Comments

You can create two types of comments:

  • Comments within SQL statements are stored as part of the application code that executes the SQL statements.

  • Comments associated with individual schema or nonschema objects are stored in the data dictionary along with metadata on the objects themselves.

Comments Within SQL Statements

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. Refer to "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, 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, employee_id, 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 salary + commission_pct */
        FROM employees 
        WHERE last_name = 'Pataballa')
  ORDER BY last_name, employee_id;

SELECT last_name,                                   -- select the name
       employee_id                                  -- employee id
       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
  ORDER BY last_name                                -- and order by last name
           employee_id                              -- and employee id.
;

Comments on Schema and Nonschema Objects

You can use the COMMENT command to associate a comment with a schema object (table, view, materialized view, operator, indextype, mining model) or a nonschema object (edition) using the COMMENT command. You can also create a comment on a column, which is part of a table schema object. Comments associated with schema and nonschema objects are stored in the data dictionary. Refer to COMMENT for a description of this form of comment.

Hints

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

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

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

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

Using Hints

A statement block can have only one comment containing hints, and that comment must follow the SELECT, UPDATE, INSERT, MERGE, or DELETE 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::=

Description of hint.gif follows
Description of the illustration hint.gif

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.

  • A global hint refers to multiple query blocks. Refer to "Specifying Multiple Query Blocks in a Global Hint" for more information.

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.

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. When you specify a hint in the query block itself to which the hint applies, you omit the @queryblock syntax.

  • 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".

Specifying Global Hints

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::=

Description of tablespec.gif follows
Description of the illustration tablespec.gif

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.

Note:

Specifying a global hint using the tablespec clause does not work for queries that use ANSI joins, because the optimizer generates additional views during parsing. Instead, specify @queryblock to indicate the query block to which the hint applies.

indexspec::=

Description of indexspec.gif follows
Description of the illustration indexspec.gif

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 Multiple Query Blocks in a Global Hint 

Oracle Database ignores global hints that refer to multiple query blocks. To avoid this issue, Oracle recommends that you specify the object alias in the hint instead of using tablespec and indexspec.

For example, consider the following view v and table t:

CREATE VIEW v AS
  SELECT e.last_name, e.department_id, d.location_id
  FROM employees e, departments d
  WHERE e.department_id = d.department_id;
 
CREATE TABLE t AS
  SELECT * from employees
  WHERE employee_id < 200;

Note:

The following examples use the EXPLAIN PLAN statement, which enables you to display the execution plan and determine if a hint is honored or ignored. Refer to EXPLAIN PLAN for more information.

The LEADING hint is ignored in the following query because it refers to multiple query blocks, that is, the main query block containing table t and the view query block v:

EXPLAIN PLAN
  SET STATEMENT_ID = 'Test 1'
  INTO plan_table FOR
    (SELECT /*+ LEADING(v.e v.d t) */ *
     FROM t, v
     WHERE t.department_id = v.department_id);

The following SELECT statement returns the execution plan, which shows that the LEADING hint was ignored:

SELECT id, LPAD(' ',2*(LEVEL-1))||operation operation, options, object_name,  object_alias
  FROM plan_table
  START WITH id = 0 AND statement_id = 'Test 1'
  CONNECT BY PRIOR id = parent_id AND statement_id = 'Test 1'
  ORDER BY id;

 ID OPERATION            OPTIONS    OBJECT_NAME   OBJECT_ALIAS
--- -------------------- ---------- ------------- --------------------
  0 SELECT STATEMENT
  1   HASH JOIN
  2     HASH JOIN
  3       TABLE ACCESS   FULL       DEPARTMENTS   D@SEL$2
  4       TABLE ACCESS   FULL       EMPLOYEES     E@SEL$2
  5     TABLE ACCESS     FULL       T             T@SEL$1

The LEADING hint is honored in the following query because it refers to object aliases, which can be found in the execution plan that was returned by the previous query:

EXPLAIN PLAN
  SET STATEMENT_ID = 'Test 2'
  INTO plan_table FOR
    (SELECT /*+ LEADING(E@SEL$2 D@SEL$2 T@SEL$1) */ *
     FROM t, v
     WHERE t.department_id = v.department_id);

The following SELECT statement returns the execution plan, which shows that the LEADING hint was honored:

SELECT id, LPAD(' ',2*(LEVEL-1))||operation operation, options,
  object_name, object_alias
  FROM plan_table
  START WITH id = 0 AND statement_id = 'Test 2'
  CONNECT BY PRIOR id = parent_id AND statement_id = 'Test 2'
  ORDER BY id;

 ID OPERATION            OPTIONS    OBJECT_NAME   OBJECT_ALIAS
--- -------------------- ---------- ------------- --------------------
  0 SELECT STATEMENT
  1   HASH JOIN
  2     HASH JOIN
  3       TABLE ACCESS   FULL       EMPLOYEES     E@SEL$2
  4       TABLE ACCESS   FULL       DEPARTMENTS   D@SEL$2
  5     TABLE ACCESS     FULL       T             T@SEL$1

See Also:

Oracle Database Performance Tuning Guide for information on the following topics:

Hints by Functional Category

Table 3-21 lists the hints by functional category and contains cross-references to the syntax and semantics for each hint. An alphabetical reference of the hints follows the table.

Table 3-21 Hints by Functional Category

Hint Link to Syntax and Semantics

Optimization Goals and Approaches

ALL_ROWS Hint

FIRST_ROWS Hint

Access Path Hints

CLUSTER Hint

--

FULL Hint

--

HASH Hint

--

INDEX Hint

NO_INDEX Hint

--

INDEX_ASC Hint

INDEX_DESC Hint

--

INDEX_COMBINE Hint

--

INDEX_JOIN Hint

--

INDEX_FFS Hint

--

INDEX_SS Hint

--

INDEX_SS_ASC Hint

--

INDEX_SS_DESC Hint

--

NATIVE_FULL_OUTER_JOIN Hint

NO_NATIVE_FULL_OUTER_JOIN Hint

--

NO_INDEX_FFS Hint

--

NO_INDEX_SS Hint

Join Order Hints

ORDERED Hint

--

LEADING Hint

Join Operation Hints

USE_HASH Hint

NO_USE_HASH Hint

--

USE_MERGE Hint

NO_USE_MERGE Hint

--

USE_NL Hint

USE_NL_WITH_INDEX Hint

NO_USE_NL Hint

Parallel Execution Hints

PARALLEL Hint

NO_PARALLEL Hint

--

PARALLEL_INDEX Hint

NO_PARALLEL_INDEX Hint

--

PQ_DISTRIBUTE Hint

Online Application Upgrade Hints

CHANGE_DUPKEY_ERROR_INDEX Hint

--

IGNORE_ROW_ON_DUPKEY_INDEX Hint

--

RETRY_ON_ROW_CHANGE Hint

Query Transformation Hints

FACT Hint

NO_FACT Hint

--

MERGE Hint

NO_MERGE Hint

--

NO_EXPAND Hint

USE_CONCAT Hint

--

REWRITE Hint

NO_REWRITE Hint

--

UNNEST Hint

NO_UNNEST Hint

--

STAR_TRANSFORMATION Hint

NO_STAR_TRANSFORMATION Hint

--

NO_QUERY_TRANSFORMATION Hint

XML Hints

NO_XMLINDEX_REWRITE Hint

--

NO_XML_QUERY_REWRITE Hint

Other Hints

APPEND Hint

APPEND_VALUES Hint

NOAPPEND Hint

--

CACHE Hint

NOCACHE Hint

--

CURSOR_SHARING_EXACT Hint

--

DRIVING_SITE Hint

--

DYNAMIC_SAMPLING Hint

--

MODEL_MIN_ANALYSIS Hint

--

MONITOR Hint

--

NO_MONITOR Hint

--

OPT_PARAM Hint

--

PUSH_PRED Hint

NO_PUSH_PRED Hint

--

PUSH_SUBQ Hint

NO_PUSH_SUBQ Hint

--

PX_JOIN_FILTER Hint

NO_PX_JOIN_FILTER Hint

--

QB_NAME Hint

--

RESULT_CACHE Hint

NO_RESULT_CACHE Hint


Alphabetical Listing of Hints

This section provides syntax and semantics for all hints in alphabetical order.

ALL_ROWS Hint

Description of all_rows_hint.gif follows
Description of the illustration all_rows_hint.gif

The ALL_ROWS hint instructs the optimizer to optimize a statement block with a goal of best throughput, which 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 = 107;

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.

APPEND Hint

Description of append_hint.gif follows
Description of the illustration append_hint.gif

The APPEND hint instructs the optimizer to use direct-path INSERT with the subquery syntax of the INSERT statement.

  • Conventional INSERT is the default in serial mode. In serial mode, direct path can be used only if you include the APPEND hint.

  • Direct-path INSERT is the default in parallel mode. In parallel mode, conventional insert can be used only if you specify the NOAPPEND hint.

The decision whether the INSERT will go parallel or not is independent of the APPEND hint.

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.

The APPEND hint is only supported with the subquery syntax of the INSERT statement, not the VALUES clause. If you specify the APPEND hint with the VALUES clause, it is ignored and conventional insert will be used. To use direct-path INSERT with the VALUES clause, refer to "APPEND_VALUES Hint".

See Also:

"NOAPPEND Hint " for information on that hint and Oracle Database Administrator's Guide for information on direct-path inserts

APPEND_VALUES Hint

Description of append_values_hint.gif follows
Description of the illustration append_values_hint.gif

The APPEND_VALUES hint instructs the optimizer to use direct-path INSERT with the VALUES clause. If you do not specify this hint, then conventional INSERT is used.

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.

The APPEND_VALUES hint can be used to greatly enhance performance. Some examples of its uses are:

  • In an Oracle Call Interface (OCI) program, when using large array binds or array binds with row callbacks

  • In PL/SQL, when loading a large number of rows with a FORALL loop that has an INSERT statement with a VALUES clause

The APPEND_VALUES hint is only supported with the VALUES clause of the INSERT statement. If you specify the APPEND_VALUES hint with the subquery syntax of the INSERT statement, it is ignored and conventional insert will be used. To use direct-path INSERT with a subquery, refer to "APPEND Hint".

See Also:

Oracle Database Administrator's Guide for information on direct-path inserts

CACHE Hint

Description of cache_hint.gif follows
Description of the illustration cache_hint.gif

(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.

CHANGE_DUPKEY_ERROR_INDEX Hint

Description of change_dupkey_error_index.gif follows
Description of the illustration change_dupkey_error_index.gif

Note:

The CHANGE_DUPKEY_ERROR_INDEX, IGNORE_ROW_ON_DUPKEY_INDEX, and RETRY_ON_ROW_CHANGE hints are unlike other hints in that they have a semantic effect. The general philosophy explained in "Hints" does not apply for these three hints.

The CHANGE_DUPKEY_ERROR_INDEX hint provides a mechanism to unambiguously identify a unique key violation for a specified set of columns or for a specified index. When a unique key violation occurs for the specified index, an ORA-38911 error is reported instead of an ORA-001.

This hint applies to INSERT, UPDATE operations. If you specify an index, then the index must exist and be unique. If you specify a column list instead of an index, then a unique index whose columns match the specified columns in number and order must exist.

This use of this hint results in error messages if specific rules are violated. Refer to IGNORE_ROW_ON_DUPKEY_INDEX Hint for details.

Note:

This hint disables both APPEND mode and parallel DML.

See Also:

IGNORE_ROW_ON_DUPKEY_INDEX Hint for information on that hint and Oracle Database Performance Tuning Guide for more information on using the online application upgrade related hints

CLUSTER Hint

Description of cluster_hint.gif follows
Description of the illustration cluster_hint.gif

(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 tables in an indexed cluster.

CURSOR_SHARING_EXACT Hint

Description of cursor_sharing_exact_hint.gif follows
Description of the illustration cursor_sharing_exact_hint.gif

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. When you specify this hint, Oracle executes the SQL statement without any attempt to replace literals with bind variables.

DRIVING_SITE Hint

Description of driving_site_hint.gif follows
Description of the illustration driving_site_hint.gif

(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.

DYNAMIC_SAMPLING Hint

Description of dynamic_sampling_hint.gif follows
Description of the illustration dynamic_sampling_hint.gif

(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

FACT Hint

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Description of the illustration fact_hint.gif

(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.

FIRST_ROWS Hint

Description of first_rows_hint.gif follows
Description of the illustration first_rows_hint.gif

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.

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 Also:

"ALL_ROWS Hint" for additional information on the FIRST_ROWS hint and statistics

FULL Hint

Description of full_hint.gif follows
Description of the illustration full_hint.gif

(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.

HASH Hint

Description of hash_hint.gif follows
Description of the illustration hash_hint.gif

(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 in a hash cluster.

IGNORE_ROW_ON_DUPKEY_INDEX Hint

Description of ignore_row_on_dupkey_index.gif follows
Description of the illustration ignore_row_on_dupkey_index.gif

Note:

The CHANGE_DUPKEY_ERROR_INDEX, IGNORE_ROW_ON_DUPKEY_INDEX, and RETRY_ON_ROW_CHANGE hints are unlike other hints in that they have a semantic effect. The general philosophy explained in "Hints" does not apply for these three hints.

The IGNORE_ROW_ON_DUPKEY_INDEX hint applies only to single-table INSERT operations. It is not supported for UPDATE, DELETE, MERGE, or multitable insert operations. IGNORE_ROW_ON_DUPKEY_INDEX causes the statement to ignore a unique key violation for a specified set of columns or for a specified index. When a unique key violation is encountered, a row-level rollback occurs and execution resumes with the next input row. If you specify this hint when inserting data with DML error logging enabled, then the unique key violation is not logged and does not cause statement termination.

The semantic effect of this hint results in error messages if specific rules are violated:

  • If you specify index, then the index must exist and be unique. Otherwise, the statement causes ORA-38913.

  • You must specify exactly one index. If you specify no index, then the statement causes ORA-38912. If you specify more than one index, then the statement causes ORA-38915.

  • You can specify either a CHANGE_DUPKEY_ERROR_INDEX or IGNORE_ROW_ON_DUPKEY_INDEX hint in an INSERT statement, but not both. If you specify both, then the statement causes ORA-38915.

As with all hints, a syntax error in the hint causes it to be silently ignored. The result will be that ORA-00001 will be caused, just as if no hint were used.

Note:

This hint disables both APPEND mode and parallel DML.

See Also:

CHANGE_DUPKEY_ERROR_INDEX Hint for information on that hint and Oracle Database Performance Tuning Guide for more information on using the online application upgrade related hints

INDEX Hint

Description of index_hint.gif follows
Description of the illustration index_hint.gif

(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;

INDEX_ASC Hint

Description of index_asc_hint.gif follows
Description of the illustration index_asc_hint.gif

(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.

INDEX_COMBINE Hint

Description of index_combine_hint.gif follows
Description of the illustration index_combine_hint.gif

(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;

INDEX_DESC Hint

Description of index_desc_hint.gif follows
Description of the illustration index_desc_hint.gif

(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

INDEX_FFS Hint

Description of index_ffs_hint.gif follows
Description of the illustration index_ffs_hint.gif

(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;

INDEX_JOIN Hint

Description of index_join_hint.gif follows
Description of the illustration index_join_hint.gif

(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;

INDEX_SS Hint

Description of index_ss_hint.gif follows
Description of the illustration index_ss_hint.gif

(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

INDEX_SS_ASC Hint

Description of index_ss_asc_hint.gif follows
Description of the illustration index_ss_asc_hint.gif

(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

INDEX_SS_DESC Hint

Description of index_ss_desc_hint.gif follows
Description of the illustration index_ss_desc_hint.gif

(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

LEADING Hint

Description of leading_hint.gif follows
Description of the illustration leading_hint.gif

(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.

MERGE Hint

Description of merge_hint.gif follows
Description of the illustration merge_hint.gif

(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
   ORDER BY e1.last_name;

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.

MODEL_MIN_ANALYSIS Hint

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Description of the illustration model_min_analysis_hint.gif

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.

MONITOR Hint

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Description of the illustration monitor_hint.gif

The MONITOR hint forces real-time SQL monitoring for the query, even if the statement is not long running. This hint is valid only when the parameter CONTROL_MANAGEMENT_PACK_ACCESS is set to DIAGNOSTIC+TUNING.

See Also:

Oracle Database Performance Tuning Guide for more information about real-time SQL monitoring

NATIVE_FULL_OUTER_JOIN Hint

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Description of the illustration native_foj_hint.gif

The NATIVE_FULL_OUTER_JOIN hint instructs the optimizer to use native full outer join, which is a native execution method based on a hash join.

See Also:

NOAPPEND Hint

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Description of the illustration noappend_hint.gif

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.

NOCACHE Hint

Description of nocache_hint.gif follows
Description of the illustration nocache_hint.gif

(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

NO_EXPAND Hint

Description of no_expand_hint.gif follows
Description of the illustration no_expand_hint.gif

(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:

The "USE_CONCAT Hint", which is the opposite of this hint

NO_FACT Hint

Description of no_fact_hint.gif follows
Description of the illustration no_fact_hint.gif

(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.

NO_INDEX Hint

Description of no_index_hint.gif follows
Description of the illustration no_index_hint.gif

(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.

NO_INDEX_FFS Hint

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Description of the illustration no_index_ffs_hint.gif

(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 "NO_INDEX Hint". For example:

SELECT /*+ NO_INDEX_FFS(items item_order_ix) */ order_id
  FROM order_items items;

NO_INDEX_SS Hint

Description of no_index_ss_hint.gif follows
Description of the illustration no_index_ss_hint.gif

(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 "NO_INDEX Hint".

See Also:

Oracle Database Performance Tuning Guide for information on index skip scans

NO_MERGE Hint

Description of no_merge_hint.gif follows
Description of the illustration no_merge_hint.gif

(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.

NO_MONITOR Hint

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Description of the illustration no_monitor_hint.gif

The NO_MONITOR hint disables real-time SQL monitoring for the query, even if the query is long running.

NO_NATIVE_FULL_OUTER_JOIN Hint

Description of no_native_foj_hint.gif follows
Description of the illustration no_native_foj_hint.gif

The NO_NATIVE_FULL_OUTER_JOIN hint instructs the optimizer to exclude the native execution method when joining each specified table. Instead, the full outer join is executed as a union of left outer join and anti-join.

NO_PARALLEL Hint

Description of no_parallel_hint.gif follows
Description of the illustration no_parallel_hint.gif

(See "Specifying a Query Block in a Hint", tablespec::=)

The NO_PARALLEL hint instructs the optimizer to run the statement serially. This hint overrides the value of the PARALLEL_DEGREE_POLICY initialization parameter. It also overrides a PARALLEL parameter in the DDL that created or altered the table. For example, the following SELECT statement will run serially:

ALTER TABLE employees PARALLEL 8;
SELECT /*+ NO_PARALLEL(hr_emp) */ last_name
  FROM employees hr_emp;

See Also:

NOPARALLEL Hint

The NOPARALLEL hint has been deprecated. Use the NO_PARALLEL hint instead.

NO_PARALLEL_INDEX Hint

Description of no_parallel_index_hint.gif follows
Description of the illustration no_parallel_index_hint.gif

(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.

See Also:

"Note on Parallel Hints" for more information on the parallel hints

NOPARALLEL_INDEX Hint

The NOPARALLEL_INDEX hint has been deprecated. Use the NO_PARALLEL_INDEX hint instead.

NO_PUSH_PRED Hint

Description of no_push_pred_hint.gif follows
Description of the illustration no_push_pred_hint.gif

(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;

NO_PUSH_SUBQ Hint

Description of no_push_subq_hint.gif follows
Description of the illustration no_push_subq_hint.gif

(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.

NO_PX_JOIN_FILTER Hint

Description of no_px_join_filter_hint.gif follows
Description of the illustration no_px_join_filter_hint.gif

This hint prevents the optimizer from using parallel join bitmap filtering.

NO_QUERY_TRANSFORMATION Hint

Description of no_query_transformatn_hint.gif follows
Description of the illustration no_query_transformatn_hint.gif

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';

NO_RESULT_CACHE Hint

Description of no_result_cache_hint.gif follows
Description of the illustration no_result_cache_hint.gif

The optimizer caches query results in the result cache if the RESULT_CACHE_MODE initialization parameter is set to FORCE. In this case, the NO_RESULT_CACHE hint disables such caching for the current query.

If the query is executed from OCI client and OCI client result cache is enabled, then the NO_RESULT_CACHE hint disables caching for the current query.

NO_REWRITE Hint

Description of no_rewrite_hint.gif follows
Description of the illustration no_rewrite_hint.gif

(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;

NOREWRITE Hint

The NOREWRITE hint has been deprecated. Use the NO_REWRITE hint instead.

NO_STAR_TRANSFORMATION Hint

Description of no_star_transformation_hint.gif follows
Description of the illustration no_star_transformation_hint.gif

(See "Specifying a Query Block in a Hint")

The NO_STAR_TRANSFORMATION hint instructs the optimizer not to perform star query transformation.

NO_STATEMENT_QUEUING Hint

Description of no_statement_queuing_hint.gif follows
Description of the illustration no_statement_queuing_hint.gif

The NO_STATEMENT_QUEUING hint influences whether or not a statement is queued with parallel statement queuing.

When PARALLEL_DEGREE_POLICY is set to AUTO, this hint enables a statement to bypass the parallel statement queue. However, a statement that bypasses the statement queue can potentially cause the system to exceed the maximum number of parallel execution servers defined by the value of the PARALLEL_SERVERS_TARGET initialization parameter, which determines the limit at which parallel statement queuing is initiated.

There is no guarantee that the statement that bypasses the parallel statement queue receives the number of parallel execution servers requested because only the number of parallel execution servers available on the system, up to the value of the PARALLEL_MAX_SERVERS initialization parameter, can be allocated.

For example:

SELECT /*+ NO_STATEMENT_QUEUING */ emp.last_name, dpt.department_name
  FROM employees emp, departments dpt
  WHERE emp.department_id = dpt.department_id;

NO_UNNEST Hint

Description of no_unnest_hint.gif follows
Description of the illustration no_unnest_hint.gif

(See "Specifying a Query Block in a Hint")

Use of the NO_UNNEST hint turns off unnesting .

NO_USE_HASH Hint

Description of no_use_hash_hint.gif follows
Description of the illustration no_use_hash_hint.gif

(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;

NO_USE_MERGE Hint

Description of no_use_merge_hint.gif follows
Description of the illustration no_use_merge_hint.gif

(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;

NO_USE_NL Hint

Description of no_use_nl_hint.gif follows
Description of the illustration no_use_nl_hint.gif

(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 > 2400;

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.

NO_XML_QUERY_REWRITE Hint

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Description of the illustration no_xml_query_rewrite_hint.gif

The NO_XML_QUERY_REWRITE hint instructs the optimizer to prohibit the rewriting of XPath expressions in SQL statements. By prohibiting the rewriting of XPath expressions, this hint also prohibits the use of any XMLIndexes for the current query. For example:

SELECT /*+NO_XML_QUERY_REWRITE*/ XMLQUERY('<A/>' RETURNING CONTENT)
  FROM DUAL;

NO_XMLINDEX_REWRITE Hint

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Description of the illustration no_xmlindex_rewrite_hint.gif

The NO_XMLINDEX_REWRITE hint instructs the optimizer not to use any XMLIndex indexes for the current query. For example:

SELECT /*+NO_XMLINDEX_REWRITE*/ count(*) 
  FROM warehouses
  WHERE existsNode(warehouse_spec, '/Warehouse/Building') = 1;

See Also:

"NO_XML_QUERY_REWRITE Hint" for another way to disable the use of XMLIndexes

OPT_PARAM Hint

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Description of the illustration opt_param_hint.gif

The OPT_PARAM hint lets you set an initialization parameter for the duration of the current query only. This hint is valid only for the following parameters: OPTIMIZER_DYNAMIC_SAMPLING, OPTIMIZER_INDEX_CACHING, OPTIMIZER_INDEX_COST_ADJ, OPTIMIZER_SECURE_VIEW_MERGING, and STAR_TRANSFORMATION_ENABLED. For example, the following hint sets the parameter STAR_TRANSFORMATION_ENABLED to TRUE for the statement to which it is added:

SELECT /*+ OPT_PARAM('star_transformation_enabled' 'true') */ *
  FROM ... ;

Parameter values that are strings are enclosed in single quotation marks. Numeric parameter values are specified without quotation marks.

ORDERED Hint

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Description of the illustration ordered_hint.gif

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 = 'Taylor'
    AND o.customer_id = c.customer_id
    AND o.order_id = l.order_id;

PARALLEL Hint

Note on Parallel Hints 

Beginning with Oracle Database 11g Release 2 (11.2.0.1), the PARALLEL and NO_PARALLEL hints are statement-level hints and supersede the earlier object-level hints: PARALLEL_INDEX, NO_PARALLEL_INDEX, and previously specified PARALLEL and NO_PARALLEL hints. For PARALLEL, if you specify integer, then that degree of parallelism will be used for the statement. If you omit integer, then the database computes the degree of parallelism. All the access paths that can use parallelism will use the specified or computed degree of parallelism.

In the syntax diagrams below, parallel_hint_statement shows the syntax for statement-level hints, and parallel_hint_object shows the syntax for object-level hints. Object-level hints are supported for backward compatibility, and are superseded by statement-level hints.

parallel_hint_statement::=

Description of parallel_hint_statement.gif follows
Description of the illustration parallel_hint_statement.gif

parallel_hint_object::=

Description of parallel_hint_object.gif follows
Description of the illustration parallel_hint_object.gif

(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. This hint overrides the value of the PARALLEL_DEGREE_POLICY initialization parameter. The hint applies to the SELECT, INSERT, MERGE, UPDATE, and DELETE portions of a statement, as well as to the table scan portion. If any parallel restrictions are violated, then the hint is ignored.

Note:

The number of servers that can be used is twice the value in the PARALLEL hint, if sorting or grouping operations also take place.

For a statement-level PARALLEL hint:

  • PARALLEL: The statement always is run parallel, and the database computes the degree of parallelism, which can be 2 or greater.

  • PARALLEL (DEFAULT): The optimizer calculates a degree of parallelism equal to the number of CPUs available on all participating instances times the value of the PARALLEL_THREADS_PER_CPU initialization parameter.

  • PARALLEL (AUTO): The database computes the degree of parallelism, which can be 1 or greater. If the computed degree of parallelism is 1, then the statement runs serially.

  • PARALLEL (MANUAL): The optimizer is forced to use the parallel settings of the objects in the statement.

  • PARALLEL (integer): The optimizer uses the degree of parallelism specified by integer.

In the following example, the optimizer calculates the degree of parallelism. The statement always runs in parallel.

SELECT /*+ PARALLEL */ last_name
  FROM employees;

In the following example, the optimizer calculates the degree of parallelism, but that degree may be 1, in which case the statement will run serially.

SELECT /*+ PARALLEL (AUTO) */ last_name
  FROM employees;

In the following example, the PARALLEL hint advises the optimizer to use the degree of parallelism currently in effect for the table itself, which is 5:

CREATE TABLE parallel_table (col1 number, col2 VARCHAR2(10)) PARALLEL 5; 

SELECT /*+ PARALLEL (MANUAL) */ col2
  FROM parallel_table;

For an object-level PARALLEL hint:

  • PARALLEL: The query coordinator should examine the settings of the initialization parameters to determine the default degree of parallelism.

  • PARALLEL (integer): The optimizer uses the degree of parallelism specified by integer.

  • PARALLEL (DEFAULT): The optimizer calculates a degree of parallelism equal to the number of CPUs available on all participating instances times the value of the PARALLEL_THREADS_PER_CPU initialization parameter.

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 calculate a degree of parallelism equal to the number of CPUs available on all participating instances times the value of the PARALLEL_THREADS_PER_CPU initialization parameter.

SELECT /*+ FULL(hr_emp) PARALLEL(hr_emp, DEFAULT) */ last_name
  FROM employees hr_emp;

Oracle ignores parallel hints on temporary tables. Refer to CREATE TABLE and Oracle Database Concepts for more information on parallel execution.

See Also:

PARALLEL_INDEX Hint

Description of parallel_index_hint.gif follows
Description of the illustration parallel_index_hint.gif

(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, full scans, and fast full 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 Also:

"Note on Parallel Hints" for more information on the parallel hints

PQ_DISTRIBUTE Hint

Description of pq_distribute_hint.gif follows
Description of the illustration pq_distribute_hint.gif

(See "Specifying a Query Block in a Hint", tablespec::=)

The PQ_DISTRIBUTE hint instructs the optimizer how to distribute rows among producer and consumer query servers. You can control the distribution of rows for either joins or for load.

Control of Distribution for Load You can control the distribution of rows for parallel INSERT ... SELECT and parallel CREATE TABLE ... AS SELECT statements to direct how rows should be distributed between the producer (query) and the consumer (load) servers. Use the upper branch of the syntax by specifying a single distribution method. The values of the distribution methods and their semantics are described in Table 3-22.

Table 3-22 Distribution Values for Load

Distribution Description

NONE

No distribution. That is the query and load operation are combined into each query server. All servers will load all partitions. This lack of distribution is useful to avoid the overhead of distributing rows where there is no skew. Skew can occur due to empty segments or to a predicate in the statement that filters out all rows evaluated by the query. If skew occurs due to using this method, then use either RANDOM or RANDOM_LOCAL distribution instead.

Note: Use this distribution with care. Each partition loaded requires a minimum of 512 KB per process of PGA memory. If you also use compression, then approximately 1.5 MB of PGA memory is consumer per server.

PARTITION

This method uses the partitioning information of tablespec to distribute the rows from the query servers to the load servers. Use this distribution method when it is not possible or desirable to combine the query and load operations, when the number of partitions being loaded is greater than or equal to the number of load servers, and the input data will be evenly distributed across the partitions being loaded—that is, there is no skew.

RANDOM

This method distributes the rows from the producers in a round-robin fashion to the consumers. Use this distribution method when the input data is highly skewed.

RANDOM_LOCAL

This method distributes the rows from the producers to a set of servers that are responsible for maintaining a given set of partitions. Two or more servers can be loading the same partition, but no servers are loading all partitions. Use this distribution method when the input data is skewed and combining query and load operations is not possible due to memory constraints.


For example, in the following direct-load insert operation, the query and load portions of the operation are combined into each query server:

INSERT /*+ APPEND PARALLEL(target_table, 16) PQ_DISTRIBUTE(target_table, NONE) */
  INTO target_table
  SELECT * FROM source_table;
 

In the following table creation example, the optimizer uses the partitioning of target_table to distribute the rows:

CREATE /*+ PQ_DISTRIBUTE(target_table, PARTITION) */ TABLE target_table
  NOLOGGING PARALLEL 16
  PARTITION BY HASH (l_orderkey) PARTITIONS 512
  AS SELECT * FROM source_table; 

Control of Distribution for Joins You control the distribution method for joins by specifying two distribution methods, as shown in the lower branch of the syntax diagram, one distribution for the outer table and one distribution for the inner table.

  • 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 3-23:

Table 3-23 Distribution Values for Joins

Distribution Description

HASH, HASH

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.

BROADCAST, NONE

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.

NONE, BROADCAST

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.

PARTITION, NONE

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.

NONE, PARTITION

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.

NONE, NONE

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 Performance Tuning Guide for more information on how Oracle parallelizes join operations

PUSH_PRED Hint

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Description of the illustration push_pred_hint.gif

(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;

PUSH_SUBQ Hint

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Description of the illustration push_subq_hint.gif

(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.

PX_JOIN_FILTER Hint

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This hint forces the optimizer to use parallel join bitmap filtering.

QB_NAME Hint

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Description of the illustration qb_name_hint.gif

(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';

RESULT_CACHE Hint

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Description of the illustration result_cache_hint.gif

The RESULT_CACHE hint instructs the database to cache the results of the current query or query fragment in memory and then to use the cached results in future executions of the query or query fragment. The hint is recognized in the top-level query, the subquery_factoring_clause, or FROM clause inline view. The cached results reside in the result cache memory portion of the shared pool.

A cached result is automatically invalidated whenever a database object used in its creation is successfully modified. This hint takes precedence over settings of the RESULT_CACHE_MODE initialization parameter.

The query is eligible for result caching only if all functions entailed in the query—for example, built-in or user-defined functions or virtual columns—are deterministic.

If the query is executed from OCI client and OCI client result cache is enabled, then RESULT_CACHE hint enables client caching for the current query.

See Also:

Oracle Database Performance Tuning Guide for information about using this hint, Oracle Database Reference for information about the RESULT_CACHE_MODE initialization parameter, and Oracle Call Interface Programmer's Guide for more information about the OCI result cache and usage guidelines

RETRY_ON_ROW_CHANGE Hint

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Description of the illustration retry_on_row_change.gif

Note:

The CHANGE_DUPKEY_ERROR_INDEX, IGNORE_ROW_ON_DUPKEY_INDEX, and RETRY_ON_ROW_CHANGE hints are unlike other hints in that they have a semantic effect. The general philosophy explained in "Hints" does not apply for these three hints.

This hint is valid only for UPDATE and DELETE operations. It is not supported for INSERT or MERGE operations. When you specify this hint, the operation is retried when the ORA_ROWSCN for one or more rows in the set has changed from the time the set of rows to be modified is determined to the time the block is actually modified.

See Also:

IGNORE_ROW_ON_DUPKEY_INDEX Hint and CHANGE_DUPKEY_ERROR_INDEX Hint for information on those hints and Oracle Database Performance Tuning Guide for more information on using the online application upgrade related hints

REWRITE Hint

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Description of the illustration rewrite_hint.gif

(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:

STAR_TRANSFORMATION Hint

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Description of the illustration star_transformation_hint.gif

(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 */ s.time_id, s.prod_id, s.channel_id
  FROM sales s, times t, products p, channels c
  WHERE s.time_id = t.time_id
    AND s.prod_id = p.prod_id
    AND s.channel_id = c.channel_id
    AND c.channel_desc = 'Tele Sales';

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:

STATEMENT_QUEUING Hint

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The NO_STATEMENT_QUEUING hint influences whether or not a statement is queued with parallel statement queuing.

When PARALLEL_DEGREE_POLICY is not set to AUTO, this hint enables a statement to be considered for parallel statement queuing, but to run only when enough parallel processes are available to run at the requested DOP. The number of available parallel execution servers, before queuing is enabled, is equal to the difference between the number of parallel execution servers in use and the maximum number allowed in the system, which is defined by the PARALLEL_SERVERS_TARGET initialization parameter.

For example:

SELECT /*+ STATEMENT_QUEUING */ emp.last_name, dpt.department_name
  FROM employees emp, departments dpt
  WHERE emp.department_id = dpt.department_id;

UNNEST Hint

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Description of the illustration unnest_hint.gif

(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 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:

USE_CONCAT Hint

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Description of the illustration use_concat_hint.gif

(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

USE_HASH Hint

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Description of the illustration use_hash_hint.gif

(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 > 2400;

USE_MERGE Hint

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Description of the illustration use_merge_hint.gif

(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.

USE_NL Hint

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.

Description of use_nl_hint.gif follows
Description of the illustration use_nl_hint.gif

(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.

USE_NL_WITH_INDEX Hint

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Description of the illustration use_nl_with_index_hint.gif

(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 > 2400;

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.