strive to update our BC Oracle support information. 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. When using hints, in some cases, you might need to specify a full set of hints to ensure the optimal execution plan. Oracle provides several tools, including SQL Tuning Advisor, SQL plan management, and SQL Performance Analyzer, to address performance problems not solved by the optimizer. The optimizer decides on a plan at run time based on statistics collected as the query executes. Wanted! Note that this table does not include the approximate query initialization parameters, which are described in "Approximate Query Initialization Parameters". Oracle technology is changing and we Burleson Using the OPTIMIZER_MODE initialization parameter, you can set the following optimizer goals: When you set the OPTIMIZER_MODE value to ALL_ROWS, the database uses the least amount of resources necessary to process all rows that the statement accessed. or have a suggestion for improving our content, we would appreciate your Wanted! For example, a statement specifies PUSH_PRED (some_view) MERGE (some_view). - Degree of Parallelism is 2 because of hint, Description of "Figure 19-1 Techniques for Influencing the Optimizer", Influencing the Optimizer with Initialization Parameters, Migrating Stored Outlines to SQL Plan Baselines, Approximate Query Initialization Parameters, Description of "Figure 19-2 Optimizer Hint". The steps for displaying a plan with hint information are the same as for displaying a plan normally. The default is Oracle Database supports more than 60 hints, each of which may have zero or more parameters. - USE_NL(t1, t2), Hint Report (identified by operation id / Query Block Name / Object Alias): For example, you may know that a certain index is more selective for certain queries. Within each object, unused hints appear at the beginning, followed by used hints. - FULL(t3) 9 - SEL$1 / T2@SEL$1 Connect SQL*Plus to the database as SYSTEM, and then query the current settings. In addition, an optional query block name can precede the tablespec syntax. Larger values result in a lower cost for full table scans, which may result in the optimizer choosing a full table scan over an index scan. Oracle Therefore, a unique key violation does not cause the INSERT to terminate or an error to be reported. The report displays the hints under the objects (for example, query blocks and tables) that appear in the plan. By default, reporting mode is off (false), which means that adaptive optimizations are enabled. ORACLE!. To identify a query block in a query, you can use an optional query block name in a hint to specify the block to which the hint applies. ServerOracle ConceptsSoftware SupportRemote Connect SQL*Plus to the database with the appropriate privileges, and then query the current optimizer mode. You can also catch regular content via Connor's blog and Chris's blog. This hint is a useful way to check for plan regressions after database upgrades. The hint FULL(t1) specified in query block SEL$2 was overridden by the hint FULL(@sel$2 T1) specified in query block SEL$1. Note that USE_NL (table1 table2) is not considered a multi- table hint because it is actually a shortcut for USE_NL (table1) and USE_NL (table2). their Oracle 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. Answer: The following are examples of using the opt_param hint with some hidden parameters: OPT_PARAM ('_optimizer_try_st_before_jppd' 'true') OPT_PARAM ('_optimizer_use_feedback' 'true') OPT_PARAM ('_optimizer_distinct_placement' 'true') OPT_PARAM ('_and_pruning_enabled' 'true') OPT_PARAM ('_optimizer_table_expansion' 'true') Oracle ignores combinations of conflicting hints, but considers other hints within the same comment. The table name within the hint should not include the schema name if the schema name is present in the statement. plansRemote The following sample SQL statement specifies a distribution of NONE for an insert into the lineitem table: Each of the following hints instructs the optimizer to use a specific SQL query transformation: STAR_TRANSFORMATION and NO_STAR_TRANSFORMATION. Determines whether the database verifies statements In a test environment, hints are useful for testing the performance of a specific access path. The exceptions are hints that may be left inside the SQL, hints such as ordered, which tell the . Database Support ORA-00001: (.) The line number of the plan line is 2, which corresponds to the first line where the table E@SEL$1 appears in the plan table. The OPTIMIZER_FEATURES_ENABLE hint acts as an umbrella parameter for enabling a series of optimizer features based on an Oracle Database release number. Remote Hints enable you to make decisions normally made by the optimizer, sometimes causing the optimizer to select a plan that it sees as higher cost. Oracle For example, the following hint directs the optimizer to pick the query plan that produces the first 10 rows from the employees table at the lowest cost: Hints can be of the following general types: Single-table hints are specified on one table or view. See "Hints for Join Orders". A statement block can have only one comment containing hints, and that comment must follow the SELECT, UPDATE, INSERT, MERGE, or DELETE keyword. Burleson Consulting Hints affected by transformations - A transformation can make some hints invalid. When you upgrade to Oracle 10g and beyond, you will find The following example specifies two hints, UNNEST and SEMIJOIN, on a subquery: In this example, the hints are specified in query block SEL$2, but SEL$2 does not appear in the final plan. What are the possible solutions to implement the recommended parameter change? Query block hints operate on single query blocks. e-mail: Burleson Consulting FormsOracle Hint usage reporting is enabled by default. If you set this parameter to 0, then the result cache is disabled. In the preceding example, no hints have line number 0, so all query blocks appeared in the final plan. Table 19-2 Initialization Parameters That Control Optimizer Behavior. UpgradesSQL If the rounded value is 0, then the feature is disabled. No U prefix exists for USE_NL(t1, t2), which means that the optimizer did use the hint for employees. The push_pred hint controls Support, SQL TuningSecurityOracle DBAOracle See "Specifying a Query Block in a Hint". The following shows the types of views on which join predicate pushdown is currently supported. The following statement uses a statement hint: Oracle Database SQL Language Reference for the most common hints by functional category. For such single-table views, an access path hint or a join hint on the view applies to the table inside the view. Oracle Database Advanced Application Developer's Guide, Oracle Database VLDB and Partitioning Guide, Oracle Database 2 Day + Data Warehousing Guide, | 6 | PX SEND BROADCAST | :TQ10000 | 4 | 88 | 2 (0)| 00:00:01 | | S->P | BROADCAST|, | 7 | TABLE ACCESS FULL| DEPARTMENTS | 4 | 88 | 2 (0)| 00:00:01 | | | |, | 9 | TABLE ACCESS FULL | EMPLOYEES | 14 | 280 | 2 (0)| 00:00:01 | Q1,01 | PCWP | |, "Optimization Approaches and Goal Hints in Views", "Access Path and Join Hints Inside Views", Description of the illustration tablespec.gif, Description of the illustration indexspec.gif. considering using the services of an Oracle support expert should OPTIMIZER_REAL_TIME_STATISTICS A SQL profile can correct suboptimal optimizer estimates discovered during SQL tuning. The optimizer uses hints to choose an execution plan for the statement unless prevented by some condition. Avoid a full table scan when an index retrieves the requested rows more efficiently. The PQ_DISTRIBUTE hint controls the distribution method for a specified join operation. Consulting StaffConsulting Therefore, if you use ENABLE NOVALIDATE RELY, some types of query rewrite might not work. false, which means real-time statistics are disabled. For views that are non-inline views, access path and join hints in the view are preserved only if the referencing query references no other tables or views (that is, if the FROM clause of the SELECT statement contains only the view). parameter. The star transformation combines the bitmap indexes on the various fact table columns. Oracle forum. By default, this parameter is true, which means adaptive plans are enabled. Oracle Database 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. - INDEX(t1) 33 - SEL$E0F432AE / T1@SEL$2 Pushing Join Predicates is a good thing, so don't ever use the no_push_pred hint because it will never improve query performance. their Oracle - PARALLEL(2) Question: What does DBA performance tuning consulting professionals. SELECT and parallel CREATE TABLE AS SELECT statements to specify how rows should be distributed between the producer (query) and the consumer (load) slaves. 10 small index range scans sounds faster than hash joining two large tables. no_push_pred 12 616 no_push_subq 12 617 no_qkn_buff 11 618 no_query_transformation 23 619 NORMAL. Oracle ignores hints if the comment containing them does not follow a DELETE, INSERT, SELECT, MERGE, or UPDATE keyword. Just Just Specify the release number as an argument to the hint. These examples show various types of hint usage reports. The PUSH_PRED(tab_name_or_alias) hint can be used to push a join predicate into an inline view, thus making the inline view a correlated subquery. Last updated: October 16, 2015 - 6:29 am UTC, Oren Nakdimon (@DBoriented), October 15, 2015 - 12:58 pm UTC, Derek Derek, October 16, 2015 - 2:33 am UTC. Because changes in the database and host environment can make hints obsolete or have negative consequences, a good practice is to test using hints, but use other techniques to manage execution plans. Hints can be specified incorrectly or associated with objects that are not present in the final plan. Scripts This chapter does not cover DBMS_STATS. A value of 10 means that the cost of an index access path is one-tenth the normal cost of an index access path. Tips ORA-00020: () ORA-00021: . Remote DBA Services At the session level, set the OPTIMIZER_ADAPTIVE_REPORTING_ONLY initialization parameter to true. LKML Archive on lore.kernel.org help / color / mirror / Atom feed * [RFC 00/10] Speculation Control feature support @ 2018-01-20 19:22 KarimAllah Ahmed 2018-01-20 19:22 ` [RFC 01/10] x86/speculation: Add basic support for IBPB KarimAllah Ahmed ` (10 more replies) 0 siblings, 11 replies; 135+ messages in thread From: KarimAllah Ahmed @ 2018-01-20 19:22 UTC (permalink / raw publish The following sections discuss the use of hints in more detail. Database automatically gathers real-time statistics during Index skip scans and index fast full scans are mutually exclusive. The default value of this parameter corresponds to the maximum I/O size that the database can perform efficiently. In this case, only 1 hint was unused. "Migrating Stored Outlines to SQL Plan Baselines" to learn how to migrate stored outlines to SQL plan baselines. ALL_ROWS is an example of a statement hint. In this case, the database gathers information required for adaptive optimization, but does not change the plans. The initial testing using this view was quite promising, as the work performed by the database to get the answer was MUCH less using t Server TrainingOracle The optimizer uses the value of this parameter to calculate the cost of full table scans and index fast full scans. Anyone In this mode, all constraints must be validated. The default value is 5%, but you can specify any percentage value between 1 and 100. Oracle Database provides initialization parameters to influence various aspects of optimizer behavior, including cursor sharing, adaptive optimization, and the optimizer mode. experience! For example, the report first shows the FULL(t1) hint, which was not used, and then FULL(@sel$2 t1), which was used. A statement block can have only one comment containing hints, but it can contain many space-separated hints. If you upgrade to a new release, and if you want to enable the features in the new release, then you do not need to explicitly set the OPTIMIZER_FEATURES_ENABLE initialization parameter. FIRST_ROWS (n) affords greater precision, because it instructs Oracle to choose the plan that returns the first n rows most efficiently. when there are lots of non-indexed predicates where the majority of them - FULL(@sel$2 t1) Determines the degree to which query rewrite is enforced. The push_subq hint is used to force all subqueries Database Support Converting the values improves cursor sharing and can affect the execution plans of SQL statements. TRUE, which is the default, enables the optimizer to utilize materialized views to enhance performance. In Outline Data of explain plan I see following for push_pred hint. thereby improving SQL optimization. I am currently on an older version of Oracle but am Total hints for statement: 4 (U - Unused (1)) Also, apart from no_push_pred hint is there any session variable or anything I could alter to disable push_pred? the number of rows. independently investigate their credentials and experience, and not rely on Oracle forum. - INDEX(t2) Database Support plansRemote This use of push_subq allows the UNION ALL/UNION view. For example, a statement specifies INDEX(employees emp_idx), where emp_idx is not a valid index name for table employees. Thus, all slaves load all partitions. To enable query optimizer features for a specific release: Log in to the database with the appropriate privileges, and then query the current optimizer features settings. For example, in the first week of 2015 you query current year and last year sales. A value of 100 infers that 100% of the index blocks are likely to be found in the buffer cache, so the optimizer adjusts the cost of an index probe or nested loop accordingly. Deferred invalidation reduces the number of cursor invalidations and spreads the recompilation workload over time. Oracle The database does not issue error messages for hints that it ignores. If the view path is specified, then the database resolves the hint from left to right, where the first view must be present in the FROM clause, and each subsequent view must be specified in the FROM clause of the preceding view. Because the parameter is expressed in blocks, it is set to a value equal to the maximum I/O size that can be performed efficiently divided by the standard block size. Performance Tuning Oracle Hints Note: Hint Analysis through SQL hint injection for Oracle data sources is not supported in DB Optimizer XE Developer. In releases before Oracle Database 19c, it could be difficult to determine why the optimizer did not use hints. servicesApplication Access to departments is serialized because a NOPARALLEL setting was applied to this table in Step 1. Performance Tuning When writing SQL code, you may know information about the data unknown to the optimizer. For backward compatibility, you may not want the execution plans to change because of new optimizer features in a new release. then use of push_subq can improve the overall performance by evaluating the subquery as Because no schema object is specified in the PARALLEL hint, the scope of the hint is the statement, not an object. alter system set "_push_join_predicate" = FALSE scope=both sid='*'; The default value is 100, which means that the optimizer evaluates indexes as an access path based on the normal cost model. When the +REPORT parameter is set, the report shows the plan the optimizer would have picked if automatic reoptimization had been enabled. The Oracle of Q: oracle index hint multiple tables example Vaibhav Code: SQL 2021-08-02 04:35:52 SELECT /*+ parallel(8) .Parallel_index Hint.Parallel hint works on tables as follows: In following queries, you are specify the parallel 4 means 4 process is used to fetch data from the employees table.SELECT /*+ parallel (employees,4) */ emp. verified. An index named emp_emp_id_pk exists on the employees.employee_id column. Optimizer defaults are adequate for most operations, but not all. For example, run the following SQL*Plus command: Set the optimizer features setting at the instance or session level. The hint usage report solves this problem. ORA-00019: . If you specify hints for access paths or join operations along with either the ALL_ROWS or FIRST_ROWS(n) hint, then the optimizer gives precedence to the access paths and join operations specified by the hints. Oracle The following example specifies two hints, one for a skip scan and one for a fast full scan: The preceding report shows that the INDEX_FFS(e) and INDEX_SS(e emp_manager_ix) hints conflict with one other. Specifies the maximum amount of cache memory that any single result can use. Verify You can influence the optimizer using several techniques, including SQL profiles, SQL Plan Management, initialization parameters, and hints. PricesHelp You can use the RETRY_ON_ROW_CHANGE hint to handle conflicting UPDATE operations during an online application upgrade. "Access Path and Join Hints on Views" and "Access Path and Join Hints Inside Views" for hint behavior with mergeable views, Oracle Database SQL Language Reference for more information on the SAMPLE option. If two or more mode hints in the referenced views conflict, then the database discards all mode hints in the views and uses the session mode, whether default or user-specified. By default, OPTIMIZER_ADAPTIVE_STATISTICS is false, which means that the following features are disabled: Sets the optimizer mode at database instance startup. For example, you may know that an index is more selective for certain queries, leading to a better plan. You can specify parallelism at the statement or object level. Before each object is a number that identifies the line in the plan where the object first appears. Predicate pushing in views containing analytic functions Tom,I have a view in my database that contains an Analytic function. I try to use push_pred() hint on the inline_1 but nothing happened, please suggest how to optimize this kind of SQL which join push pred is not working. In Oracle Database, adaptive query optimization is the process by which the optimizer adapts an execution plan based on statistics collected at run time. To force the use of the index emp_job_ix for the table e3, you can use one of the following statements: To avoid this issue, Oracle recommends that you specify a query block in the hint using the @SEL notation: Example 19-4 Using Global Hints with NO_MERGE. For example, an adaptive plan always choose the default plan, but the database collects information about which plan the database would use if the parameter were set to false. By passing the STATUS_CODE predicate into the view, now it can use a simple INDEX RANGE SCAN operation on the large STATUS table. Adjusts the cost of index probes. . The following sample statement creates a table and specifies the PARTITION distribution method: In contrast, a NONE distribution combines the query and load operation into each slave. value of a join predicate and "push" it into the main view, Therefore, if bcol BETWEEN is more restrictive (rejects a higher percentage of the rows) than ccol BETWEEN, then the last join becomes easier (with fewer rows) if tabb is joined before tabc. Also look at push_subq in which alias is not necessary to give. For example, a statement specifies FULL(employees) INDEX(employees), but an index scan and full table scan are mutually exclusive. Errata? If you decide to use hints with views, then the following sections describe the behavior. For example, in Example 19-3 the job_history table has a single-column index on the employee_id column and a concatenated index on employee_id and start_date columns. You need to mention the alias or alias given to a sub query int the hint to join and to force the optimizer. The following example specifies an index range hint for the emp_manager_ix index: The following query of the plan table specifies the format value of TYPICAL, which shows only unused hints: The U in the preceding hint usage report indicates that the INDEX_RS hint was not used. And of course, keep up to date with AskTOM via the official twitter account. Optimizer hints are special comments in a SQL statement that pass instructions to the optimizer. 0 - SEL$5 When set to FORCE, the database stores all results in the cache. 4 - SEL$5DA710D3 / T1@SEL$2 ( The hr.employees aa should be hr.employees ) no aa. Controls the cost analysis of an index probe with a nested loop. The number of partitions being loaded is greater than or equal to the number of load slaves. A stored outline is a legacy technique that serve a similar purpose to SQL plan baselines. it could very well be that the default level of parallelism is more efficient than the level you're forcing in the hint (and the difference in response suggests that the hint is being applied; it just isn't producing the results you expected), or that table statistics are stale, or that the volume of data you're dealing with doesn't justify the Excel-DB, Oracle Database Tips by Donald BurlesonApril 2, 2015. 18 - SEL$1 / T3@SEL$1 UNNEST. 1 - SEL$1 Feel free to ask questions on our TuningEmergency All rights reserved by A hint is a commented instruction in a SQL statement. For example, you can set optimizer goals using both initialization parameters and hints. SQL profiles also technically include hints. The table T1@SEL$1 appears in the same query block at line 5 of the plan. Modify the spfile for the database. If OPTIMIZER_ADAPTIVE_REPORTING_ONLY is set to true, then adaptive optimization runs in reporting-only mode. initialization parameter is set to true, Oracle This chapter explains how to use hints to instruct the optimizer to use specific approaches. Oracle does not recommend explicitly setting the OPTIMIZER_FEATURES_ENABLE initialization parameter to an earlier release. The default setting is Hints beginning with the keyword PARALLEL indicate the degree of parallelism for the query. At the instance level, optimize for response time. Total hints for statement: 5 (U - Unused (2)) The following statement uses a single-table hint: Multitable hints are like single-table hints except that the hint can specify multiple tables or views. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. plan. Controls whether the database uses the SQL query result cache for all queries, or only for the queries that are annotated with the result cache hint. The join order can have a significant effect on the performance of a SQL statement. The optimizer can use adaptive statistics when query predicates are too complex to rely on base table statistics alone. If multiple hints appear in the same hint block, and if one hint has a syntax error, then the optimizer honors all hints before the hint with an error and ignores hints that appear afterward. Hints that specify a table use the following syntax, where view specifies a view name and table specifies the name or alias of the table: tablespec::= If the number of sessions is extremely large, then the multiblock read count value decreases to avoid the buffer cache getting flooded with too many table scan buffers. For example, if you specify a hint in a query that selects against a complex view, then this hint is not honored, because it is not pushed inside the view. Oracle PostersOracle Books The following hints instructs the optimizer to use a specific join operation for a table: Use of the USE_NL and USE_MERGE hints is recommended with any join order hint. Oracle hint: PUSH_PRED. An adaptive plan always chooses the default plan, but the database collects information about the execution as if the parameter were set to false. Total hints for statement: 10 (U - Unused (2), N - Unresolved (1), E - Syntax error (1)) FORCE enables the query rewrite feature of the optimizer and directs the optimizer to rewrite queries using materialized views even when the estimated query cost of the unoptimized query is lower. In STALE_TOLERATED mode, the optimizer uses materialized views that are valid but contain stale data as well as those that contain fresh data. Conversely, the no_push_subq hint can be used to You can use the ORDERED or STAR hint to force the join order. You can use the following techniques to influence the optimizer: Parameters influence many types of optimizer behavior at the database instance and session level. Check out more PL/SQL tutorials on our LiveSQL tool. For example, run the following SQL statement to configure only this session to optimize for throughput: Oracle Database Reference to learn about the OPTIMIZER_MODE initialization parameter. To enable query rewrite when constraints are in NOVALIDATE mode, the integrity level must be TRUSTED or STALE_TOLERATED. clause and the QB_NAME The optimizer goal is the prioritization of resource usage by the optimizer. In Oracle 12c you can achieve the same using Lateral Inline Views, without TABLE, MULTISET and user-defined types: SELECT * "Influencing the Optimizer with Initialization Parameters". Oracle Database provides initialization parameters to influence various aspects of optimizer behavior, including cursor sharing, adaptive optimization, and the optimizer mode. Set to FORCE to enable the creation of a new cursor when sharing an existing cursor, or when the cursor plan is not optimal. Setting the parameter to FALSE causes the optimizer to ignore the INMEMORY property of tables during the optimization of SQL statements. If it is, please let us know via a Comment, https://asktom.oracle.com/pls/apex/f?p=100:11:0::NO::P11_QUESTION_ID:3469884600671. You intend to create a join view of employees and job_history that contains a nested query block. The value TYPICAL displays only the hints that are not used in the final plan, whereas the value ALL displays both used and unused hints. Also, such hints can result in unexpected execution plans. 5 - SEL$5DA710D3 / T1@SEL$1 When setting this parameter, consider how the result cache handles PL/SQL functions. The OPTIMIZER_FEATURES_ENABLE initialization parameter is set to 12.1.0.1 or later. To specify a hint for a table in a view or subquery, the global hint syntax is preferable. SupportAnalysisDesignImplementationOracle predicate pushing and it is related to the _optimizer_extend_jppd_view_types Catalog oracle sql . SELECT /*+ NO_MERGE (v) PUSH_PRED (v) */ * FROM hr.employees e, ( SELECT manager_id FROM hr.employees ) v WHERE e.manager_id = v.manager_id (+) AND e.employee_id = 100; That double alias might cause it to fail. Separate multiple hints by at least one space, as in the following statement: Hints enable you to make decisions normally made by the optimizer. . This value is platform-dependent and is 1 MB for most platforms. feedback. When mapping is complete, each query server performs the join between a pair of resulting partitions. Specifies the number of blocks that are read in a single I/O during a full table scan or index fast full scan. Oracle hints are optimizer directives that are used in SQL de-bugging to change the default execution plan for SQL. INDEX and USE_NL are examples of single-table hints. PortalApp Enables or disables the query rewrite feature of the optimizer. The queryblock identifier can either be system-generated or user-specified. 911RAC ServerOracle ConceptsSoftware SupportRemote The following figure shows the principal techniques for influencing the optimizer. Hints beginning with NO_PARALLEL disable parallelism. Performance Tuning. Hints control a wide range of behavior. The optimizer honors INDEX(t1) and FULL(t2), but ignores MERG(v) and USE_NL(t2). The following example shows how to tune join order effectively: Choose the driving table and the driving index (if any). hint. Feel free to ask questions on our Oracle Database Reference to learn about optimizer features enabled when you set OPTIMIZER_FEATURES_ENABLE to different release values. plan. Copyright 1996 - 2020 If PUSH_JOIN_PREDICATE is FALSE, you can use the PUSH_JOIN_PRED hint to force pushing of a join predicate into the view. For example, the preceding report shows hints that apply to the following distinct objects: T1@SEL$2, and T1@SEL$1. If there is no top-level optimizer mode hint, then the database uses mode hints in referenced views as long as all mode hints in the views are consistent. Access path and join hints can appear in a view definition. Oracle Database Reference to learn about OPTIMIZER_MODE. The optimizer uses the instructions encoded in hints to choose an execution plan for a statement, unless a condition prevents the optimizer from using the hint. Burleson Because PL/SQL function result cache invalidation does not track all kinds of dependencies (such as on sequences, SYSDATE, SYS_CONTEXT, and package variables), indiscriminate use of the query result cache on queries calling such functions can result in changes to results, that is, incorrect results. U - FULL(t1) / hint overridden by another in parent query block The NO_PUSH_PRED hint instructs the optimizer not to push a join predicate into the view. 25 - SEL$E0F432AE / T2@SEL$2 The primary application is interactive, so you want to set the optimizer goal for the database instance to minimize response time. qualifications. Oracle Database PL/SQL Packages and Types Reference, /*+ INDEX (employees emp_department_ix)*/, Hint Report (identified by operation id/Query Block Name/Object Alias) See "About the Life Cycle of Shared Cursors". Oracle Database SQL Language Reference to learn more about EXPLAIN PLAN. The syntax for the push_subq hint is: And of course, keep up to date with AskTOM via the official twitter account. A transformation can make some hints invalid. The last two conditions are join conditions. The hint report shows which hints were used and ignored, and typically explains why hints were ignored. Controls adaptive plans. The OPTIMIZER_ADAPTIVE_REPORTING_ONLY initialization parameter is set to false (default). Remote DBA Services The input data is evenly distributed across the partitions being loaded. When a unique key violation is encountered during a single-table INSERT operation, a row-level rollback occurs and execution resumes with the next input row. The hint NO_ORDER_SUBQ specified in SEL$2 is not a valid hint. If you find an error 0 - STATEMENT All rights reserved by U - NLJ_PREFETCH(t2) The disadvantage of hints is the extra code that must be managed, checked, and controlled. You want to preserve the optimizer behavior from the earlier release. - NO_MERGE You must specify the table to be accessed exactly as it appears in the statement. For batch applications such as Oracle Reports, optimize for best throughput. in the query block to be executed at the earliest possible place in the The following statement uses the no_push_pred hint in the outer query block and has to specify the name of the alias of the inner query block into which the predicate is not to be pushed. You can use the push_pred hint with a nested loops join into a view, when the view is the outer table in the join. Scripting on this page enhances content navigation, but does not change the content in any way. TuningEmergency If a hint specifies an unavailable access path, then the optimizer ignores it. e-mail: Burleson Consulting You can also catch regular content via Connor's blog and Chris's blog. Puhsing predicates into subqueries containing aggregates is always a challenge (for the optimizer and for us) because you never know if by doing you might change the result. Instead of joining a large table, Oracle can use a join predicate pushdown. DEFERRED allows an application to take advantage of the reduced cursor invalidation for DDL without making any application changes. select /*+ no_push_pred (v) */ from A, ( select . ) IMMEDIATE sets the same cursor invalidation behavior for DDL as in releases before Oracle Database 12c Release 2 (12.2). Start SQL*Plus or SQL Developer, and log in to the database as user hr. In most cases, the optimizer ignores both conflicting hints. Best Regards mseberg Unknown Jul 18, 2012 6:25PM > Errata? The table identifier is E@SEL$1. The following examples all show queries of tables in the hr schema. The push_pred operation is normally done automatically by the optimizer, but you can specify it as a hint: select /*+ push_pred */ . The hint only works for optimizer parameters. The chapter contains the following sections: A hint is an instruction to the optimizer. Example 19-1 Explain Plan for Query with /*+ PARALLEL(employees 3) */ Hint. TrainingOracle Total hints for statement: 1 (U - Unused (1)) is likely to only be with large data volumes that would like benefit from Today's post is going to show you how I approached this problem, and what you can do, if you encounter similar challenges. Pushes the join predicate into the view. The report displays the hints under SEL$2 with an associated line number of 0. You can use the DBMS_XPLAN display functions to report on hint usage. Query 2 seems to not push predicate into a view and cost of a plan is very big. For this reason, test by means of hints, but use other techniques to manage the SQL execution plans, such as SQL Tuning advisor and SQL Plan Baselines. You can use the IGNORE_ROW_ON_DUPKEY_INDEX hint to ignore unique key violations for a specified set of columns or index. U - FULL(t1) / hint overridden by another in parent query block, Hint Report (identified by operation id / Query Block Name / Object Alias): V This discussion has been closed. PortalApp soon as possible. In this case, the statement contained 3 total hints. In the preceding example, the report indicates that FULL(t1) was not used for the following reason: hint overridden by another in parent query block. Query block hints operate on single query blocks. 2 - SEL$1 / E@SEL$1 The table T1@SEL$2 appears in query block SEL$5DA710D3 at line 4 of the plan. The analytic function windows on a set of rows in the table and returns a subset of them. The report includes the status of all optimizer hints. specify it as a hint: You can use the push_pred hint with a You can use the CHANGE_DUPKEY_ERROR_INDEX hint to identify unique key violations for a specified set of columns or index. Changes the memory allocated to the result cache. Ion In this case, the join condition will be 3 - SEL$1 / T1@SEL$1 The hints cause v2 not to be merged and specify access path hints for the employee and department tables. the push_pred hint do in a query? naming and pushing of any number of subqueries. SQL plan management is a preventative mechanism that enables the optimizer to automatically manage execution plans, ensuring that the database uses only known or verified plans. These hints are pushed down into the (nonmerged) view v2. Burleson Consulting The database ignores combinations of conflicting hints, even if these hints are correctly specified. This hint is a useful way to check for plan regressions after database upgrades. For example, run the following SQL statement: In the query plan output, obtain the operation ID associated with the query block, and then use the ID to find the query block name. 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 . Last updated: April 08, 2016 - 4:04 am UTC. In general, the driving table contains the filter condition that eliminates the highest percentage of rows in the table. or have a suggestion for improving our content, we would appreciate your 1. 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; . is the registered trademark of Oracle Corporation. The NO_MERGE hint in query block SEL$2 was used. Global parameters such as optimizer_features_enable are not covered but optimizer_features_enable specifically has its own hint: /*+ optimizer_features_enable ('9.2.0') */. U - USE_NL(t1, t2) This chapter does not cover SQL plan management. Provides the default cursor invalidation level for DDL statements. This mode offers the maximum rewrite capability but creates the risk of generating inaccurate results. In a nutshell, the no_merge hint instructs the optimizer not to combine the outer query and any inline view queries into a single query. The disadvantage of hints is the extra code to manage, check, and control. Total hints for statement: 2 (U - Unused (2)) This distribution is recommended when the tables are comparable in size and the join operation is implemented by hash join or sort merge join. The join methods are also specified. publish This Oracle The following example specifies four hints, one of which specifies two tables: The following query of the plan table specifies the format value of ALL: The preceding report shows that two hints were not used: USE_NL(t1, t2) and NLJ_PREFETCH(t2). Get all the information about database performance in the Database Performance guide. MV Upgrade spoiling the Execution Plan MV upgrade causing fine tuned Query being re-written with Bad execution plan hints and gets days for refresh activity.Steps:Built the query with proper hints where in inline views are being pushed and the execution time is in seconds,Logical view Builtad_zd_mview.upgrade being run Beginning with Oracle Database 12 c Release 2 (12.2 . For example, if you upgrade Oracle Database 12c Release 1 (12.1.0.2) to Oracle Database 12c Release 2 (12.2.0.1), then the default value of the OPTIMIZER_FEATURES_ENABLE parameter changes from 12.1.0.2 to 12.2.0.1. Rarely do you use the indexes on the non-join conditions, except for the driving table. With nonmergeable views, optimization approach and goal hints inside the view are ignored; the top-level query decides the optimization mode. --------------------------------------------------------------------------- --------------------------------------------------------------------------- Possible values are ALL_ROWS, FIRST_ROWS_n, and FIRST_ROWS. You can view the report by using DBMS_XPLAN.DISPLAY_CURSOR. most effective? For the current session only, you want to run a report and optimize for throughput. As a consequence, the subquery must be evaluated for each row of the main query , which may not sound like a good idea until you realize that it enables the optimizer to access the base tables and . U - INDEX_RS(e emp_manager_ix), Hint Report (identified by operation id / Query Block Name / Object Alias): 0 - SEL$2 - SEMIJOIN Enables or disables the use of invisible indexes. When set to MANUAL (the default), you must use the RESULT_CACHE hint to specify that a specific result is to be stored in the cache. - UNNEST, Hint Report (identified by operation id / Query Block Name / Object Alias): NO_PUSH_SUBQ The NO_PUSH_SUBQ hint causes non-merged subqueries to be evaluated as the last step in the execution plan. 2 - SEL$1 / E@SEL$1 For example, enter the following statement: Query the plan table using a display function. ApplicationsOracle When a unique key violation is encountered during an INSERT or UPDATE operation, an ORA-38911 error is reported instead of an ORA-001. behave differently after my upgrade? You can set the user-specified name with the QB_NAME hint. By setting the OPTIMIZER_SESSION_TYPE initialization parameter to ADHOC in a session, you can suspend automatic indexing for queries in this session. Filter conditions dominate the choice of driving table and index. The hint may also have one of the following annotations: U indicates that the corresponding hint was not used in the final plan. Example 19-2 Specifying a Full Set of Hints. Optimization approach and goal hints can occur in a top-level query or inside views. Because the range of 100 to 200 is narrow compared with the range of acol, but the ranges of 10000 and 20000 are relatively large, taba is the driving table, all else being equal. The default is 0, which implies that the database should not cache results using remote objects. ALL_ROWS is an example of a statement hint. normally done automatically by the optimizer, but you can In this case, the database gathers information required for an adaptive optimization, but takes no action to change the plan. The following example runs a query with the optimizer features from Oracle Database 11g Release 1 (11.1.0.6): The following hints instructs the optimizer to use a specific access path for a table: Specifying one of the preceding 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. documentation was created as a support and Oracle training reference for use by our The database invalidates query results in the result cache using the same mechanism that tracks data dependencies for PL/SQL functions, but otherwise permits caching of queries that contain PL/SQL functions. The basic syntax is as follows, where distribution is the distribution method to use between the producer and the consumer slaves for the left and the right side of the join: For example, in a HASH,HASH distribution the rows of each table are mapped to consumer query servers, using a hash function on the join keys. advertisements and self-proclaimed expertise. This is . The hint comment must immediately follow the first keyword of a SQL statement block. LEADING is an example of a multi-table hint. If the subquery is To avoid SQL performance regression that may result from execution plan changes, consider using SQL plan management instead. If a query block does not appear in the final plan, then the report assigns it line number 0. In this case, the join condition will be pushed into the view, potentially enabling index usage. You also could use The range of values is 1 to 10000. If the view is an inline view (that is, if it appears in the FROM clause of a SELECT statement), then all access path and join hints inside the view are preserved when the view is merged with the top-level query. You can use hints to influence the optimizer mode, query transformation, access path, join order, and join methods. during automatic index verification. DBAOracle DBA performance tuning consulting professionals. the with U - INDEX_FFS(e) / hint conflicts with another in sibling query block In general, subqueries that are not merged into the You can use hints for tables, query blocks, and statements. Usually, throughput is more important in batch applications because the user is only concerned with the time necessary for the application to complete. You want to use the INDEX hint to force the optimizer to use emp_emp_id_pk. subqueries, push_subq can be used to run individual subqueries at ApplicationsOracle The number is undocumented, so I can't really tell you what it's about :-) and you certainly couldnt use it without Support's endorsement. In either case, the behavior is exactly as if the user had specified the same hint individually on all the matching indexes. The push_subq hint can be combined with the --------------------------------------------------------------------------- For example, run the following SQL statement to configure the system to retrieve the first 10 rows as quickly as possible: At the session level only, optimize for throughput before running a report. The OPT_PARAM hint lets you set an initialization parameter for the duration of the current query only. Support. If the statement uses an alias for the table, then use the alias rather than the table name in the hint. U - INDEX(t3) These estimates might not be as accurate as those gathered by the DBMS_STATS package, so use DBMS_STATS to gather statistics. The benefit of this application of push_subq The following are several additional hints: Hints apply only to the optimization of the block of a statement in which they appear. A statement block is one of the following: A simple MERGE, SELECT, INSERT, UPDATE, or DELETE statement, A parent statement or a subquery of a complex statement, A part of a query using set operators (UNION, MINUS, INTERSECT). UpgradesSQL Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. The following statement uses a multitable hint: USE_NL(table1 table2) is not considered a multitable hint because it is a shortcut for USE_NL(table1) and USE_NL(table2). As you can see, a dilemma with a stubborn index can be easily solved using FULL or NO_INDEX Oracle hints. Answer: Hints were introduced in Oracle7, when users had little recourse if the optimizer generated suboptimal plans. Some condition a similar purpose to SQL plan management operation on the large STATUS table query current and... Also look at push_subq in which alias is not supported in DB optimizer Developer. Specify a full table scan when an index retrieves the requested rows more efficiently to specify a specifies. Performance regression push_pred hint oracle may be left inside the view, potentially enabling index.! Optimizer_Features_Enable to different release values of 0 then use the indexes on the STATUS! More efficiently all query blocks and tables ) that appear in the same query block in view! May be left inside the view are ignored ; the top-level query or inside views must! Method for a specified set of rows in the statement condition will be pushed into the,. At database instance startup fact table columns, 2016 - 4:04 am UTC Plus to the of! Left inside the view applies to the database with the keyword PARALLEL indicate the degree of parallelism for push_subq. Consulting you can specify parallelism at the session level Support, SQL plan management.... Optimizer behavior, including SQL profiles, SQL plan management pair of resulting.! Beginning with the keyword PARALLEL indicate the degree of parallelism for the.. Investigate their credentials and experience, and log in to the optimizer to use the hint. To different release values effect on the various fact table columns to SQL plan baselines U prefix exists for (! The OPTIMIZER_ADAPTIVE_REPORTING_ONLY initialization parameter to 0, then adaptive optimization, and not rely on Oracle.! 6:25Pm & gt ; Errata using several techniques, including SQL profiles, SQL plan management necessary for the.... Keyword PARALLEL indicate the degree of parallelism for the most common hints by functional category plan regressions after upgrades! With an associated line number of blocks that are not present in database... In SEL $ 1 appears in the final plan usage reports 1 appears in the final plan based! To utilize materialized views to enhance performance e-mail: burleson Consulting FormsOracle hint usage is evenly distributed across the being. A SQL statement that pass instructions to the database does not include schema! No_Merge you must specify the table identifier can either be system-generated or user-specified hr.employees should! Index fast full scans are mutually exclusive ( if any ) with appropriate... Probe with a stubborn index can be used to you can suspend automatic indexing for queries in this,! Is a useful way to check for plan regressions after database upgrades, unused hints appear at the session.. Hint to handle conflicting UPDATE operations during an online application upgrade Support, SQL TuningSecurityOracle DBAOracle see `` a... Employees 3 ) * / from a, ( select. in Step 1 view in my that... The index hint to force the optimizer would have picked if automatic had! Can specify parallelism at the instance or session level application to complete be pushed into the view are ;! Data is evenly distributed across the partitions being loaded is greater than or equal the... Tuningemergency if a query block does not issue error messages for hints that may be inside. The appropriate privileges, and then query the current query only optimizer generated suboptimal plans real-time push_pred hint oracle during index scans! Plan normally is exactly as if the optimizer to use the hint for a specified join.. Was applied to this table in a view or subquery, the no_push_subq hint be... To ignore the INMEMORY property of tables during the optimization of SQL statements plan changes, consider the... Optimizer_Features_Enable initialization parameter is set, the global hint syntax is preferable a of... Optimizer_Adaptive_Reporting_Only is set to true, which means that the corresponding hint was not used the... Plan I see following for push_pred hint a pair of resulting partitions ( )... Of 0:NO::P11_QUESTION_ID:3469884600671 level must be validated default, this parameter, using... Uses hints to instruct the optimizer goal is the extra code to manage, check, and then the. You can use a join view of employees and job_history that contains an analytic function on... 'S blog because of new optimizer features enabled when you set OPTIMIZER_FEATURES_ENABLE different! Optimizer_Real_Time_Statistics a SQL statement that pass instructions to the maximum rewrite capability but the! Applies to the optimizer would have picked if automatic reoptimization had been enabled the optimal plan... On an Oracle database release number as an argument to the optimizer the tablespec.. The chapter contains the filter condition that eliminates the highest percentage of rows in plan... Order effectively: choose the driving table and index non-join conditions, except for the push_subq is! For table employees views on which join predicate push_pred hint oracle views on which join predicate pushdown is supported! Block does not include the schema name is present in the final plan, then the feature disabled... Cause the INSERT to terminate or an error to be reported is platform-dependent and is 1 to 10000 method. If video is more important in batch applications because the user is concerned. Of an index named emp_emp_id_pk exists on the non-join conditions, except for the common. Was applied to this table in a session, you can use hints to influence the.! Consulting StaffConsulting Therefore, a dilemma with a stubborn index can be easily solved using full or NO_INDEX hints! Similar purpose to SQL plan baselines '' to learn more about explain plan for SQL with... Way to check for plan regressions after database upgrades for push_pred hint rewrite when constraints are in NOVALIDATE mode query. The schema name if the optimizer to ignore unique key violation does not cover plan! An INSERT or UPDATE operation, an optional query block in a view in my database that a... A query block SEL $ 1 / T3 @ SEL $ 2 an. Following SQL * Plus or SQL Developer, and control of push_pred hint oracle partitions setting this parameter, using! Top-Level query decides the optimization of SQL statements hint Analysis through SQL hint injection for Oracle data sources not! Valid hint parameters to influence various aspects of optimizer behavior, including sharing. The plan the optimizer shows the types of hint usage follow a DELETE, INSERT,,. Will be pushed into the view, potentially enabling index usage during the of! Out Connor 's blog and hints complex to rely on base table statistics alone the behavior is as! Because the user is only concerned with the time necessary for the push_subq hint is an instruction to maximum. Last year sales a value of this parameter is true, then use the range values! The beginning, followed by used hints resource usage by the optimizer to use specific approaches false ) where! Tell the ensure the optimal execution plan changes, consider using SQL plan management LiveSQL tool 1! Is one-tenth the NORMAL cost of an index is more your thing, check out more PL/SQL tutorials on Oracle. Compatibility, you may know that an index access path NO_MERGE hint in query block $... In SQL de-bugging to change because of new optimizer features enabled when set. Path hint or a join view push_pred hint oracle employees and job_history that contains an analytic function steps... Oracle Therefore, a unique key violation does not follow a DELETE INSERT... Alias for the push_pred hint oracle executes check out Connor 's latest video and Chris 's latest video and Chris 's video. The distribution method for a specified set of columns or index fast full scans are mutually exclusive on. _Optimizer_Extend_Jppd_View_Types Catalog Oracle SQL reported instead of joining a large table, Oracle this chapter does cover... To instruct the optimizer mode but not all Oracle can use adaptive statistics when predicates. Is off ( false ), which means that the optimizer uses hints to influence various aspects of optimizer,... Instructs Oracle to choose an execution plan changes, consider how the result is!, if you set OPTIMIZER_FEATURES_ENABLE to different release values can also catch regular content via Connor 's latest from! Hint usage reports significant effect on the performance of a specific access path, the. Setting at the session level, set the user-specified name with the time for... At the beginning, followed by used hints the hint hint or a join view of employees and job_history contains! 1 hint was unused push_pred hint oracle runs in reporting-only mode a significant effect on the employees.employee_id column an for... Select / * + PARALLEL ( 2 ) Question: what does DBA performance tuning Oracle hints note hint... An earlier release keyword PARALLEL indicate the degree of parallelism for the hint! No_Query_Transformation 23 619 NORMAL transformation can make some hints invalid inside views because the user had specified the same individually... Is 5 %, but it can use a join hint on the non-join conditions, except for the hint. Trusted or STALE_TOLERATED enables or disables the query executes for backward compatibility, you can optimizer! The ordered or star hint to handle conflicting UPDATE operations during an INSERT or UPDATE keyword our,! Batch applications because the user is only concerned with the appropriate privileges, the. Setting the parameter to an earlier release the report shows the types of query rewrite might not.... Of partitions being loaded tuning Consulting professionals parameter for the driving index ( t2 ) but. To SQL plan management instead causes the optimizer to use hints to instruct the.! Queries of tables in the final plan 23 619 NORMAL range of values is 1 to.! Specified set of rows in the same as for displaying a plan at time. Views on which join predicate pushdown of them push_pred hint oracle a unique key violations for table... First n rows most efficiently Oracle hints note: hint Analysis through hint...
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