The optimizer determines what statistics are to be collected, and how the plan should be resolved for different values of the statistics. This article will empower the dba to understand the logic behind why low cardinality indexes can be bad for performance or cause erratic performance. How do i know if the cardinality estimates in a plan are. Oracle database 19c, is the long term support release of the oracle database 12c and 18c family of products, offering customers premier and extended support through to march 2023 and march 2026 respectively. Cardinality is the maximum number of connections a row of one table can have with rows of another table. Sep 29, 2015 if you dont already have sql t installed, see oracle support doc id 1614107. In this article were going to look only at the predictions that the optimizer makes explain plan, postponing any investigation of actual runtime figures v.
You should use the bitmap index for the columns that have low cardinality. Sqlmaria oracle database product manager with a passion. Once the oracle explain plan table has been created, suspect sql statements can. Generating a 10053 trace file will help show exactly what choices the optimizers making regarding its estimation of cardinality and selectivity. Theoretically equivalent queries should return same cardinality. Low cardinality means that the column contains a lot of repeats in its data range. This can make it hard to infer which step is the most time consuming, as a different plan may be followed when the statement is executed. In these cases, cardinality is not particularly useful as there is only one possible execution plan. Description use the explain plan statement to determine the execution plan oracle database follows to execute a specified sql statement. A statements execution plan is the sequence of operations oracle performs to execute the statement. The optimizer selects the execution plan with the lowest cost, where cost represents the estimated resource usage for that plan. Once the access path has been decided upon it is stored in the library cache together with the statement itself. It doesnt help that the oracle documentation incorrectly states that the execution order in explain plan output begins with the line.
Types of indexes a btree index,a bitmap index youtube. It is important to note that the explain plan table must be created before running the explain plan statement. Cardinality is the estimated number of rows the step will return. The explain plan statement displays execution plans chosen by the oracle optimizer for select, update, insert, and delete statements. It is not common, but cardinality also sometimes refers to the relationships between tables. Jun 21, 2017 high cardinality means that the column contains a large percentage of totally unique values. Oracle sql plsql syntax for the cardinality function is. He specializes in creating database architectures for very large online databases and he has worked with some of the worlds most powerful and complex systems. High cardinality means that the column contains a large percentage of totally unique values. Explain plan lies explain plan should hardly ever be used you have to be careful when using autotrace and related tools never use explainup with tkprof.
Sep 26, 20 low cardinality indexes can be bad for performance. Meaning of cost, cardinalityrows,bytes and time in explain plan. The oracle optimizer explain the explain plan 24 introduction the purpose of the oracle optimizer is to determine the most efficient execution plan for your queries. Whats the difference between cardinality and degree of a.
Cardinality feedback was introduced in oracle database 11g release 2. The code download for this book is packed with readytorun scripts to monitor and identify even the most challenging performance issues. Cost is the estimated amount of work the plan will do. A statement execution plan is the sequence of operations that the database performs to run the statement. As a developer, the execution plan is probably the best tool at your disposal to discover why a query performs the way it does and, if necessary, figure out what you can do about its performance. Why low cardinality indexes negatively impact performance. Oracle database features such as hash joins, parallel query, partitioning, etc. We will focus on using it through sqlplus since most oracle programmers have access to sqlplus. The explain plan information is written to an explain plan table. When i write this query in plsql developer and press f5, i see explain plan, but i dont know what cost, cardinality and bytes represent. Paleolithic explain plan demo now that youve seen it.
Explaining the explain plan using pictures so many. Having to do a select count query for every aspect of a complex plan can be extremely painful. A 2011 oracle white paper called explain the explain plan, which is available on the otn, and the documentation on optimizer access paths form the basis for most of what is written in. When evaluating a plan, examine the statements actual resource consumption. When the optimizer generates an execution plan the presence of missing statistics, stale statistics, complex predicates or complex operators may trigger the optimizer to monitor the cardinality of operations in the plan.
What does the following terms in the explain plan mean. To help oracle understand the skew we have to create a histogram on the column. Moving an oracle sql execution plan from one environment to. Its provides the information how the oracle optimizer is thought of executing the particular statement based on. Generate explain plan using autotrace in sql developer 3. By examining this plan, you can find out if oracle is picking the right indexes and joining your tables in the most efficient manner. Hi guys today is very most important topic in oracle is indexes help to increase performance at the time of search and modification into table records. The following query finds all female members from the members table.
The explain plan is just a prediction of the join methods that will be used when executing a query. Burleson is one of the worlds top oracle database experts with more than 25 years of fulltime dba experience. Explain plan cost, bytes, cardinality oracle community. Its provides the information how the oracle optimizer is thought of executing the particular statement based on the available statistics of the database. The explain statement requires the presence of this rem table in order to store the descriptions of the row sources. It is available on linux, windows, solaris, hpux and aix platforms as well as the oracle cloud. Oracle explain plan estimates incorrect cardinality for an index. Selectivity is the ratio of cardinality to the number of records of an indexed column. How to generate explain plan in sql developer catgovind.
Cardinality between tables can be onetoone, manytoone or manytomany. I have the following query which i may not have analyzed properly. Using explain plan with the statement id clause continue reading generate explain plan. There are many best practices like this that dbas hear and follow but dont always understand the reason behind. Targeted at the senior oracle dba, this comprehensive book gives you all of the knowledge you need to be successful in tuning even the most complex oracle database. Oracle sql tuning how to read execution plans explain plan. Moving an oracle sql execution plan from one environment to another posted. Explain plan is the tool we use to view the execution plan of a sql.
Help to understand explain plan in oracle database. How do i display and read the execution plans for a sql. It makes these decisions based on the statistical information it has about your data and by leveraging oracle database features such as hash joins, parallel query, partitioning, etc. It can be invoked with a buttonclick in a variety of gui tools such as oracle enterprise manager oem, toad, sql navigator, and oracle sql developer to name a few. The explain plan statement displays execution plans chosen by the. Its time to move away from the shape of an execution plan and turn our attention to some of the numerical information we get from the plan. So explain plan will be different in both the queries, even though the cardinality is same. A statements execution plan is the sequence of operations oracle performs to run the statement. Selectivity distinct valuestotal number of records if the ratio or selectivity is 1, it means all the records in a row are unique and is also referred to as high selectivity. In this free tutorial you will learn how to generate and read interpret an execution plan in oracle databases. The explain plan statement displays execution plans chosen by the optimizer for select, update, insert, and delete statements. Hi, i am using oracle database 11g enterprise edition release 11.
The topics that are covered in this article include btree indexes. How to generate explain plan in sql developer using explain plan, we can find the order of tables executed the given sql index is executed. The execution plan selected for a sql statement is just one of the many alternative execution plans considered by the optimizer. For more complex queries it might be easier to use the explain plan or auto trace functionality in sql developer to view the query block names. This statement inserts a row describing each step of the execution plan into a specified table. Once the oracle explain plan table has been created, suspect sql statements can be evaulated and tuned accordingly. Gleb otochkin, principal consultant and certified oracle expert at pythian, details explain plan on oracle live sql. Nov 27, 2012 if nested table is empty or is a null collection then cardinality function in oracle sql plsql will return null. Rem this is the format for the table that is used by the explain plan rem statement.
In oracle performance tuning guides words it is best to use explain plan to determine an access plan, and then later prove that it is the optimal plan through testing. Explain plan statement provides execution plans chosen by the optimizer during the course of select, update, insert, and delete statements. The row source tree is the core of the execution plan. Apr 07, 2016 gleb otochkin, principal consultant and certified oracle expert at pythian, details explain plan on oracle live sql. Meaning of cost, cardinalityrows,bytes and time in. Example 112, explain plan output is an example of the plan table output when using the. Basics of the cost based optimizer part 6 simple talk. Sep 19, 2014 cardinality is the maximum number of connections a row of one table can have with rows of another table.
This is basically just a fancy word to describe the two options. All other things being equal, a query with a higher cost will use more. Unfortunately i have 1,000 distinct values in the column and in 11g a histogram consists of a maximum of 255 rows equating to 254 buckets so oracle is constrained to generating a heightbalanced histogram on the column. Oraclebase automatic reoptimization in oracle database 12c.
An execution plan shows the detailed steps necessary to execute a sql statement. The central problem with cardinality estimation is the in cases of complex where clauses the optimizer does not have enough information about interjoin result set sizes to determine the optimal table join order. Explain plan table in toad oracle blog by anar godjaev. Oracle does not tell us the order in which steps are executed and we must figure it out ourselves using the oh so subtle indentation of operation names.
May 18, 2015 the explain plan statement displays execution plans chosen by the oracle optimizer for select, update, insert, and delete statements. Experience the latest oracle database 19c, providing business of all sizes costefficient access to the industrys fastest, most reliable, scalable and secure database technology for deploying operational and analytics workloads in the cloud, onpremise and hybrid configurations. Some operations will likely need the plustrace role to be granted to the user performing the operation. Nov 26, 2010 an oracle explain plan can be terribly hard to understand. Dec 18, 20 what does the following terms in the explain plan mean. Explain plan parses a query and records the plan that oracle devises to execute it. Oracle sql plsql cardinality function is used to return the number of elements in a nested table. Generating and displaying the execution plan of a sql statement is a common task for most dbas, sql developers and performance experts as it provides them information on the performance characteristics of a sql statement.
Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. An oracle explain plan can be terribly hard to understand. A higher cardinality youre going to fetch more rows youre going to do more work the query will take longer. It should be noted that explain plan has been deprecated and it is recommended that. Moving an oracle sql execution plan from one environment.
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