oracle cardinality hint on join

Remember that we included a P_NUM_ROWS parameter in our pipelined function. SupportAnalysisDesignImplementationOracle The following bug has been raised for the problem: Bug 27989222 : WRONG CARDINALITY ESTIMATION WITH STALE DIRECTIVES WHEN FIX 21802552 IS ENABLED. All rights reserved by An explain plan *command* doesn't peek. Think again. If it is, please let us know via a Comment, http://www.cis.unisa.edu.au/oracle/server.101/b10759/functions014.htm#i1269375, http://docs.oracle.com/docs/cd/B19306_01/server.102/b14211/ex_plan.htm#sthref1416, http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:8764517459743#1929856400346576884, http://www.oracle.com/technetwork/issue-archive/2009/09-jan/o19asktom-086775.html. Oracle Academy. So Tom! Oracle ignores combinations of conflicting hints, but considers other hints within the same comment. Remote DBA Services select student_id, student_name, cardinality(SUBJECT) AS SUBJECTS_NUMBER, city First we will query our function to see how many rows it actually generates, as follows. Note: Oracle Database Tips by Donald Burleson. First, we'll modify our EMPLOYEES_PIPED pipelined function to include a P_NUM_ROWS parameter, as follows. So instead of properly adjusting the two-table formula to the multi-table case, Oracle uses it in a naive way: instead of operating on values from the intermediate data set on one side, and the table being joined on the other, it simply takes NDVs and other stats from the tables referenced in the join predicates, and plugs them into the standard formula. Can you post the exact SQL statement you are using that includes the CARDINALITY hint (perhaps there is a syntax error) Share 1) The row estimate after the two tables are joined is the same as the lower of the two estimates (1.4M rows). Sign . any predicate where the first part is constant and evaluates to false, isn't it equivalent to not specifying that predicate all? Tuning: The Definitive Reference" by Donald K. Burleson. You can verify the safety before doing such though by running an explain plan on that query with the HINT, and without the HINT, and comparing costs. 911RAC ServerOracle ConceptsSoftware SupportRemote ApplicationsOracle 516), Help us identify new roles for community members, Help needed: a call for volunteer reviewers for the Staging Ground beta test, 2022 Community Moderator Election Results. If we trace the optimisation of our query with a 10053 event, we can see further evidence that our interface type is being used by the CBO, as follows. The original query submitted had hard-coded dates instead of bind variables (which I asked them to change) but to replicate the issue, I am keeping them here. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. Developed Interacted BI reports using Buttons and Bookmarks to achieve the cardinality UI Show more Show less Cognizant 5 years 7 months . Merge Join cartesian is prefered only for the small tables which have a very small number of rows. You also explained why using parallel, an index might not be used. ALL RIGHTS RESERVED. As its name suggests, this hint enables us to supply the cardinality for a rowsource in a query. Share and learn SQL and PL/SQL; free access to the latest version of Oracle Database! Oracle INNER JOIN with USING clause Besides the ON clause, it is possible to use the USING clause to specify which columns to test for equality when joining tables. Despite the fact that we only want to query two names from the EMPLOYEES table, Oracle has chosen a hash semi-join. the answer is always -- BENCHMARK IT.. ops$tkyte@ORA920> create table t ( weight int, pet_id int, setup_date date ); Table created. See the Query Hints topic in the SQL Server documentation for full details. . Hints and SQL*Plus Auto trace report for query optimization-Created new procedures, functions, triggers . 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Challenges of a small company working with an external dev team from another country, Aligning vectors of different height at bottom. Wanted! In such situations, the join cardinality is much harder to estimate accurately. . Oracle ignores hints if the comment containing them does not follow a DELETE, INSERT, SELECT, MERGE, or UPDATE keyword. At this point having cardinality statistics for the function can be critical to enable the CBO to derive an efficient execution plan. For pipelined functions with small resultsets, this 100% sample might be trivial compared with the effect that the wrong cardinality could have on the overall execution plan. You may need to use this hint with a global temporary table, for instance. In Software in Silicon (Sample Code & Resources). Tips We can loosely categorize them as execution hints, and this is a known limitation of the hint usage report. Using the hint, we have supplied the exact cardinality of our pipelined function to the CBO. So the predicate takes the form, are the estimated card= values in the plan for all of the other steps anywhere near "correct", if the result set is small - it would opt for nested loops -big - hash join. This function does nothing with the collection itself; it is merely a wrapper over it. 2022 - EDUCBA. This feature enables us to design our own cardinality calculations (as a prescribed method in the interface type) and then associate them with our table or pipelined functions. I am not sure I understood your response. Oracle But pushing this to the 3-table entabled join case doesnt really have such solid foundations. I understand that the CBO does not know of the correlation between the two tables. UNIXOracle To use the Extensible Optimiser with a table or pipelined function, we require three components: We will create each of these components below. This feature in Oracle allows you to increase the value of an expression at the compile time . As there were no bind variables used, I can't see how CBO could be influenced by peeking. These hints are used internally by Oracle but they haven't been included in the public documentation. Unfortunately, some boundary conditions apparently havent been handled properly since implementing this change. Scripts This is not correct. As it turns out, this assumption is not accurate in this case, as there is a strong correlation between COL3 and the other two columns (I spotted it by noticing that removing a join on COL3 doesnt change the rowcount significantly, despite the fact that it has a relatively high NDV). Likewise, an extended column group will do the same thing without requiring the index on top. I mentioned in an earlier blog post that in Oracle Database 19c we introduced a hint usage report for DBMS_XPLAN. By closing this banner, scrolling this page, clicking a link or continuing to browse otherwise, you agree to our Privacy Policy, Explore 1000+ varieties of Mock tests View more, Special Offer - Oracle Training (14 Courses, 8+ Projects) Learn More, 360+ Online Courses | 50+ projects | 1500+ Hours | Verifiable Certificates | Lifetime Access, Oracle Training (14 Courses, 8+ Projects), Oracle DBA Database Management System Training (2 Courses), All in One Financial Analyst Bundle- 250+ Courses, 40+ Projects, To 10 Oracle Database Interview Questions, Oracle Apps Technical Interview Questions. Consulting StaffConsulting publish We can see that the CBO uses a cardinality of 8,168 rows for our pipelined function. Remote Instead, it will be used by the CBO as we will see shortly. This time the optimiser has the correct cardinality due to our interface type. Example : For an alternative implementation of our Extensible Optimiser method, we could remove the P_NUM_ROWS parameter and instead use a lookup table to store representative cardinalities for all of our table or pipelined functions. PricesHelp Oracle SQL Join Cardinality Oracle Tips by Burleson Consulting. What I at least THINK I've observed is that the term gets confused with selectivity, which I think is used in 2 different ways. The Extensible Optimiser feature is the most flexible method to use at this stage and is usable in all versions of 10g. The OPT_ESTIMATE hint can be used to re-factor the cardinalities for tables (this includes views and table functions), indexes and joins. The second question is more interesting. Oracle High log file sync waits? Our implementation is very similar to our previous example. In my opinion, it is a bug. from my_student; As we see in the above screenshot the query returns the student id and the measure of elements in each row of the subject column. The Oracle of Instead, it used bind variable expressions. The reason why is in the trace file, as the following excerpt demonstrates. There is an undocumented cardinality hint you can try: Doh. The number of records retrieved is 3500. Catalog 2) If it thinks that joining to B reduces the data set this much, why doesnt it try to do this earlier, making the intermediate data set smaller? The ODCIStatsTableFunction is the method where we can be creative, although in fact our implementation is very simple. This Oracle Perhaps, but according to the optimizer, joining A to V before joining to B is a better strategy, because it allows to reduce eliminate 80% of rows before performing the expensive join to B. The syntax of this new query hint resembles the following: <query_hint > ::= { USE HINT (N'key' [ [, ].n ]) } This update also introduces the following hint options that can be used with the USE HINT argument. This is CARDINALITY (table n): This hint instructs Oracle to use n as the table, rather than rely on its own stats. I am aware of the cardinality and opt_estimate hints and have used them successfully but only for static tables, not inline views or CTEs. Let us now create a query where we get the cardinality of subjects only for students from Agra city. are statistics current & correct for all tables & indexes involved? The workload was reduced because Oracle was simply trying to get the pipelined function's rowcount. Fourth and finally, we must associate the function with the interface type, as follows. Let us now look at the below examples to understand better the COMMIT statement. from forthcoming book "Oracle I.e. Get college- and career-ready with Oracle Academy. Many times however, we use cardinality meaning "distinct cardinality" when discussing selectivity. As we can see in the above screenshot the output returns the student name, student id, number of subjects of the students who are from the city of Agra. rev2022.12.7.43084. Centinul said to use this hint carefully - How can I assure that the hint is used carefully?I mean present and future explain plan will be optimized plan? I normally dont write reviews but I had to say this, the answers could have been better if it were written with simple vocabulary. This is very similar to our previous example so doesn't need to be explained in any great detail. Cardinality is an advanced hint in SQL that lets the optimizer calculate the number of rows that will be returned before the SQL statement is run. 1) Getting join cardinalities right for multi-column joins is a tricky business If not, this oracle-developer.net article is recommended background reading. Oracle Totally works. As stated earlier, table functions are typically used to support variable in-lists, so we will see an example of how this wrapper method can assist in this scenario. I agree that there is a lot of room for improvement in the readability of the blog. Answer: The inline hint is the opposite of the materialize hint. Given that the optimiser uses a default (and usually incorrect) value for cardinality, it follows that this could lead to sub-optimal execution plans. Thanks to Patrick Jolliffe for pointing that out. Copyright 1996 - 2020 Or to be more accurate, I reserve this approach for root cause analysis after a solution has been offered if time allows. Copyright 1996 - 2020 What is inline hint in Oracle? There are cases when transitive closure can be used to disentangle a join, e.g. Syntax: CARDINALITY( column ) Parameters: column: It is used to specify the column in the nested table whose cardinality needs to be returned. That wasn't the question. There is no single constant. How to create id with AUTO_INCREMENT on Oracle? USE_NL / NO_USE_NL. So That explains that question about the diffference between estimates when using parallel hint and not using it. In the query with the parallel hint, there is a note saying: "dynamic statistics used: dynamic sampling (level=7)". their Oracle Would the US East Coast raise if everyone living there moved away? I.e. As far as previous releases are concerned, the same test case runs correctly on 12.1. Lets take another look at the simplified query and its plan: So we have a view (successfully merged by the CBO) V consisting of 4 tables, V1, V2, V3 and V4, and two tables, A and B. LinuxMonitoringRemote supportRemote In my previous post I showed an example of how a querys performance can be improved using the waste minimization technique. Actually, in a way the optimizer is correct: indeed, joining to B does reduce the dataset considerably, but less than the CBO suggests (2 orders of magnitude instead of 5). In this article, we discussed the definition of the CARDINALITY function in Oracle. Part of theSQL Server 2022 blog series. I did notice that the execution plan was producing some pretty incorrect cardinality estimates. If we repeat the query with a dynamic sampling level of just 2 and trace the CBO using the 10053 event, we can see quite clearly how the CBO can be so accurate at lower sampling levels. By using a dynamic sampling hint on INDEX without parallel hint, I was able to get a better estimate of the number of rows returned. Suppose I have a query typically used in web-based reporting tools that allow for optional search filters. And of course, keep up to date with AskTOM via the official twitter account. What are different types of hints in Oracle? Related research on SQL tuning with cardinality feedback Also see these related notes on cardinality estimation: ORACLE CBO EXTENSIBLE OPTIMIZER CARDINALITY Inside Oracle SQL optimization This table consists of both non nested column as well as a nested column. Feel free to ask questions on our I guess they're in some way related? Strengthen Your Skills. We can see that it took less time and fewer resources than our actual query, yet it accessed the same number of rows. DBA performance tuning consulting professionals. using the WITH clause): Here are some working examples of What mechanisms exist for terminating the US constitution? 17.8M in INVOICE_LINE table and 1.4M in INVOICE table. THE CERTIFICATION NAMES ARE THE TRADEMARKS OF THEIR RESPECTIVE OWNERS. A subquery is a SELECT statement that is nested within another SELECT statement and which return intermediate results. We can see that this query completed extremely quickly (this was due to having Autotrace set to traceonly so that the resultset wasn't spooled to screen and a SQL*Plus arraysize of 500). With this in mind, we will use the hint in our standard query, as follows. qualifications. As a student you can get free access to a wealth of . Your email address will not be published. Previous Post Optimizer Adaptive Features and Upgrading to Oracle Database 12c Release 2 or Oracle Database 18c First of all, the test case is based on the assumption that optimizer_dynamic_sampling is set to 2: Also, before running the test Im verifying that optimizer adaptive statistics have been enabled: Then, Im creating three tables t1, t2 and t3. Do Spline Models Have The Same Properties Of Standard Regression Models? What is cardinality hint in Oracle? the issue is, the plan is generated before the query is executed. The bug ist still here tested on 19.7.0.0.200414. Can the UVLO threshold be below the minimum supply voltage? This table consists of three columns. it takes the various parameter settings in place and using the built in model -- assigns costs. I think I've also seen it as how many rows come back on average from the total so that in the case of a 100 distinct row table, a selectivity of .01 would "be good", or as you approach zero that is "good" as oppossed to 1. cardinality in that context is a function that returns the count of the elements in a collection. First, we will execute the query without the COLLECTION_WRAPPER function, as follows. You could confirm this by generating an explain plan of the query. As we can see, the CBO has picked up the cardinality value that we passed to our pipelined function and used it to optimise our SQL. This also demonstrates that the COLLECTION_WRAPPER and COLLECTION_WRAPPER_OT objects can be re-used for all queries that include small collections (as in-lists or otherwise). Rob. Separating columns of layer and exporting set of columns in a new QGIS layer. plansRemote experience! We will now create a pipelined function, as follows. Its worth noting that I intentionally gathered statistics on the empty tables t2 and t3 before loading any data. So, in this example, we will find the cardinality of the nested column subject which is present in table my_student. This Oracle It was about the 1=2 (or any expression that evaluates to false and has a OR in the predicate). Feel free to ask questions on our Is it safe to enter the consulate/embassy of the country I escaped from as a refugee? We have now completed our setup for the Extensible Optimiser. The first one to hard parse sets the plan. In fact, as Jonathan Lewis noticed in Cost Based Fundamentals, if you use transitive closure to legally re-arrange multi-table join predicates, you often would be getting different results! Second, we will create a small function that will receive and return a collection of our generic VARCHAR2_NTT type. But changing optimizer_dynamic_sampling to 4 will sample table t in the query (with predicates on c1 and c2) without the dynamic sampling hint. from my_student where city ='Agra'; As we can see in the above query we have given the city as Agra in where condition. So, in the case of a query like: After reading up on this hint I found some queries (over skewed data in very large tables) performed waay much faster when using the hint /*+ dynamic_sampling(3) */. >The original query submitted had hard-coded dates instead of bind variables. Excel-DB, Oracle Database Tips by Donald BurlesonDecember 4, 2015. Solution In this Document Goal Solution My Oracle Support provides customers with access to over a million knowledge articles and a vibrant support community of peers and Oracle experts. Despite the fact that we shouldn't use this hint in production code, we will see the effect it has on our query below. As we can see, the result is that Oracle uses a hash join, as it did when we used the CARDINALITY hint above. Anyway, there's not exactly a similar hint in SQL Server to Oracle's cardinality hint, and regarding the parallel query hint, you can specify that in SQL Server with the MAXDOP hint like so: As its name suggests, this hint enables us to supply the cardinality for a rowsource in a query. Your email address will not be published. Oracle 911RAC I was looking for different oracle questions, always this website came up first, so I would read this answer and then had to move to other websites as I dont comprehend the style of writing that much. In your case it appears that the cardinality hint is estimating the number of rows in the dp_files table to be 100. The optimizer recognizes hints only when using the cost-based approach. servicesApplication This is because the optimiser has used the 8,168 heuristic cardinality for our collection. The estimates for each individual table are pretty correct. By using an in-line view, SUM and NVL, Oracle executed a more complicated version of a COUNT(*) from the function to determine its rowcount. Oracle To be clear, it's the cardinality of the inline view I'm trying to influence, not that of a constituent table. The raw trace file tells us that the values of the bind variables were 0 for the default NVL arguments in the outer query block and 1 for the C1 and C2 expressions in the in-line view. . from global_temp_table; You can buy it direct from the publisher for 30%-off and get instant Unfortunately, most of the settings cannot be easily changed, at least not without some additional investments both in terms of money (switching to the premium WordPress plan) and time. With a cardinality of 8,168 for the pipelined function, the optimiser has chosen a sort-merge join. First, we will create a simple collection type to support any variable in-list of string values. The type's cardinality method is invoked by the CBO during query optimisation to determine the rowcount for the pipelined or table function. The resulting trace file (formatted with TKProf) shows two SQL statements for our query. oracle-developer.net 2002-2022 copyright Adrian Billington all rights reserved | original template by SmallPark | last updated 25 January 2016. an association between the function and the type. Support. He's worked with Oracle technology since 1988 and joined Oracle in 1996. advertisements and self-proclaimed expertise. An index join is often more expensive than simply scanning the most selective index and subsequently probing the table. The OPT_ESTIMATE hint is used in SQL Profiles to scale the CBO's estimates up or down according to the correction factors it determined during a SQL Tuning Task (note that during a SQL Tuning Task, Oracle compares actual rowsource cardinalities for a query with the estimates it used to derive the original plan. We also discussed the syntax of the function and also the working of the function. Later on, we went through different examples to understand the implementation of the function better. We will use the ORDER BY clause for the same. The Ion tool is an easy way to analyze Oracle SQL performance and Ion also allows you to spot hidden SQL performance trends. This time, the optimiser is aware that there are only two elements in the variable in-list collection and has opted for a nested loops join accordingly. The actual rows returned is 17.8M, as I would have expected based on the fact that the invoice_id and delivery_date must match in both tables. don't sample: Is this answer out of date? It contains the numeric number equal to the number of elements in each row of the nested table column and in case there is no element than it returns NULL in the result set. For more information, see Hints (Transact-SQL) - Query. Think you need a certain kind of background to join Oracle? Of course, you can also set it only for a SQL by hint. You can also force it at a certain value via hint. I can force an index range scan using using INVOICE_IX4 (by adding an index hint) but I cannot force the fast full scan using INVOICE_IX1. The cardinality hint in Oracle is for when you have predicates that are too complex for Oracle's Cardinality Estimator, but your query looks rather simple. SupportApps You can also go through our suggested articles to learn more . Let us look at the query without using DESC with ORDER BY. However, there might be times when we need to join a pipelined or table function to a set of other rowsources (tables, views etc). How do I limit the number of rows returned by an Oracle query after ordering? The CARDINALITY hint is simple to use (although quite limited) yet for some reason remains undocumented. When one talks about an index selectivity, I've seen it as - number of distinct rows divided by the total number of rows (cardinality of the table?) These execution statistics are then shown next to the original Optimizer estimates in the execution plan if you use the function DBMS_XPLAN.DISPLAY_CURSOR to display the plan. Let us look at the data present in the my_student table. A page item :x determines whether or not the collection is populated. Check out more PL/SQL tutorials on our LiveSQL tool. Instead, the optimizer decides himself not to do the transformation as a result of a wrong cardinality estimate which, in turn, is caused by the adjustment-to-zero issue. Expertise through exercise! I was left with something that ran pretty well. The duplicate data and the fact that there are no complex transformations (which is usually the reason for using a pipelined function in the first place) is irrelevant in this case as we simply need a representative rowsource for our examples. Its a very interesting question, and it deserves a separate post so that it could be explored in detail. The cardinalityhint is used in two general cases, complex joins and dynamically created tables like global temporary tables (and possibly using materializations using the WITH clause): Here are some working examples of the cardinality hint: select /*+ cardinality( gtt, 500 ) */ stuff What is cardinality hint in Oracle? Oracle Let us now look into how it works in oracle. There are several guidelines for using the parallel hint: The target table or index must be doing a full-scan operation, A starting point for the parallel degree us cpu_count-1 (the truly fastest degree is determined empirically via timing the query. This is a heuristic value (rather than an estimate) that the optimiser uses for all table/pipelined functions. their Oracle It is assumed that readers are comfortable with pipelined and table functions. you need some reparsing - because you want to have potentially different plans each time. We have many different programmes for a variety of different interests. It would be good to avoid the parsing step forced by the use of dynamic SQL. because using the ORDERED hint allows you to specify Let us first take a look at the table my_student. Connect and share knowledge within a single location that is structured and easy to search. Using Autotrace to generate an Explain Plan, we will re-query the function to see how many rows the CBO thinks it returns. If you are correctly specifying the CARDINALITY hint and that doesn't change performance, that would imply that the problem is not with the optimizer's cardinality estimates. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Enter your email address to follow this blog and receive notifications of new posts by email. 1) cardinality hint is good, you can also gather statistics on it. Excel-DB, How does Oracle estimate table join cardinality? keys was introduced a while ago and the main way to create them was by using the intention action Store Relation at the JOIN clause in your query. Besides that, I reengineered the test case, so that no hint is used any more to prevent unnesting. Most of the posters saying against using Cardinality, But, there must be some good reasons of using this hint. Its a constant scrolling up-down left-right. e-mail: Burleson Consulting In fact, a one-row estimation is often a cue for a completely wrong calculation. Also, what is cardinality hint in Oracle? An undocumented yet often used hint to aid Oracle when statistics are unavailable or inaccurate is CARDINALITY ( tab_name_or_alias number_of_rows ) . These estimates rely on two underlying things: statistics about the data and a set of assumptionsknown as the model. These hints allow you to instruct the optimizer to use a hash join, a sort-merge join, or nested loops, respectively. As we can see in the above screenshot the output returns the name arranged in descending order along with the number of subjects for each name using the cardinality function. joins and dynamically created tables like global temporary tables (and possibly We will filter the EMPLOYEES table by a variable in-list of names. To test it, we will repeat our sample query but without any hints, as follows. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. Oracle ServerOracle ConceptsSoftware SupportRemote Find centralized, trusted content and collaborate around the technologies you use most. In our example, we have simply exploited the Extensible Optimiser feature to create our own alternative to the CARDINALITY hint. Oracle Academy. In this scenario, we are going to restrict the result set of the SELECT statement using the WHERE condition. [MongoDB] . 3) The cardinality estimates are actually lower if I do not use the parallel hint. did you read the error message? To see the effect of the Extensible Optimiser in this case, we will repeat the query but with the COLLECTION_WRAPPER function, as follows. The first question is easier. Continue to be careful with the workaround, as it can cause problems elsewhere. Join operation hints are also paired: USE_HASH / NO_USE_HASH. If it is, please let us know via a Comment, http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:3779680732446#15740265481549, https://www.oracle.com/technetwork/issue-archive/2009/09-jan/o19asktom-086775.html. independently investigate their credentials and experience, and not rely on More Information This update is included in Service Pack 1 for SQL Server 2016. I often face an issue writing Oracle SQL queries, where the optimizer makes questionable choices re cardinality. Burleson Consulting You said the "1=2 goes away" but it doesn't appear to. Why is this?". With the introduction of dynamic sampling in 10g , there is no need for cardinality hint. Burleson it will not read the first N, it'll randomly sample. Which is what I would expect to happen even with the parallel hint. In these cases, cardinality is not particularly useful as there is only one possible execution plan. Burleson Consulting sql oracle Share Improve this question Follow edited May 18, 2017 at 10:24 SynozeN Technologies However, for delivery_date, the date range predicate is pushed from INVOICE to INVOICE_LINE based on the fact that I have demanded the delivery_date on both tables is the same. In principle there are three common ways in which Oracle can operate a parallelOracle can operate a parallel We do this as follows. We can now add our interface type body, as follows. Another case can be of using the ORDER BY without using the DESC keyword. Remote DBA Services It's one of the flaws of using it. Another Capital puzzle (Initially Capitals), Alternative idiom to "ploughing through something" that's more sad and struggling. We will see examples of this below. With this technique, we could avoid hard-coding cardinalities and modify them over time in the lookup table as needed. How to influence oracle cardinality of inline view / CTE result? Usage. It is because of the difficulties of this nature that I am not a fan of the cardinality-centric approach to SQL tuning problems. instead of a simple two-table join we have an entangled join, where A is joined to the result of (V1,V2) join via columns from both V1 and V2. All rights reserved. What do bi/tri color LEDs look like when switched at high speed? Therefore, rephrasing the query with a join will allow the optimizer to use the most efficient execution plan for the query. Running Oracle 12.1.0.2.0 on Solaris Sparc 11.3. Even at level 2, the CBO has the exact cardinality. Connor and Chris don't just spend all day on AskTOM. It is undoubtedly a significant enhancement compared to the previous release, where the information was stored in the result cache. When we execute the function with a SELECT statement the function first checks whether the column is a nested column or not and then it measures the number of elements present. The tracefile also contains a second query of our pipelined function, as follows. publish First, we can see the query itself, as follows. */ Tips Let us look at the table data once before we apply the function. Better to run it and use dbms_xplan.display_cursor, Is this answer out of date? documentation was created as a support and Oracle training reference for use by our Because all data is available in the indexes, no table access is needed. ForumClass independently investigate their credentials and experience, and not rely on the optmizer . Moreover, those newly created SPDs containing the result of DS will supersede the SPDs for DS itself, and in doing so completely eliminate DS in future optimizations: Anyway, given the optimizer estimated the cardinality perfectly and those estimates are successfully being applied to the subsequent executions, the following question inevitably arises: What went wrong with the on-going optimization process? The notes section of the DBMS_XPLAN output tells us that dynamic sampling was used for our query. I guess that your other readers would also be happy with such change. In this article I would like to give you idea about Oracle Hints and Oracle Hints Examples in detail. CARDINALITY Help Center SQL Language Reference Database Oracle Oracle Database Release 18 SQL Language Reference Table of Contents Search Download Table of Contents Title and Copyright Information Preface Changes in This Release for Oracle Database SQL Language Reference 1 Introduction to Oracle SQL 2 Basic Elements of Oracle SQL 3 Pseudocolumns ApplicationsOracle I posted it again to clarify. Incidentally, our type could also be associated with other functions (assuming they also had a single parameter named P_NUM_ROWS). - 1 Sql Plan Directive used for this statement, DYNAMIC_SAMPLING JOIN CARDINALITY MISESTIMATE SUPERSEDED, DYNAMIC_SAMPLING_RESULT VERIFY CARDINALITY ESTIMATE USABLE, Join Card adjusted from 444734.345351 to 0.000000 due to adaptive dynamic sampling. Why do we always assume in problems that if things are initially in contact with each other then they would be like that always? Should I presume that the optimizer has determined that it is better to parallelize and use a full table scan rather than use any available indexes to filter the INVOICE records? not with the collection, I asked "Besides, given a predicate like (1=2 or c in (set)), i.e. Would you agree with this observation? PricesHelp Table functions are commonly used as a mechanism to bind variable in-lists passed as collections into SQL queries (for an example, see this article). ); Code language: SQL (Structured Query Language) (sql) The above text is an excerpt Performance Tuning The blockchain tech to build in a crypto winter (Ep. Verify Consequently, you might observe some catastrophically wrong join cardinality estimates (and bad execution plans too) when dynamic sampling is performed. Finding CARDINALITY of Column As we have discussed that we can find the cardinality of a nested column using the CARDINALITY function. the plans are different (that is permitted, there is no reason that might now happen). The following illustrates the syntax of the INNER JOIN with the USING clause. So, in this example, we will find the cardinality of the nested column subject which is present in table my_student. UpgradesSQL Th first set of output is from production and the second set of values from testing. Tell me more. We will complete this article with an example of a table function. stuff e-mail: Burleson Consulting Required fields are marked *. Examples The following example shows the number of elements in the nested table column ad_textdocs_ntab of the sample table pm.print_media : And of course, keep up to date with AskTOM via the official twitter account. I'm thinking it may involve the qb_name hint and have had some success with this, but not in this particular case. gets it wrong. This was a little bit . The only thing that would filter records is trying to match ODS_INVOICE_ID in both tables. select student_id, CARDINALITY(subject) Making statements based on opinion; back them up with references or personal experience. What I'm wondering is how to tell the optimizer that I know for example that even though it thinks t2 will result in 2 million rows, in fact, it might be more like 1 thousand. How is join cardinality estimated? Hadoop, Data Science, Statistics & others. The ODCIStatsTableFunction method simply picks up the parameter we pass to our pipelined function, optionally uses it to calculate a cardinality value and then passes it on to the CBO. The CARDINALITY hint is simple to use (although quite limited) yet for some reason remains undocumented. optimizers inability to estimate the inter-table join result set. DBAOracle hint is one of the most popular SQL tuning hints, In the absence of column histograms, the Oracle CBO must be able to guess on information, and sometimes the guess is wrong. Apart from being a supported method, another benefit over the CARDINALITY hint is that initial value of P_NUM_ROWS could be passed as a variable rather than hard-coded as above (it must be known in advance when used in the CARDINALITY hint). What I mean by that is the optimizer estimated just one row while 449K were returned. Nice. USE_MERGE / NO_USE_MERGE. I wanted the plan to use a NL operation for the "X" table but it was always going to a hash join (reason being a high cardinality estimate of 700K for the "minus sub-query") before I added the cardinality hint . As far as I know " The cardinality hint is used in two general cases, complex joins and dynamically created tables like global temporary tables" - but I think the hint can be used for other reason as well. The cardinality hint is used in two general cases, complex Two of these methods are unsupported (as of 11g Release 1) and for this reason, their use in production code is discouraged. . Consequently, you might observe some catastrophically wrong join cardinality estimates (and bad execution plans too) when dynamic sampling is performed. As a consequence of this shameless manipulation, SQL plan directives (SPD) for DS will be created at the next execution, like for example: As you can see, SPDs for DS were indeed generated. JOINS Chapter 5: HINTS Chapter 6: BASICS Chapter 7: ROW COUNTS and RUN TIMES Chapter 8: EXADATA LAB: Reverse Engineering the Oracle SQL CTE (Common Table Expression) where no data, Avoid Full Table Scan with subquery or analytic function in view. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. With a more suitable cardinality, the optimiser has chosen an alternative access path (a hash join in this case). Parameters separate by space (optionally by comma). - - - lpi . 1) it'll use bind variable peeking with all that entails and implies. 1 2 3 SELECT /*+ CARDINALITY (gtt, 1000) */ gtt.gtt_id, dname FROM dept d, global_temp_tab gtt WHERE d.deptno = gtt.deptno; Of the two supported methods, the DYNAMIC_SAMPLING hint is a new feature of 11.1.0.7 and has some limitations and performance implications. ForumClass access to the code depot of Oracle tuning scripts. This default value appears to be dependant on the default database block size (this example is from a database with an 8kb block size). You can also catch regular content via Connor's blog and Chris's blog. Calculating expected value from quantiles. I'm thinking it may involve the qb_name hint and have had some success with this, but not in this particular case. The important point to note is that while measuring the elements inside each row of a nested column it does not check whether the elements are duplicate or not. This is one reason why the ORDERED hint is one of the most popular SQL tuning hints, because using the ORDERED hint allows . This is to be expected of course, because we have supplied the same information to the optimiser, albeit by a different method. The above output is from v$sql_plan. strive to update our BC Oracle support information. For more information on the Extensible Optimiser and how it can be used to generate a range of statistics for functions, read pl/sql functions and cbo costing on this website and the further reading references provided therein. However, I couldnt find any information on Metalink about it. 2) When dealing with entangled multi-table joins its even trickier SQL executes innermost subquery first, then next level. Answer: The full hint is an optimizer directive used to force a full table scan of the specified table. Yes. The reason for that is that Oracle doesnt have any special treatment for multi-table joins: it simply joins one table at a time. The model determines how we interpret . The second method uses the OPT_ESTIMATE hint. By doing so, Im misleading the optimizer to come up with wrong join cardinalities. The use of the cardinality hint compensates for the Site design / logo 2022 Stack Exchange Inc; user contributions licensed under CC BY-SA. The majority of the examples in this article will be based on a join between the DEPARTMENTS and EMPLOYEES data in the supplied HR schema. Oracle ignores hints containing syntax errors, but considers other correctly specified hints within the same comment. so that as you approach 1, you have a selective index. OK my actual use case is for an Apex report region where a list of ids is stored in a collection. PL/SQL reference manual from the Oracle documentation library. the absence of column histograms, Oracle CBO must be DBA performance tuning consulting professionals. Oracle. Avoid Correlated Subqueries (query line: 7) : A correlated subquery is a subquery that contains a reference (column: PROGRAM_NO) to a table that also appears in the outer query. The CARDINALITY function is supported in the various versions of the Oracle/PLSQL, including, Oracle 12c, Oracle 11g and Oracle 10g. The Oracle of . Table and pipelined functions are often used as simple rowsources (for example, SELECT * FROM TABLE(dbms_xplan.display)). We will now create an interface object type, beginning with the specification, as follows. Oracle Tuning Power Scripts: With 100+ High Performance SQL Scripts . To counter this, we can wrap our collection in a call to the wrapper function and enable the CBO to get the correct cardinality, as follows. Hash joins support input swapping, which we have discussed when we talked about left-deep and right-deep join trees . Just All legitimate Oracle experts Oracle For this reason, they should not be used in production code. As indicated in the title, the issue has been introduced in the release 12.2. These hints can be passed to the server, but the server ignores them. Oracle Consider the following: The actual query text is not that important, it's more the structure. Note that this function pipes over 20,000 records by inflating the EMPLOYEES dataset by a factor of 200 (this volume of data will show the impact of using correct cardinality statistics more clearly). 1. I cannot afford this investment at the moment, but Ill keep your comment in mind, and if an opportunity presents itself, Ill work on improving the blogs design. advertisements and self-proclaimed expertise. What's the benefit of grass versus hardened runways? NOT a problem with the optimizer, as no amount of metadata will help when but, if you wanted the "dynamic cardinality" to be used - we'd have to hard parse it to see if a different plan were warranted. You need a comprehensive source of information and in-depth tips and techniques for using the new technology. Because theres no fundamental reason why it should give the correct one. TrainingOracle Support, SQL TuningSecurityOracle However, two of these hints are unsupported and the third is potentially costly. Let us create a SELECT statement for the same. Note that you can adjust join cardinalities between 2 tables in order to make that join not to be used. I changed alter system to alter session in the workaround, as switching off the fix globally can produce side-effects elsewhere. The answer can be found in the predicate section: the hash join on line 17 references 3 distinct tables: A, V1 and V2. This recursive query was executed by Oracle because we used the DYNAMIC_SAMPLING hint. Third, we will create an interface type specification to be associated with our simple COLLECTION_WRAPPER function, as follows. Tuning: The Definitive Reference. Also, the difference between the good and the bad plan is dramatic. Thank you very much! TuningEmergency Oracle forum. On databases with larger default block sizes, the CBO will use a larger heuristic value for the cardinality. If you find an error If the optimizer thinks that joining to B reduces the datasets so strongly, why doesnt it do it at an earlier stage? 4) Sometimes transitive closure can allow to disentangle join predicates if such transformation is possible, it can improve the accuracy of the join cardinality estimate. Database Support Database Support Some important points to note about this are as follows: The interface type body is where we code our cardinality calculation, as follows. Scripts By using a dynamic sampling hint on INDEX without parallel hint, I was able to get a better estimate of the number of rows returned. Unfortunately, some boundary conditions apparently haven't been handled properly since implementing this change. the only stats that where missing were the column stats, everything else (table, index) were there. Targeting an individual query Try levels 8, 10, and 11 to see if dynamic statistics help. Oracle technology is changing and we servicesApplication document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); This site uses Akismet to reduce spam. After a couple of months I've been asked to leave small comments on my time-report sheet, is that bad? Anyone BI, Oracle and Qlikview.-Worked on legacy data replication tool to Kafka stream migration. Parallel query will enable a higher level of dynamic sampling unless your session is using a non-default value for optimizer_dynamic_sampling. Our sample query but without any hints, because using the with clause:! Add our interface type, as follows my time-report sheet, is that bad versions of the and. Tools that allow for optional search filters dates Instead of bind variables Connor Chris! A or in the workaround, as follows for multi-column joins is a SELECT statement using the ORDER without. Guess they 're in some way related using it INVOICE_LINE table and 1.4M in INVOICE table with. Without using the where condition typically used in web-based reporting tools that allow for optional filters! Is executed are unsupported and the third is potentially costly test it we. Confirm this by generating an explain plan * command * does n't appear to parallel hint and had! Earlier blog post that in Oracle or UPDATE keyword containing them does not follow a DELETE,,! The Oracle of Instead, it will not read the first part is constant and evaluates to,. The CBO has the correct one make that join not to be 100 this function nothing... Modify them over time in the release 12.2 the cardinalities for tables ( and bad execution too! Very similar to our previous example so does n't peek switching off the fix globally can produce elsewhere... Its worth noting that I intentionally gathered statistics on it cardinality is much harder to estimate the inter-table join set! And possibly we will create a small company working with an external dev team from another country, Aligning of... One reason why the ORDERED hint allows you to specify let us first take a at. Conditions apparently havent been handled properly since implementing this change sad and struggling forced by the use the! A refugee from table ( dbms_xplan.display ) ) higher level of dynamic sampling in,... I mean by that is the optimizer to use ( although quite )... I have a very small number of rows returned by an Oracle query after ordering switching off fix. Used to disentangle a join, or nested loops, respectively * command * does n't peek the cardinality. Our is it safe to enter the consulate/embassy of the posters saying against cardinality... Share knowledge within a single location that is structured and easy to search estimating the number of rows SQL problems... Introduction of dynamic sampling is performed column subject which is what I would like to you! And using the ORDERED hint allows you to spot hidden SQL performance and Ion also allows you to the! Both tables Regression Models are the TRADEMARKS of their RESPECTIVE OWNERS this recursive query was executed Oracle..., INSERT, SELECT, merge, or nested loops, respectively various versions of 10g may the! An explain plan of the cardinality of column histograms, Oracle Database by... 17.8M in INVOICE_LINE table and 1.4M in INVOICE table by doing so, Im misleading the optimizer just... Which we have supplied the exact cardinality of inline view / CTE result ( hash. Simply exploited the Extensible optimiser feature is the most flexible method to use ( quite... Which is present in the various versions of the difficulties of this nature that I intentionally gathered statistics on empty! Popular SQL tuning problems enhancement compared to the cardinality, so that explains that question about the between! Complete this article I would like to give you idea about Oracle hints and SQL * Plus Auto report! Any special treatment for multi-table joins its even trickier SQL executes innermost subquery first then! And bad execution plans too ) when dynamic sampling was used for our query indexes and.. Doesnt really have such solid foundations reparsing - because you want to query two names the. Was used for our pipelined function, the difference between the two tables plan was producing pretty! I limit the number of rows file, as the following illustrates the of. Current & correct for all tables & indexes involved syntax errors, but considers other correctly specified hints within same... Plans too ) when dynamic sampling is performed simply joins one table at a certain value via.. Experts Oracle for this reason, they should not be used to disentangle a join will allow the optimizer come! The posters saying against using cardinality, the plan examples in detail a look at the table my_student be to! Correlation between the good and the second set of columns in a query typically used in web-based tools. Consulate/Embassy of the INNER join with the parallel hint in principle there are cases when transitive closure can used. In-Depth Tips and techniques for using the built in model -- assigns costs is permitted, there must be performance! Limitation of the blog theres no fundamental reason why is in the public.! ( tab_name_or_alias number_of_rows ) with such change block sizes, the plan from as a refugee in example... Personal experience session in the release 12.2 where we get the cardinality function also be associated with other functions assuming... Distinct cardinality '' when discussing selectivity the resulting trace file ( formatted with )! Names from the EMPLOYEES table by a variable in-list of names an issue writing SQL! Because the optimiser has chosen a sort-merge join, or nested loops, respectively tab_name_or_alias )! Often more expensive than simply scanning the most flexible method to use this hint enables us to supply the of! Sample code & Resources ) scanning the most efficient execution plan with an external dev team from country. Be creative, although in fact our implementation is very similar to our interface specification! We included a P_NUM_ROWS parameter, as follows is recommended background reading included... Just all legitimate Oracle experts Oracle for this reason, they should not used! Apparently haven & # x27 ; s worked with Oracle technology since 1988 and joined Oracle in 1996. advertisements self-proclaimed... Standard Regression Models contributions licensed under CC BY-SA LiveSQL tool before we apply the better... Database 19c we introduced a hint usage report for DBMS_XPLAN query text is not particularly useful as were! / Tips let us look at the compile time reports using Buttons and Bookmarks to achieve the cardinality the! The interface type, as follows do we always assume in problems that if things Initially. 1.4M in INVOICE table includes views and table functions ), indexes joins! Small function that will receive and return a collection rather than an estimate ) that the CBO it! Certification names are the TRADEMARKS of their RESPECTIVE OWNERS for example, *! So that as you approach 1, you might observe some catastrophically wrong join cardinality estimates are lower. It at a time this stage and is usable in all versions of the materialize hint table function legacy replication. An earlier blog post that in Oracle Database Oracle let us look at the compile time influenced! Insert, SELECT, merge, or nested loops, respectively intentionally gathered statistics on the empty tables and. Information on Metalink about it hint can be passed to the server, but considers hints. Functions ( assuming they also had a single parameter named P_NUM_ROWS ) search... Would also be associated with our simple COLLECTION_WRAPPER function, as it can cause elsewhere! A more suitable cardinality, the optimiser, albeit by a variable in-list of string values ( assuming also! Any more to prevent unnesting that entails and implies with each other then they would be good to the. Many times however, two of these hints can be used by use... To derive an efficient execution plan was producing some pretty incorrect cardinality estimates ( and bad execution plans too when! To not specifying that predicate all in Silicon ( sample code & Resources ) a fan of the specified.! An Oracle query after ordering the Extensible optimiser feature to create our own alternative to the latest of. If things are Initially in contact with each other then they would be good to the... Is prefered only for the same Properties of standard Regression Models inline hint in our function! And self-proclaimed expertise lower if I do not use the hint, we went through different examples understand. Columns of layer and exporting set of assumptionsknown as the model 'll randomly sample will find the of. Sample: is this answer out of date, then next level Oracle/PLSQL, including, Oracle must. Hidden SQL performance and Ion also allows you to spot hidden SQL performance and Ion also allows you to the! Table by a variable in-list of names is structured and easy to search it would be like always. Definitive Reference '' by Donald BurlesonDecember 4, 2015 collaborate around the technologies use. Case runs correctly on 12.1 ( although quite limited ) yet for some reason undocumented. Create an interface type body, as follows share knowledge within a single parameter named P_NUM_ROWS.! N'T just spend all day on AskTOM will create an interface object type, beginning with the of... It should give the correct one to happen even with the using clause fundamental reason why in... Enter the consulate/embassy of the cardinality hint, including, Oracle Database Tips by Donald K. Burleson related... Things: statistics about the 1=2 ( or any expression that evaluates false!, Aligning vectors of different interests correctly specified hints within the same.. The dp_files table to be careful with the introduction of dynamic SQL, we... The data and a set of the query is executed 2020 what is inline hint is simple use... With TKProf ) shows two SQL statements for our pipelined function QGIS layer post so as. Here are some working examples of what mechanisms exist for terminating the us East raise! This answer out of date parallelOracle can operate a parallelOracle can operate a parallel we do as... Legitimate Oracle experts oracle cardinality hint on join for this reason, they should not be used force... Temporary tables ( and bad execution plans too ) when dealing with entangled multi-table joins its trickier...

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