index. This hint does not change the default order of the index, and therefore does not specify anything more than the INDEX_SS
hint. However, you can use the INDEX_SS_ASC
hint to specify ascending range scans explicitly should the default behavior change.
See Also:
Oracle Database Performance Tuning Guide for information on index skip scans(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The INDEX_SS_DESC
hint instructs the optimizer to perform an index skip scan for the specified table. If the statement uses an index range scan and the index is ascending, then Oracle scans the index entries in descending order of their indexed values. In a partitioned index, the results are in descending order within each partition. For a descending index, this hint effectively cancels out the descending order, resulting in a scan of the index entries in ascending order.
Each parameter serves the same purpose as in the "INDEX Hint". For example:
SELECT /*+ INDEX_SS_DESC(e emp_name_ix) */ last_name FROM employees e WHERE first_name = 'Steven';
See Also:
Oracle Database Performance Tuning Guide for information on index skip scans(See "Specifying a Query Block in a Hint", tablespec::=)
The LEADING
hint instructs the optimizer to use the specified set of tables as the prefix in the execution plan. This hint is more versatile than the ORDERED
hint. For example:
SELECT /*+ LEADING(e j) */ * FROM employees e, departments d, job_history j WHERE e.department_id = d.department_id AND e.hire_date = j.start_date;
The LEADING
hint is ignored if the tables specified cannot be joined first in the order specified because of dependencies in the join graph. If you specify two or more conflicting LEADING
hints, then all of them are ignored. If you specify the ORDERED
hint, it overrides all LEADING
hints.
(See "Specifying a Query Block in a Hint", tablespec::=)
The MERGE
hint lets you merge views in a query.
If a view's query block contains a GROUP BY
clause or DISTINCT
operator in the SELECT
list, then the optimizer can merge the view into the accessing statement only if complex view merging is enabled. Complex merging can also be used to merge an IN
subquery into the accessing statement if the subquery is uncorrelated.
For example:
SELECT /*+ MERGE(v) */ e1.last_name, e1.salary, v.avg_salary FROM employees e1, (SELECT department_id, avg(salary) avg_salary FROM employees e2 GROUP BY department_id) v WHERE e1.department_id = v.department_id AND e1.salary > v.avg_salary;
When the MERGE
hint is used without an argument, it should be placed in the view query block. When MERGE
is used with the view name as an argument, it should be placed in the surrounding query.
The MODEL_MIN_ANALYSIS
hint instructs the optimizer to omit some compile-time optimizations of spreadsheet rules—primarily detailed dependency graph analysis. Other spreadsheet optimizations, such as creating filters to selectively populate spreadsheet access structures and limited rule pruning, are still used by the optimizer.
This hint reduces compilation time because spreadsheet analysis can be lengthy if the number of spreadsheet rules is more than several hundreds.
The NOAPPEND
hint instructs the optimizer to use conventional INSERT
by disabling parallel mode for the duration of the INSERT
statement. Conventional INSERT
is the default in serial mode, and direct-path INSERT
is the default in parallel mode.
(See "Specifying a Query Block in a Hint", tablespec::=)
The NOCACHE
hint instructs the optimizer to place the blocks retrieved for the table 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. For example:
SELECT /*+ FULL(hr_emp) NOCACHE(hr_emp) */ last_name FROM employees hr_emp;
The CACHE
and NOCACHE
hints affect system statistics table
scans(long
tables)
and table
scans(short
tables)
, as shown in the V$SYSSTAT
view.
See Also:
Oracle Database Performance Tuning Guide for information on automatic caching of tables, depending on their size(See "Specifying a Query Block in a Hint")
The NO_EXPAND
hint instructs the optimizer not to consider 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. For example:
SELECT /*+ NO_EXPAND */ * FROM employees e, departments d WHERE e.manager_id = 108 OR d.department_id = 110;
See Also:
Oracle Database Performance Tuning Guide for a discussion of OR-expansion
the "USE_CONCAT Hint", which is the opposite of this hint
(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_FACT
hint is used in the context of the star transformation. It instruct the optimizer that the queried table should not be considered as a fact table.
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The NO_INDEX
hint instructs the optimizer not to use one or more indexes for the specified table. For example:
SELECT /*+ NO_INDEX(employees emp_empid) */ employee_id FROM employees WHERE employee_id > 200;
Each parameter serves the same purpose as in "INDEX Hint" with the following modifications:
If this hint specifies a single available index, then the optimizer does not consider a scan on this index. Other indexes not specified are still considered.
If this hint specifies a list of available indexes, then the optimizer does not consider a scan on any of the specified indexes. Other indexes not specified in the list are still considered.
If this hint specifies no indexes, then the optimizer does not consider a scan on any index on the table. This behavior is the same as a NO_INDEX
hint that specifies a list of all available indexes for the table.
The NO_INDEX
hint applies to function-based, B-tree, bitmap, cluster, or domain indexes. If a NO_INDEX
hint and an index hint (INDEX
, INDEX_ASC
, INDEX_DESC
, INDEX_COMBINE
, or INDEX_FFS
) both specify the same indexes, then the database ignores both the NO_INDEX
hint and the index hint for the specified indexes and considers those indexes for use during execution of the statement.
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The NO_INDEX_FFS
hint instructs the optimizer to exclude a fast full index scan of the specified indexes on the specified table. Each parameter serves the same purpose as in the "INDEX Hint". For example:
SELECT /*+ NO_INDEX_FFS(items item_order_ix) */ order_id FROM order_items items;
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The NO_INDEX_SS
hint instructs the optimizer to exclude a skip scan of the specified indexes on the specified table. Each parameter serves the same purpose as in the "INDEX Hint".
See Also:
Oracle Database Performance Tuning Guide for information on index skip scans(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_MERGE
hint instructs the optimizer not to combine the outer query and any inline view queries into a single query.
This hint lets you have more influence over the way in which the view is accessed. For example, the following statement causes view seattle_dept
not to be merged.:
SELECT /*+NO_MERGE(seattle_dept)*/ e1.last_name, seattle_dept.department_name FROM employees e1, (SELECT location_id, department_id, department_name FROM departments WHERE location_id = 1700) seattle_dept WHERE e1.department_id = seattle_dept.department_id;
When you use the NO_MERGE
hint in the view query block, specify it without an argument. When you specify NO_MERGE
in the surrounding query, specify it with the view name as an argument.
(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_PARALLEL
hint overrides a PARALLEL
parameter in the DDL that created or altered the table. For example:
SELECT /*+ NO_PARALLEL(hr_emp) */ last_name FROM employees hr_emp;
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The NO_PARALLEL_INDEX
hint overrides a PARALLEL
parameter in the DDL that created or altered the index, thus avoiding a parallel index scan operation.
The NOPARALLEL_INDEX
hint has been deprecated. Use the NO_PARALLEL_INDEX
hint instead.
(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_PUSH_PRED
hint instructs the optimizer not to push a join predicate into the view. For example:
SELECT /*+ NO_MERGE(v) NO_PUSH_PRED(v) */ * FROM employees e, (SELECT manager_id FROM employees ) v WHERE e.manager_id = v.manager_id(+) AND e.employee_id = 100;
(See "Specifying a Query Block in a Hint")
The NO_PUSH_SUBQ
hint instructs the optimizer to evaluate nonmerged subqueries as the last step in the execution plan. Doing so can improve performance if the subquery is relatively expensive or does not reduce the number of rows significantly.
This hint prevents the optimizer from using parallel join bitmap filtering.
(See "Specifying a Query Block in a Hint")
The NO_REWRITE
hint instructs the optimizer to disable query rewrite for the query block, overriding the setting of the parameter QUERY_REWRITE_ENABLED
. For example:
SELECT /*+ NO_REWRITE */ sum(s.amount_sold) AS dollars FROM sales s, times t WHERE s.time_id = t.time_id GROUP BY t.calendar_month_desc;
The NO_QUERY_TRANSFORMATION
hint instructs the optimizer to skip all query transformations, including but not limited to OR
-expansion, view merging, subquery unnesting, star transformation, and materialized view rewrite. For example:
SELECT /*+ NO_QUERY_TRANSFORMATION */ employee_id, last_name FROM (SELECT * FROM employees e) v WHERE v.last_name = 'Smith';
(See "Specifying a Query Block in a Hint")
The NO_STAR_TRANSFORMATION
hint instructs the optimizer not to perform star query transformation.
(See "Specifying a Query Block in a Hint")
Use of the NO_UNNEST
hint turns off unnesting .
(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_USE_HASH
hint instructs the optimizer to exclude hash joins when joining each specified table to another row source using the specified table as the inner table. For example:
SELECT /*+ NO_USE_HASH(e d) */ * FROM employees e, departments d WHERE e.department_id = d.department_id;
(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_USE_MERGE
hint instructs the optimizer to exclude sort-merge joins when joining each specified table to another row source using the specified table as the inner table. For example:
SELECT /*+ NO_USE_MERGE(e d) */ * FROM employees e, departments d WHERE e.department_id = d.department_id ORDER BY d.department_id;
(See "Specifying a Query Block in a Hint", tablespec::=)
The NO_USE_NL
hint instructs the optimizer to exclude nested loops joins when joining each specified table to another row source using the specified table as the inner table. For example:
SELECT /*+ NO_USE_NL(l h) */ * FROM orders h, order_items l WHERE l.order_id = h.order_id AND l.order_id > 3500;
When this hint is specified, only hash join and sort-merge joins are considered for the specified tables. However, in some cases tables can be joined only by using nested loops. In such cases, the optimizer ignores the hint for those tables.
The NO_XML_QUERY_REWRITE
hint instructs the optimizer to prohibit the rewriting of XPath expressions in SQL statements. For example:
SELECT /*+NO_XML_QUERY_REWRITE*/ XMLQUERY('<A/>') FROM dual;
The ORDERED
hint instructs Oracle to join tables in the order in which they appear in the FROM
clause. Oracle recommends that you use the LEADING
hint, which is more versatile than the ORDERED
hint.
When you omit the ORDERED
hint from a SQL statement requiring a join, the optimizer chooses the order in which to join the tables. You might want to use the ORDERED
hint to specify a join order if you know something that the optimizer does not know about the number of rows selected from each table. Such information lets you choose an inner and outer table better than the optimizer could.
The following query is an example of the use of the ORDERED
hint:
SELECT /*+ORDERED */ o.order_id, c.customer_id, l.unit_price * l.quantity FROM customers c, order_items l, orders o WHERE c.cust_last_name = :b1 AND o.customer_id = c.customer_id AND o.order_id = l.order_id;
(See "Specifying a Query Block in a Hint", tablespec::=)
The PARALLEL
hint instructs the optimizer to use the specified number of concurrent servers for a parallel operation. The hint applies to the SELECT
, INSERT
, MERGE
, UPDATE
, and DELETE
portions of a statement, as well as to the table scan portion.
Note:
The number of servers that can be used is twice the value in thePARALLEL
hint, if sorting or grouping operations also take place.If any parallel restrictions are violated, then the hint is ignored.
The integer
value specifies the degree of parallelism for the specified table. Specifying DEFAULT
or no value signifies that the query coordinator should examine the settings of the initialization parameters to determine the default degree of parallelism. In the following example, the PARALLEL
hint overrides the degree of parallelism specified in the employees
table definition:
SELECT /*+ FULL(hr_emp) PARALLEL(hr_emp, 5) */ last_name FROM employees hr_emp;
In the next example, the PARALLEL
hint overrides the degree of parallelism specified in the employees
table definition and instructs the optimizer to use the default degree of parallelism determined by the initialization parameters.
SELECT /*+ FULL(hr_emp) PARALLEL(hr_emp, DEFAULT) */ last_name FROM employees hr_emp;
Oracle ignores parallel hints on temporary tables. Please refer to CREATE TABLE and Oracle Database Concepts for more information on parallel execution.
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The PARALLEL_INDEX
hint instructs the optimizer to use the specified number of concurrent servers to parallelize index range scans for partitioned indexes.
The integer
value indicates the degree of parallelism for the specified index. Specifying DEFAULT
or no value signifies that the query coordinator should examine the settings of the initialization parameters to determine the default degree of parallelism. For example, the following hint indicates three parallel execution processes are to be used:
SELECT /*+ PARALLEL_INDEX(table1, index1, 3) */
(See "Specifying a Query Block in a Hint", tablespec::=)
The PQ_DISTRIBUTE
hint instructs the optimizer how to distribute rows of joined tables among producer and consumer query servers. Such distribution can improve the performance of parallel join operations.
outer_distribution
is the distribution for the outer table.
inner_distribution
is the distribution for the inner table.
The values of the distributions are HASH
, BROADCAST
, PARTITION
, and NONE
. Only six combinations table distributions are valid, as described in Table 2-22:
Table 2-22 Distribution Hint Combinations
Distribution | Description |
---|---|
|
The rows of each table are mapped to consumer query servers, using a hash function on the join keys. When mapping is complete, each query server performs the join between a pair of resulting partitions. This distribution is recommended when the tables are comparable in size and the join operation is implemented by hash-join or sort merge join. |
|
All rows of the outer table are broadcast to each query server. The inner table rows are randomly partitioned. This distribution is recommended when the outer table is very small compared with the inner table. As a general rule, use this distribution when the inner table size multiplied by the number of query servers is greater than the outer table size. |
|
All rows of the inner table are broadcast to each consumer query server. The outer table rows are randomly partitioned. This distribution is recommended when the inner table is very small compared with the outer table. As a general rule, use this distribution when the inner table size multiplied by the number of query servers is less than the outer table size. |
|
The rows of the outer table are mapped using the partitioning of the inner table. The inner table must be partitioned on the join keys. This distribution is recommended when the number of partitions of the outer table is equal to or nearly equal to a multiple of the number of query servers; for example, 14 partitions and 15 query servers. Note: The optimizer ignores this hint if the inner table is not partitioned or not equijoined on the partitioning key. |
|
The rows of the inner table are mapped using the partitioning of the outer table. The outer table must be partitioned on the join keys. This distribution is recommended when the number of partitions of the outer table is equal to or nearly equal to a multiple of the number of query servers; for example, 14 partitions and 15 query servers. Note: The optimizer ignores this hint if the outer table is not partitioned or not equijoined on the partitioning key. |
|
Each query server performs the join operation between a pair of matching partitions, one from each table. Both tables must be equipartitioned on the join keys. |
For example, given two tables r
and s
that are joined using a hash join, the following query contains a hint to use hash distribution:
SELECT /*+ORDERED PQ_DISTRIBUTE(s HASH, HASH) USE_HASH (s)*/ column_list
FROM r,s
WHERE r.c=s.c;
To broadcast the outer table r
, the query is:
SELECT /*+ORDERED PQ_DISTRIBUTE(s BROADCAST, NONE) USE_HASH (s) */ column_list
FROM r,s
WHERE r.c=s.c;
See Also:
Oracle Database Concepts for more information on how Oracle parallelizes join operations(See "Specifying a Query Block in a Hint", tablespec::=)
The PUSH_PRED
hint instructs the optimizer to push a join predicate into the view. For example:
SELECT /*+ NO_MERGE(v) PUSH_PRED(v) */ * FROM employees e, (SELECT manager_id FROM employees ) v WHERE e.manager_id = v.manager_id(+) AND e.employee_id = 100;
(See "Specifying a Query Block in a Hint")
The PUSH_SUBQ
hint instructs the optimizer to evaluate nonmerged subqueries at the earliest possible step in the execution plan. Generally, subqueries that are not merged are executed as the last step in the execution plan. If the subquery is relatively inexpensive and reduces the number of rows significantly, then evaluating the subquery earlier can improve performance.
This hint has no effect if the subquery is applied to a remote table or one that is joined using a merge join.
This hint forces the optimizer to use parallel join bitmap filtering.
(See "Specifying a Query Block in a Hint")
Use the QB_NAME
hint to define a name for a query block. This name can then be used in a hint in the outer query or even in a hint in an inline view to affect query execution on the tables appearing in the named query block.
If two or more query blocks have the same name, or if the same query block is hinted twice with different names, then the optimizer ignores all the names and the hints referencing that query block. Query blocks that are not named using this hint have unique system-generated names. These names can be displayed in the plan table and can also be used in hints within the query block, or in query block hints. For example:
SELECT /*+ QB_NAME(qb) FULL(@qb e) */ employee_id, last_name FROM employees e WHERE last_name = 'Smith';
(See "Specifying a Query Block in a Hint")
The REWRITE
hint instructs 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.
Oracle does not consider views outside of the list. If you do not specify a view list, then Oracle searches for an eligible materialized view and always uses it regardless of the cost of the final plan.
See Also:
Oracle Database Concepts and Oracle Database Advanced Replication for more information on materialized views
Oracle Data Warehousing Guide for more information on using REWRITE
with materialized views
The RULE
hint disables the use of the optimizer. This hint is not supported and should not be used.
(See "Specifying a Query Block in a Hint")
The STAR_TRANSFORMATION
hint instructs the optimizer to 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. For example:
SELECT /*+ STAR_TRANSFORMATION */ * FROM sales s, times t, products p, channels c WHERE s.time_id = t.time_id AND s.prod_id = p.product_id AND s.channel_id = c.channel_id AND p.product_status = 'obsolete';
Even if the hint is specified, there is no guarantee that the transformation will take place. The optimizer generates the subqueries only if it seems reasonable to do so. If no subqueries are generated, then there is no transformed query, and the best plan for the untransformed query is used, regardless of the hint.
See Also:
Oracle Data Warehousing Guide for a full discussion of star transformation.
Oracle Database Reference for more information on the STAR_TRANSFORMATION_ENABLED
initialization parameter.
(See "Specifying a Query Block in a Hint")
The UNNEST
hint instructs the optimizer to unnest and merge the body of the subquery into the body of the query block that contains it, allowing the optimizer to consider them together when evaluating access paths and joins.
Before a subquery is unnested, the optimizer first verifies whether the statement is valid. The statement must then must pass heuristic and query optimization tests. The UNNEST
hint instructs the optimizer to check the subquery block for validity only. If the subquery block is valid, then subquery unnesting is enabled without checking the heuristics or costs.
See Also:
"Collection Unnesting: Examples" for more information on unnesting nested subqueries and the conditions that make a subquery block valid
Oracle Database Performance Tuning Guide for additional information on subquery unnesting
(See "Specifying a Query Block in a Hint")
The USE_CONCAT
hint instructs the optimizer to transform combined OR
-conditions in the WHERE
clause of a query into a compound query using the UNION
ALL
set operator. Without this hint, this transformation occurs only if the cost of the query using the concatenations is cheaper than the cost without them. The USE_CONCAT
hint overrides the cost consideration. For example:
SELECT /*+ USE_CONCAT */ * FROM employees e WHERE manager_id = 108 OR department_id = 110;
See Also:
the "NO_EXPAND Hint", which is the opposite of this hint and Oracle Database Performance Tuning Guide for a discussion ofOR
-expansion(See "Specifying a Query Block in a Hint", tablespec::=)
The USE_HASH
hint instructs the optimizer to join each specified table with another row source using a hash join. For example:
SELECT /*+ USE_HASH(l h) */ * FROM orders h, order_items l WHERE l.order_id = h.order_id AND l.order_id > 3500;
(See "Specifying a Query Block in a Hint", tablespec::=)
The USE_MERGE
hint instructs the optimizer to join each specified table with another row source using a sort-merge join. For example:
SELECT /*+ USE_MERGE(employees departments) */ * FROM employees, departments WHERE employees.department_id = departments.department_id;
Use of the USE_NL
and USE_MERGE
hints is recommended with the LEADING
and ORDERED
hints. The optimizer uses those hints when the referenced table is forced to be the inner table of a join. The hints are ignored if the referenced table is the outer table.
The USE_NL
hint instructs the optimizer to join each specified table to another row source with a nested loops join, using the specified table as the inner table.
(See "Specifying a Query Block in a Hint", tablespec::=)
The USE_NL
hint instructs the optimizer to join each specified table to another row source with a nested loops join, using the specified table as the inner table.
Use of the USE_NL
and USE_MERGE
hints is recommended with the LEADING
and ORDERED
hints. The optimizer uses those hints when the referenced table is forced to be the inner table of a join. The hints are ignored if the referenced table is the outer table.
In the following example, where a nested loop is forced through a hint, orders
is accessed through a full table scan and the filter condition l.order_id = h.order_id
is applied to every row. For every row that meets the filter condition, order_items
is accessed through the index order_id
.
SELECT /*+ USE_NL(l h) */ h.customer_id, l.unit_price * l.quantity FROM orders h ,order_items l WHERE l.order_id = h.order_id;
Adding an INDEX
hint to the query could avoid the full table scan on orders
, resulting in an execution plan similar to one used on larger systems, even though it might not be particularly efficient here.
(See "Specifying a Query Block in a Hint", tablespec::=, indexspec::=)
The USE_NL_WITH_INDEX
hint instructs the optimizer to join the specified table to another row source with a nested loops join using the specified table as the inner table. For example:
SELECT /*+ USE_NL_WITH_INDEX(l item_product_ix) */ * FROM orders h, order_items l WHERE l.order_id = h.order_id AND l.order_id > 3500;
The following conditions apply:
If no index is specified, then the optimizer must be able to use some index with at least one join predicate as the index key.
If an index is specified, then the optimizer must be able to use that index with at least one join predicate as the index key.