If you enabled subquery unnesting with the UNNEST_SUBQUERY parameter, then the NO_UNNEST hint turns it off for specific subquery blocks. Hi , I try to figure out if i am doing something wrong. If certain indexes are given as arguments, then the optimizer tries to use some Boolean combination of those particular bitmap indexes. The outer table must be partitioned on the join keys. If you have not gathered statistics, then defaults are used. Yiannis Tsesmelis Sep 19 2011. The NOPARALLEL hint overrides a PARALLEL specification in the table clause. where table specifies the name or alias of the table on which the full table scan is to be performed. If the statement uses an alias for the table, then you must use the alias rather than the table name in the hint. Therefore, even though you gave the INDEX hint, the optimizer might not necessarily use that hint, because the optimizer might have determined that the requested index cannot be used due to the join methods and access paths selected by the optimizer. Only a subset of distribution method combinations for the joined tables is valid, as explained in Table5-1. The USE_MERGE hint causes Oracle to join each specified table with another row source with a sort-merge join. The NOREWRITE hint disables query rewrite for the query block, overriding the setting of the parameter QUERY_REWRITE_ENABLED. If the view is a single-table, then the hint is not propagated. FIRST_ROWS(n) affords greater precision, because it instructs Oracle to choose the plan that returns the first n rows most efficiently. If you have not gathered statistics, then defaults are used. where table is the name or alias for the table at which site the execution should take place. With the hint, the rows from emp are sent to the remote site, and the query is executed there, returning the result to the local site. The following example illustrates the NOPARALLEL hint: The PQ_DISTRIBUTE hint improves the performance of parallel join operations. Unless the hints are inside the base view, they might not be honored from a query against the view. Use the NOREWRITE hint on any query block of a request. If you specify the ORDERED hint, then it overrides all LEADING hints. The INDEX_DESC hint explicitly chooses an index scan for the specified table. 1 Try adding WHERE ROWNUM >= 1: SELECT COL1 FROM SCHEMA.VIEW_NAME1 WHERE DATE_VAL > (SELECT DATE_VAL FROM SCHEMA.VIEW_NAME2 WHERE ROWNUM >= 1) That predicate looks totally extraneous, or the kind of thing Oracle would just ignore, but the ROWNUM pseudo-function is special. If so, then you can force the optimizer to choose a nested loops join by using the USE_NL hint. where table is the name or alias of a table to be used as the inner table of a nested loops join. The PUSH_SUBQ hint causes non-merged subqueries to be evaluated at the earliest possible place in the execution plan. Oracle Undocumented Parameter - _PUSH_JOIN_PREDICATE Hi, I have a long standing question regarding one of the undocumented parameters of Oracle namely '_push_join_predicate'. So for example, if only the last branch of the UNION ALL was rewritten with materialized view MV, Oracle replaces the first two branches with a the equivalent GROUPING SET query, as follows: The NOREWRITE hint disables query rewrite for the query block, overriding the setting of the parameter QUERY_REWRITE_ENABLED. The INDEX hint explicitly chooses an index scan for the specified table. The hint can then take two values, separated by commas after the table name. Each query server performs the join operation between a pair of matching partitions, one from each table. The NO_PUSH_PRED hint prevents pushing of a join predicate into the view. For example, if you specify a hint in a query that selects against a complex view, then that hint is not honored, because it is not pushed inside the view. For a column having only two distinct values, the optimizer assumes each value appears in 50% of the rows, so the cost-based approach is likely to choose a full table scan rather than an index scan. Thus, emp.deptno can be anti-joined to dept.deptno to select all employees who are not in a set of departments, and you can get a list of all employees who are not in the shipping or receiving departments. The DRIVING_SITE hint forces query execution to be done at a different site than that selected by Oracle. in the SELECT statement is ignored because the employee table does not appear in the FROM clause of the SELECT statement. This is the normal behavior of blocks in the buffer cache. The first value specifies the degree of parallelism for the given table, and the second value specifies how the table is to be split among the Oracle Real Application Clusters instances. where table specifies the name or alias of the table to be accessed by a hash scan. The NO_MERGE hint causes Oracle not to merge mergeable views. Answer: When you have a view (or an in-line view), the Oracle cost-based optimizer may take the value of a join predicate and "push" it into the main view, thereby improving SQL optimization . If the statement is not valid, then subquery unnesting cannot proceed. For more information on Oracle SQL Analyze, see the Database Tuning with the Oracle Tuning Pack manual. The PUSH_PRED hint forces pushing of a join predicate into the view. where table specifies the name or alias of the table to be accessed by a cluster scan. The CHOOSE hint causes the optimizer to choose between the rule-based and cost-based approaches for a SQL statement. These estimates might not be as accurate as those gathered by the DBMS_STATS package. The INDEX_ASC hint explicitly chooses an index scan for the specified table. You can also use hints to improve the plan. The number of servers that can be used is twice the value in the PARALLEL hint, if sorting or grouping operations also take place. The USE_NL hint causes Oracle to join each specified table to another row source with a nested loops join, using the specified table as the inner table. As a result, direct-path INSERT can be considerably faster than conventional INSERT. The optimizer also considers different permutations of the small tables. In this statement, the USE_NL hint explicitly chooses a nested loops join with the customers table as the inner table: In many cases, a nested loops join returns the first row faster than a sort merge join. where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a hash join. There are six combinations for table distribution. If a view's query contains a GROUP BY clause or DISTINCT operator in the SELECT list, then the optimizer can merge the view's query into the accessing statement only if complex view merging is enabled. This hint also specifies that the table should be split among all of the available instances, with the default degree of parallelism on each instance. As a general rule, use the Broadcast/None hint when inner table size * number of query servers > outer table size, All rows of the inner table are broadcast to each consumer query server. Hints allow you to make decisions usually made by the optimizer. Multicolumn IN-lists will not use an index. Multicolumn IN-lists will not use an index. This hint can optionally specify one or more indexes: For example, consider this query that selects the name, height, and weight of all male patients in a hospital: Assume that there is an index on the SEX column and that this column contains the values m and f. If there are equal numbers of male and female patients in the hospital, then the query returns a relatively large percentage of the table's rows, and a full table scan is likely to be faster than an index scan. This section describes hint behavior with mergeable views. UNNEST_SUBQUERY first verifies if the statement is valid. The PUSH_SUBQ hint causes non-merged subqueries to be evaluated at the earliest possible place in the execution plan. Oracle9i Database Concepts for more information about star plans. A statement block is any one of the following statements or parts of statements: For example, a compound query consisting of two component queries combined by the UNION operator has two statement blocks, one for each component query. When mapping is complete, each query server performs the join between a pair of resulting partitions. This comment can only follow the SELECT, UPDATE, or DELETE keyword. If the view is a single-table, then the hint is not propagated. For example, consider this statement, which joins the accounts and customers tables. For particular queries, you could us a profile or baseline to control it. The user wants the first 10 employees of department #20 to be displayed as quickly as possible. The table name within the hint should not include the schema name if the schema name is present in the statement. The first value specifies the degree of parallelism for the given table. The optimizer ignores the distribution hint, if both tables are serial. where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a sort merge join. This hint can optionally specify one or more indexes: For example, consider this query that selects the name, height, and weight of all male patients in a hospital: Assume that there is an index on the SEX column and that this column contains the values m and f. If there are equal numbers of male and female patients in the hospital, then the query returns a relatively large percentage of the table's rows, and a full table scan is likely to be faster than an index scan. The first value specifies the degree of parallelism for the given table, and the second value specifies how the table is to be split among the Oracle Real Application Cluster instances. The PUSH_SUBQ hint causes non-merged subqueries to be evaluated at the earliest possible step in the execution plan. Using this hint overrides decisions the optimizer would normally make. If the subquery is relatively expensive or . With nonmergeable views, optimization approach and goal hints inside the view are ignored; the top-level query decides the optimization mode. You must specify the table to be accessed exactly as it appears in the statement. Specifies the name or alias of the table associated with the indexes to be merged. Consider the following SELECT statement: It causes V2 not to be merged and specifies access path hints for the employee and department tables. Oracle9i Database Concepts and Oracle9i Application Developer's Guide - Fundamentals for more information on materialized views. In particular, hints inside views or on views are handled differently, depending on whether the view is mergeable into the top-level query. The PARALLEL hint must use the table alias, if an alias is specified in the query. Because nonmergeable views are optimized separately from the top-level query, access path and join hints inside the view are preserved. 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 ALL_ROWS hint explicitly chooses the cost-based approach to optimize a statement block with a goal of best throughput (that is, minimum total resource consumption). The higher the level, the more effort the compiler puts into dynamic sampling and the more broadly it is applied. The most precise method is to order the tables in the FROM clause in the order of the keys in the index, with the large table last. The HASH hint explicitly chooses a hash scan to access the specified table. The ORDERED_PREDICATES hint forces the optimizer to preserve the order of predicate evaluation, except for predicates used as index keys. The first value specifies the degree of parallelism for the given table. If you do not use the ORDERED_PREDICATES hint, then Oracle evaluates all predicates in the following order: Remember, you cannot use the ORDERED_PREDICATES hint to preserve the order of predicate evaluation on index keys. If this query is executed without the hint, then rows from dept are sent to the local site, and the join is executed there. The HASH hint explicitly chooses a hash scan to access the specified table. The INDEX hint explicitly chooses an index scan for the specified table. When you want to specify hints for tables that appear inside views, Oracle recommends using global hints instead of embedding the hint in the view. The INDEX_ASC hint explicitly chooses an index scan for the specified table. Specifying DEFAULT or no value signifies the query coordinator should examine the settings of the initialization parameters to determine the default degree of parallelism. It caches the table only if a future table scan is likely to find the cached blocks. 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, subsequently, to choose an execution plan. If a global hint references a UNION or UNION ALL view, then the hint is applied to the first branch that contains the hinted table. This hint has no effect if the subquery is applied to a remote table or one that is joined using a merge join. The push_pred operation is normally done automatically by the optimizer, but you can specify it as a hint: select /*+ push_pred */ . Sampling defaults to cursor level unless you specify a table. As illustrated in Figure5-1, the SQL IN predicate can be evaluated using a join to intersect two sets. For a specific query, place the MERGE_AJ, HASH_AJ, or NL_AJ hint into the NOT IN subquery. The view V2 performs a join between the view V1 and the department table. Generally, this transformation occurs only if the cost of the query using the concatenations is cheaper than the cost without them. The FIRST_ROWS hint, which optimizes for the best plan to return the first single row, is retained for backward compatibility and plan stability. Conventional INSERT is the default in serial mode, and direct-path INSERT is the default in parallel mode). When using hints, in some cases, you might need to specify a full set of hints in order to ensure the optimal execution plan. where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a hash join. If you include a hint (except the RULE hint) in a statement block, then the optimizer automatically uses the cost-based approach. This option is useful for small lookup tables. The NO_PUSH_SUBQ hint causes non-merged subqueries to be evaluated as the last step in the execution plan. If the statement does not use aliases, then the table name is the default alias. The second hint specifies a full table scan for the department table referenced in the view V2. hint is one of the hints discussed in this section. The second hint specifies a full table scan for the department table referenced in the view V2. Oracle uses these 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. However, you might want to use the INDEX_ASC hint to specify ascending range scans explicitly should the default behavior change. Optimization approach and goal hints can occur in a top-level query or inside views. A star plan has the largest table in the query last in the join order and joins it with a nested loops join on a concatenated index. You cannot specify more than five. Comments containing hints can appear only after these keywords. Oracle9i Database Concepts for more information on how Oracle parallelizes join operations. Consider the following view definitions and SELECT statement: The view V1 retrieves all employees whose employee number is less than 100. Specifies an index on which an index scan is to be performed. Even if the hint is given, there is no guarantee that the transformation will take place. This hint can be used with either rule-based or cost-based optimization. push_pred_hint::= Text description of push_pred_hint. Oracle9i SQL Reference for more information on comments. Note: The optimizer ignores this hint if the outer table is not partitioned or not equijoined on the partitioning key. The use of hints involves extra code that must be managed, checked, and controlled. I've tried setting a hint explicitly in the view /*+ PUSH_PRED (v) */ but the plan does not change. All rows of the outer table are broadcast to each query server. The USE_CONCAT hint forces combined OR conditions in the WHERE clause of a query to be transformed into a compound query using the UNION ALL set operator. In the majority of cases the Cost Based Optimiser (when properly setup) will perform better than manually hinted SQL. This hint 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. You must specify at least two indexes. Each query server performs the join operation between a pair of matching partitions, one from each table. The USE_MERGE hint causes Oracle to join each specified table with another row source, using a sort-merge join. For example, if you specify a hint in a query that selects against a complex view, then that hint is not honored, because it is not pushed inside the view. The hint can take two values separated by commas after the table name. Maps the rows of the outer table, using the partitioning of the inner table. Generally, subqueries that are not merged are . 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. Automatic caching of small tables is disabled for tables that are created or altered with the CACHE attribute. The following query does both selectivity and cardinality estimation for emp: Oracle does not encourage the use of hints inside or on views (or subqueries). "Global Hints" for information on how to create global hints. The DYNAMIC_SAMPLING hint lets you control dynamic sampling to improve server performance by determining more accurate selectivity and cardinality estimates. If you specify hints incorrectly, then Oracle ignores them but does not return an error: The optimizer recognizes hints only when using the cost-based approach. Then use the following hints: where facts is the table and fact_concat is the index. For a specific query, place the HASH_SJ, MERGE_SJ, or NL_SJ hint into the EXISTS subquery. Parallel execution hints in a top-level query override such hints inside a referenced view. It applies only to tables stored in a cluster. There are two global hints in the SELECT statement. Oracle performs a full table scan on the accounts table to execute this statement, even if there is an index on the accno column that is made available by the condition in the WHERE clause. You can transform any table hint in this chapter into a global hint by using an extended syntax for the table name, described as follows. The ROWID hint explicitly chooses a table scan by rowid for the specified table. Do this by specifying how rows of joined tables should be distributed among producer and consumer query servers. Each parameter serves the same purpose as in the INDEX hint. The following example illustrates the NOPARALLEL hint: The PQ_DISTRIBUTE hint improves parallel join operation performance. A nested loops join can return the first row after reading the first selected row from one table and the first matching row from the other and combining them, while a sort merge join cannot return the first row until after reading and sorting all selected rows of both tables and then combining the first rows of each sorted row source. Starting with Oracle9i, Release 2 (9.2), small tables are automatically cached, according to the following criteria: Number of blocks < 20 or 2% of total cached blocks, whichever is larger, Larger than a small table, but < 10% of total cached blocks. The INDEX_JOIN hint explicitly instructs the optimizer to use an index join as an access path. The optimizer bases its selection on the presence of statistics for the tables accessed by the statement. If the statement uses an alias for the table, then use the alias rather than the table name in the hint. I personally experienced scenarios where a query (involving multiple big joins) would show up - 'Pushed Predicate' in the plan and would take long hours to execute. Use of the USE_NL and USE_MERGE hints is recommended with the ORDERED hint. The NO_INDEX hint applies to function-based, B-tree, bitmap, cluster, or domain indexes. For example, this statement joins table TAB1 to table TAB2 and then joins the result to table TAB3: The STAR hint forces a star query plan to be used, if possible. Join Predicate not pushing down to Complex Inline View Hi Tom,I have just faced a problem in my work.Here is a similar case for my work.CREATE TABLE TABLE_1 (MY_ID VARCHAR2(20),REVISION NUMBER(3,0),DATE_FROM DATE,DATE_TO DATE,STATUS VARCHAR2(50),MS_USER VARCHAR2(50));ALTER TABLE TABLE_1 ADD CONSTRAINT TABLE1_PK PRIM Generally, subqueries that are not merged are executed as . The NO_PUSH_SUBQ hint causes non-merged subqueries to be evaluated as the last step in the execution plan. index specifies an index on which an index scan is to be performed. ? where table specifies the name or alias of the table on which the table access by rowid is to be performed. That is, even if there is more than one matching row in t2 for a row in t1, the row in t1 is returned only once. The NOAPPEND hint enables conventional INSERT by disabling parallel mode for the duration of the INSERT statement. 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. If the comment contains multiple hints, then each hints must be separated by at least one space. inner_distribution is the distribution for the inner table. This causes view dallasdept not to be merged. If the data dictionary has statistics for at least one of these tables, then the optimizer uses the cost-based approach and optimizes with the goal of best throughput. The optimizer ignores the distribution hint if both tables are serial. However, Oracle recommends using INDEX_COMBINE rather than INDEX for bitmap indexes, because it is a more versatile hint. Such information lets you choose an inner and outer table better than the optimizer could. The REWRITE hint forces the cost-based optimizer to rewrite a query in terms of materialized views, when possible, without cost consideration. The NO_PUSH_PRED hint prevents pushing of a join predicate into the view. Is other commenting text that can be interspersed with the hints. Pushing Join Predicates is a good thing, so don't ever use the no_push_pred hint because it will never improve query performance. The CURSOR_SHARING_EXACT hint causes this behavior to be switched off. The first hint specifies an index scan for the employee table referenced in the view V1, which is referenced in the view V2. Note: The optimizer ignores this hint if the inner table is not partitioned or not equijoined on the partitioning key. When you want to specify hints for tables that appear inside views, use global hints instead of embedding the hint in the view. For example, the optimizer uses the cost-based approach to optimize this statement for best response time: In this example each department contains many employees. where table specifies the name or alias of the table to be accessed by a hash scan. Even if the hint is given, there is no guarantee that the transformation will take place. By default, hints do not propagate inside a complex view. The inner table must be partitioned on the join keys. Assume that these tables are not stored together in a cluster: Because the default goal of the cost-based approach is best throughput, the optimizer chooses either a nested loops operation, a sort-merge operation, or a hash operation to join these tables, depending on which is likely to return all the rows selected by the query more quickly. In this particular example, we have specified the exact join order to be used via the ORDERED hint, as well as the join methods to be used on the different tables. The ORDERED_PREDICATES hint forces the optimizer to preserve the order of predicate evaluation, except for predicates used as index keys. The SELECT statement retrieves rows from the view V2 restricting it to the department whose number is 20. The PARALLEL hint lets you specify the desired number of concurrent servers that can be used for a parallel operation. Each hint described in this section suggests a SQL query transformation. Answer: When you have a view (or an in-line view), the Oracle cost-based optimizer may take the value of a join predicate and "push" it into the main view, thereby improving SQL optimization. Then, for each branch of the UNION ALL, Oracle tries a rewrite with a materialized view. The statement must then must pass a heuristic test. The following section describes this in detail. Trying to get predicate pushing Ross Campbell, May 14, 2007 - 4:32 pm UTC When the NO_MERGE hint is used without an argument, it should be placed in the view query block. v_load_base does not contain analytic functions. Comments containing hints can appear only after these keywords. Use of the USE_NL and USE_MERGE hints is recommended with the ORDERED hint. "How the CBO Optimizes SQL Statements for Fast Response" for an explanation of the difference between FIRST_ROWS(n) and FIRST_ROWS. The statement must then must pass a heuristic test. The NO_FACT hint is used in the context of the star transformation to indicate to the transformation that the hinted table should not be considered as a fact table. Name or alias of a table to be used as the inner table of a join. You are correct, the VIEW PUSHED PREDICATE operation means "The view thus becomes correlated and must be evaluated for each row of the outer query block". DELETE, INSERT, SELECT, and UPDATE are keywords that begin a statement block. This is a specific kind of predicate pushing, the join predicate pushdown transformation. The NOREWRITE hint disables the use of function-based indexes. where table specifies the name or alias of the table on which the dynamic sampling is to be performed and integer is a value from 0 to 10 indicating the degree of sampling. Using this hint overrides decisions the optimizer would normally make. Optimizer hints can be categorized as follows: The hints described in this section let you choose between the cost-based and the rule-based optimization approaches. The NO_PUSH_PRED hint prevents pushing of a join predicate into the view. For such single-table views, an access path hint or a join hint on the view applies to the table inside the view. The SQL Analyze tool (available with the Oracle Tuning Pack), provides a graphical user interface for working with optimizer hints. (Your database is in serial mode if you are not using Enterprise Edition. If you specify two or more LEADING hints on different tables, then all of them are ignored. Therefore, use the DBMS_STATS package to gather statistics. If you specify this hint in any of these statements, then the optimizer uses the cost-based approach and optimizes for best throughput. Complex merging can also be used to merge an IN subquery into the accessing statement if the subquery is uncorrelated. Subquery unnesting unnests and merges the body of the subquery into the body of the statement that contains it, allowing the optimizer to consider them together when evaluating access paths and joins. In the hint you must specify a table exactly the same way it appears in the statement. For example, the optimizer uses the cost-based approach to optimize this statement for best response time: In this example each department contains many employees. The PARALLEL hint lets you specify the desired number of concurrent servers that can be used for a parallel operation. If this query is executed without the hint, then rows from dept are sent to the local site, and the join is executed there. The INDEX_FFS hint causes a fast full index scan to be performed rather than a full table scan. The NOAPPEND hint enables conventional INSERT by disabling parallel mode for the duration of the INSERT statement. When NO_MERGE is used with the view name as an argument, it should be placed in the surrounding query. The predicate is not pushed. If you decide, nonetheless, to use hints with views, the following sections describe the behavior in each case. As an application designer, you might know information about your data that the optimizer does not know. Use the NO_PUSH_PRED hint to prevent pushing of a join predicate into the view. The outer table rows are randomly partitioned. Consider the INDEX hint in the following SELECT statement: The INDEX hint applies to the employee table in the first branch of the UNION ALL view v, not to the employee table in the second branch. where table specifies the name or alias of the table on which the full table scan is to be performed. Oracle9i Data Warehousing Guide for more information on parallel execution. Hints provide a mechanism to direct the optimizer to choose a certain query execution plan based on the following criteria: Hints (except for the RULE hint) invoke the cost-based optimizer (CBO). For a specific query, place the HASH_SJ, MERGE_SJ, or NL_SJ hint into the EXISTS subquery. So eventually I discovered the answer: apparently the optimizer's decision to materialize a CTE ('With' block) will break the optimizer's ability to perform Cost-Based Query Transformation which in turn breaks the ability to perform certain operations like Join Predicate Pushdown (which is the View Pushed Predicate in the Explain Plan seen above) and Complex View Merge (a method of re-writing . A star plan has the largest table in the query last in the join order and joins it with a nested loops join on a concatenated index. For this reason, hints in the first component query apply only to its optimization, not to the optimization of the second component query. Note: The optimizer ignores this hint if the outer table is not partitioned or not equijoined on the partitioning key. Usually, if you analyze the tables, then the optimizer selects an efficient star plan. Because Oracle's default behavior for a range scan is to scan index entries in ascending order of their indexed values, this hint does not specify anything more than the INDEX hint. The USE_CONCAT hint turns off IN-list processing and OR-expands all disjunctions, including IN-lists. This hint is recommended when the outer table is very small compared to the inner table. If you do not use the ORDERED_PREDICATES hint, then Oracle evaluates all predicates in the order specified by the following rules. If you do not specify a view list, then Oracle searches for an eligible materialized view and always uses it regardless of its cost. . You can set the value of DYNAMIC_SAMPLING to a value from 0 to 10. no_push_pred_hint::= Text description of the illustration no_push_.gif PUSH_SUBQ. Oracle can replace literals in SQL statements with bind variables, if it is safe to do so. The push_pred hint is on Jonathan Lewis' list of the Big Five Hints. With the cost-based approach, this also includes the goal of best throughput or best response time. Assume that these tables are not stored together in a cluster: Because the default goal of the cost-based approach is best throughput, the optimizer chooses either a nested loops operation or a sort-merge operation to join these tables, depending on which is likely to return all the rows selected by the query more quickly. Specifies an index on which an index scan is to be performed. SELECT Statement Hints. That is, even if there is more than one matching row in t2 for a row in t1, the row in t1 is returned only once. Each hint described in this section suggests a SQL query transformation. As mentioned, you cannot use the ORDERED_PREDICATES hint to preserve the order of predicate evaluation on index keys. Each hint described in this section suggests a join operation for a table. These hints are pushed down into the (nonmerged) view V2. 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 both the NO_INDEX hint and the index hint are ignored for the specified indexes and the optimizer considers the specified indexes. The syntax of this hint is: . This hint is most often used to reduce the number of possible permutations for a query and make optimization faster. 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. When you want to specify hints for tables that appear inside views, use global hints instead of embedding the hint in the view. Maps the rows of each table to consumer query servers, using a hash function on the join keys. Therefore, use the DBMS_STATS package to gather statistics. The REWRITE hint forces the cost-based optimizer to rewrite a query in terms of materialized views, when possible, without cost consideration. If any parallel restrictions are violated, then the hint is ignored. 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: To broadcast the outer table r, the query is: The PARALLEL_INDEX hint specifies the desired number of concurrent servers that can be used to parallelize index range scans for partitioned indexes. This hint is useful if you are using distributed query optimization. The plus sign must immediately follow the comment delimiter (no space is permitted). This option is useful for small lookup tables. For a description of the tablespec syntax, see "Specifying Global Table Hints". Each hint described in this section suggests an access path for a table. Specifying one of these hints causes the optimizer to choose the specified access path only if the access path is available based on the existence of an index or cluster and on the syntactic constructs of the SQL statement. Also, do not specify schema names in the hint even if they are specified in the FROM clause. This hint lets the user have more influence over the way in which the view is accessed. In this statement, the INDEX hint explicitly chooses an index scan on the sex_index, the index on the sex column: The INDEX hint applies to IN-list predicates; it forces the optimizer to use the hinted index, if possible, for an IN-list predicate. The NO_INDEX hint explicitly disallows a set of indexes for the specified table. However, such hints can result in unexpected plans. CREATE TABLE t0 (id number, ref_id number); CREATE . Specifying DEFAULT or no value signifies the query coordinator should examine the settings of the initialization parameters (described in a later section) to determine the default degree of parallelism. If a global hint references a UNION or UNION ALL view, then the hint is applied to the first branch that contains the hinted table. The USE_CONCAT hint forces combined OR conditions in the WHERE clause of a query to be transformed into a compound query using the UNION ALL set operator. Use this hint in the WHERE clause of SELECT statements. Hints apply only to the optimization of the statement block in which they appear. If the statement does not use aliases, then the table name is the default alias. Hints apply only to the optimization of the statement block in which they appear. Usually, the optimizer considers using OR expansion and uses this method if it decides that the cost is lower than not using it. Is a keyword that begins a statement block. Unless the hints are inside the base view, they might not be honored from a query against the view. The EXPAND_GSET_TO_UNION hint is used for queries containing grouping sets (such as queries with GROUP BY GROUPING SET or GROUP BY ROLLUP). As a result, direct-path INSERT can be considerably faster than conventional INSERT. However, if a very small percentage of the hospital's patients are male, then the query returns a relatively small percentage of the table's rows, and an index scan is likely to be faster than a full table scan. If you omit the ORDERED hint from a SQL statement performing a join, then the optimizer chooses the order in which to join the tables. 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: To broadcast the outer table r, the query is: The PARALLEL_INDEX hint specifies the desired number of concurrent servers that can be used to parallelize index range scans for partitioned indexes. The ALTER SESSION SET OPTIMIZER_MODE statement does not affect SQL that is run from within PL/SQL. Access path and join hints on referenced views are ignored, unless the view contains a single table (or references an Additional Hints view with a single table). In this statement, the USE_NL hint explicitly chooses a nested loops join with the customers table as the inner table: In many cases, a nested loops join returns the first row faster than a sort merge join. The second value specifies how the table is to be split among the Oracle Real Application Cluster instances. In direct-path INSERT, data is appended to the end of the table, rather than using existing space currently allocated to the table. If you do not use the ORDERED_PREDICATES hint, then Oracle evaluates all predicates in the order specified by the following rules. The hint can take two values, separated by commas after the table name. For example, consider this statement, which joins the accounts and customers tables. NO_PUSH_PRED. Optimizer . PARALLEL, NOPARALLEL, PARALLEL_INDEX, and NOPARALLEL_INDEX hints inside views are preserved when the view is merged with the top-level query. index specifies an index on which an index scan is to be performed. If you specify two or more LEADING hints on different tables, then all of them are ignored. The NOPARALLEL_INDEX hint overrides a PARALLEL attribute setting on an index to avoid a parallel index scan operation. Oracle9i Data Warehousing Guide for more information on parallel execution, Oracle9i Data Warehousing Guide for more information on parallel execution. The hint passes instructions to the optimizer as a suggested execution plan for the SQL statement. If a SQL statement has a hint specifying an optimization approach and goal, then the optimizer uses the specified approach regardless of the presence or absence of statistics, the value of the OPTIMIZER_MODE initialization parameter, and the OPTIMIZER_MODE parameter of the ALTER SESSION statement. Symptoms Query in 12c executed with 'UNION ALL PUSHED PREDICATE' while in 19c 'UNION ALL' This hint can be used with either rule-based or cost-based optimization. Is one of the hints discussed in this section. If you know that the value in the WHERE clause of the query appears in a very small percentage of the rows, then you can use the INDEX hint to force the optimizer to choose an index scan. Maps the rows of each table to consumer query servers using a hash function on the join keys. The FACT hint is used in the context of the star transformation to indicate to the transformation that the hinted table should be considered as a fact table. The CHOOSE hint causes the optimizer to choose between the rule-based and cost-based approaches for a SQL statement. With nonmergeable views, optimization approach and goal hints inside the view are ignored: the top-level query decides the optimization mode. For example: MERGE_AJ uses a sort-merge anti-join, HASH_AJ uses a hash anti-join, and NL_AJ uses a nested loop anti-join. The NO_PUSH_PRED hint prevents pushing of a join predicate into the view. You can use hints to specify the following: The use of hints involves extra code that must be managed, checked, and controlled. The chapter contains the following sections: Hints let you make decisions usually made by the optimizer. You can use hints to specify the following: Note: Slowness when predicate not pushed in UNION ALL 19c (Doc ID 2890837.1) Last updated on AUGUST 30, 2022 Applies to: Oracle Database - Enterprise Edition - Version 19.13. and later Information in this document applies to any platform. A statement block can have only one comment containing hints. The ROWID hint explicitly chooses a table scan by rowid for the specified table. The CACHE and NOCACHE hints affect system statistics "table scans(long tables)" and "table scans(short tables)", as shown in the V$SYSSTAT view. The most precise method is to order the tables in the FROM clause in the order of the keys in the index, with the large table last. where table is the name or alias for the table at which site the execution should take place. There are two global hints in the SELECT statement. The NO_PUSH_PRED hint prevents pushing of a join predicate into the view. Causes Oracle to interpret the comment as a list of hints. In general, hints take precedence over table clauses. The use of hints involves extra code that must be managed, checked, and controlled. Chapter3, "Gathering Optimizer Statistics", Database Tuning with the Oracle Tuning Pack, Hints for Optimization Approaches and Goals, "How the CBO Optimizes SQL Statements for Fast Response", Oracle9i Application Developer's Guide - Fundamentals. As a general rule, use the None/Broadcast hint when inner table size * number of query servers < outer table size. Oracle does not consider views outside of the list. Maps the rows of the inner table using the partitioning of the outer table. For example, the optimizer uses the cost-based approach to optimize this statement for best throughput: The hints FIRST_ROWS(n) (where n is any positive integer) or FIRST_ROWS instruct Oracle to optimize an individual SQL statement for fast response. Oracle9i Database Concepts for more information about star plans. This leads to a major decrease in performance. Consider the following view definitions and SELECT statement: The view V1 retrieves all employees whose employee number is less than 100. The following example tries to demonstrate this hint. However, such hints can result in unexpected execution plans. This chapter explains how to use hints to force various approaches. If a hint specifies an unavailable access path, then the optimizer ignores it. Do this by specifying how rows of joined tables should be distributed among producer and consumer query servers. The NO_MERGE hint causes Oracle not to merge mergeable views. This is because you can define views in one context and use them in another. Barring the use of frequency histograms, the number of occurrences of each distinct column value is not available to the optimizer. where table is a table to be joined to the row source resulting from joining the previous tables in the join order using a sort merge join. Chapter3, "Gathering Optimizer Statistics" for more information on default values. As you can see, push_pred work with outer join. These estimates might not be as accurate as those gathered by the DBMS_STATS package. FIRST_ROWS(n) affords greater precision, because it instructs Oracle to choose the plan that returns the first n rows most efficiently. The CURSOR_SHARING_EXACT hint causes this behavior to be switched off. If the subquery is relatively expensive or does not reduce the number of rows significantly, then it improves performance to evaluate the subquery last. The global hint syntax also applies to unmergeable views. This converts the subquery into a special type of join between t1 and t2 that preserves the semantics of the subquery. no_push_subq_hint::= Text description of no_push_subq_hint. . Predicates: Without user-defined functions, . A more general method is to use the STAR hint. Query hint no_push_pred - Oracle Forums SQL & PL/SQL Query hint no_push_pred 893411 Jan 26 2012 edited Jan 26 2012 Hi, I want to prevent oracle to do a VIEW PUSHED PREDICATE, because it slows down my query. The INDEX_DESC hint explicitly chooses an index scan for the specified table. If it is valid, then subquery unnesting is enabled without Oracle checking the heuristics. To force cardinality estimation even for an analyzed table, you can use a further hint, dynamic_sampling_est_cdn, as in the following example: This forces cardinality estimation for emp, even if the table is analyzed. If no indexes are given as arguments for the INDEX_COMBINE hint, then the optimizer uses whatever Boolean combination of bitmap indexes has the best cost estimate for the table. Specifies an index on which an index scan is to be performed (optional). Pushing predicates Oracle 9i Christo Kutrovsky, November 10, 2004 - 3:59 pm UTC Tom, My specific case is a little complicated, so I will just simplify it here. July 13, 2018 pushing predicates Filed under: CBO, Execution plans, Hints, Oracle Jonathan Lewis @ 1:05 pm BST Jul 13,2018 All rows of the inner table are broadcast to each consumer query server. In the following example, the CACHE hint overrides the table's default caching specification: The NOCACHE hint specifies that the blocks retrieved for the table are placed at the least recently used end of the LRU list in the buffer cache when a full table scan is performed. When NO_MERGE is used with the view name as an argument, it should be placed in the surrounding query. no_push_pred_hint::= Text description of no_push_pred_hint . Hints provide a mechanism to direct the optimizer to choose a certain query execution plan based on the following criteria: Hints (except for the RULE hint) invoke the cost-based optimizer (CBO). Use the EXPLAIN PLAN statement to identify the distribution chosen by the optimizer. How can i make the optimizer push the predicate also within a view . 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, subsequently, to choose an execution plan. Use hints to determine the access path for any SQL statements submitted from within PL/SQL. If the subquery block is valid, then subquery unnesting is enabled without Oracle's checking the heuristics. Complex merging is not cost-based; that is, the accessing query block must include the MERGE hint. For example, the following query will not result in any dynamic sampling if emp is analyzed: The cardinality statistic is used, if it exists. A general rule is: Use the Broadcast/None hint if the size of the inner table * number of query servers > size of the outer table. The MERGE hint lets you merge a view on a per-query basis. (Your database is in serial mode if you are not using Enterprise Edition. If you decide, nonetheless, to use hints with views, the following sections describe the behavior in each case. The outer table must be partitioned on the join keys. The NO_INDEX hint explicitly disallows a set of indexes for the specified table. A subquery is evaluated as a semi-join only with these limitations: The hints described in this section determine how statements are parallelized or not parallelized when using parallel execution. Based on this information, you might be able to choose a more efficient execution plan than the optimizer. Chapter3, "Gathering Optimizer Statistics" for more information on default values. The optimizer only generates the subqueries if it seems reasonable to do so. The STAR hint applies when there are at least three tables, the large table's concatenated index has at least three columns, and there are no conflicting access or join method hints. For example: SELECT /*+ NO_MERGE(v) NO_PUSH_PRED(v) */ * FROM employees e, (SELECT manager_id FROM employees) v WHERE e.manager . Oracle9i Database Concepts and Oracle9i Application Developer's Guide - Fundamentals for more information on materialized views. In terms of disabling predicate pushing session or instance wide, you'd be into hidden parameter territory there - once again, you'd need to liaise with Support to get their blessing on that one. The CLUSTER hint explicitly chooses a cluster scan to access the specified table. Alternatively, the SQL NOT IN predicate can be evaluated using an anti-join to subtract two sets. The hints described in this section allow you to choose between the cost-based and the rule-based optimization approaches. When the NO_MERGE hint is used without an argument, it should be placed in the view query block. For more information on the SAMPLE option, see Oracle9i Database Concepts and Oracle9i Database Reference. MERGE_AJ uses a sort-merge anti-join, HASH_AJ uses a hash anti-join, and NL_AJ uses a nested loop anti-join. Alternatively, the SQL predicate NOT IN can be evaluated using an anti-join to subtract two sets. ---- query 1---. This is controlled with the CURSOR_SHARING startup parameter. The outer table rows are randomly partitioned. An SQL Hint is a type of comment added to a Select, Insert, Update or Delete SQL statement. This converts the subquery into a special type of join between t1 and t2 that preserves the semantics of the subquery. The STAR_TRANSFORMATION hint makes the optimizer use the best plan in which the transformation has been used. The USE_HASH hint causes Oracle to join each specified table with another row source with a hash join. The inner table rows are randomly partitioned. The problem can be solved by rewriting the view in two separate views but that kind of defeats the purpose of creating a view. You must specify the table to be accessed exactly as it appears in the statement. For example, you might know that a certain index is more selective for certain queries. Each hint described in this section suggests an access path for a table. I have a performance problem on a UNION ALL view. Specifying one of these hints causes the optimizer to choose the specified access path only if the access path is available based on the existence of an index or cluster and on the syntactic constructs of the SQL statement. My DB is win32 11.2.0.1.0. Use this hint in the WHERE clause of SELECT statements. Each parameter serves the same purpose as in the INDEX hint. The MERGE hint lets you merge a view for each query. See Also: PARALLEL, NOPARALLEL, PARALLEL_INDEX, and NOPARALLEL_INDEX hints inside views are preserved when the view is merged with the top-level query. For the same reason, access path hints on the view in the top-level query are ignored. Optimization approach and goal hints can occur in a top-level query or inside views. no_push_pred_hint::= Text description of the illustration no_push_pred_hint.gif. In Example5-1, the ORDERED hint specifies the exact join order to be used; the join methods to be used on the different tables are also specified. from aview aview -- the aview is a view containing also a union all. The STAR hint applies when there are at least three tables, the large table's concatenated index has at least three columns, and there are no conflicting access or join method hints. Without the hint, the optimizer could make a cost-based decision to use the best plan generated without the transformation, instead of the best plan for the transformed query. A statement block is any one of the following statements or parts of statements: For example, a compound query consisting of two component queries combined by the UNION operator has two statement blocks, one for each component query. Hi, all. The FULL hint explicitly chooses a full table scan for the specified table. Complex merging can also be used to merge an IN subquery into the accessing statement if the subquery is uncorrelated. To g The CACHE hint specifies that the blocks retrieved for the table are placed at the most recently used end of the LRU list in the buffer cache when a full table scan is performed. Parallel execution hints on the view in a top-level query override such hints inside a referenced view. The APPEND hint always follows the INSERT keyword, and the PARALLEL hint can follow the INSERT keyword. Specifying DEFAULT or no value signifies that the query coordinator should examine the settings of the initialization parameters (described in a later section) to determine the default degree of parallelism. The real queries use about 10 different tables instead of just 3. This hint is useful if you are using distributed query optimization. Each hint described in this section suggests a join operation for a table. For a description of the queryblock syntax, see "Specifying a Query Block in a Hint". Improves the performance of parallel join operation between a pair of matching partitions, one each! At the earliest possible step in the table on which an index scan.... Improves the performance of parallel join operations HASH_AJ, or NL_SJ hint into the in. Of SELECT statements optimizer only generates the subqueries if it decides that the transformation will place. Global hint syntax also applies to function-based, B-tree, bitmap, cluster, or keyword... 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