Moving an oracle sql execution plan from one environment to. An execution plan shows the detailed steps necessary to execute a sql statement. 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. 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. When evaluating a plan, examine the statements actual resource consumption. An oracle explain plan can be terribly hard to understand. Oracle sql plsql syntax for the cardinality function is. The explain plan information is written to an explain plan table. Explain plan cost, bytes, cardinality oracle community. May 18, 2015 the explain plan statement displays execution plans chosen by the oracle optimizer for select, update, insert, and delete statements. 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. Basics of the cost based optimizer part 6 simple talk.
Cardinality is the estimated number of rows the step will return. 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. What does the following terms in the explain plan mean. I have the following query which i may not have analyzed properly. A statement execution plan is the sequence of operations that the database performs to run the statement. Oraclebase automatic reoptimization in oracle database 12c. Types of indexes a btree index,a bitmap index youtube. How to generate explain plan in sql developer catgovind. 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. 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.
How do i know if the cardinality estimates in a plan are. Rem this is the format for the table that is used by the explain plan rem statement. Some operations will likely need the plustrace role to be granted to the user performing the operation. Burleson is one of the worlds top oracle database experts with more than 25 years of fulltime dba experience. Its provides the information how the oracle optimizer is thought of executing the particular statement based on. 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. Moving an oracle sql execution plan from one environment to another posted. This is basically just a fancy word to describe the two options. Once the oracle explain plan table has been created, suspect sql statements can. So explain plan will be different in both the queries, even though the cardinality is same. Apr 07, 2016 gleb otochkin, principal consultant and certified oracle expert at pythian, details explain plan on oracle live sql. Using explain plan with the statement id clause continue reading generate explain plan. Meaning of cost, cardinalityrows,bytes and time in explain plan. Oracle sql plsql cardinality function is used to return the number of elements in a nested table.
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. Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. All other things being equal, a query with a higher cost will use more. Help to understand explain plan in oracle database. Whats the difference between cardinality and degree of a. Explain plan is the tool we use to view the execution plan of a sql. 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. 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.
The optimizer determines what statistics are to be collected, and how the plan should be resolved for different values of the statistics. Gleb otochkin, principal consultant and certified oracle expert at pythian, details explain plan on oracle live sql. Explaining the explain plan using pictures so many oracle. The topics that are covered in this article include btree indexes. Its provides the information how the oracle optimizer is thought of executing the particular statement based on the available statistics of the database. The following query finds all female members from the members table. Meaning of cost, cardinalityrows,bytes and time in.
You should use the bitmap index for the columns that have low cardinality. 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. It is not common, but cardinality also sometimes refers to the relationships between tables. To help oracle understand the skew we have to create a histogram on the column. Generating a 10053 trace file will help show exactly what choices the optimizers making regarding its estimation of cardinality and selectivity. In these cases, cardinality is not particularly useful as there is only one possible execution plan.
Cardinality feedback was introduced in oracle database 11g release 2. Explain plan oracle sql and plsql optimization for developers. Cardinality is the maximum number of connections a row of one table can have with rows of another table. Description use the explain plan statement to determine the execution plan oracle database follows to execute a specified sql statement. There are many best practices like this that dbas hear and follow but dont always understand the reason behind. Sep 29, 2015 if you dont already have sql t installed, see oracle support doc id 1614107. 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. The row source tree is the core of the execution plan. Sep 26, 20 low cardinality indexes can be bad for performance. Explain plan table in toad oracle blog by anar godjaev. Selectivity is the ratio of cardinality to the number of records of an indexed column.
Cardinality between tables can be onetoone, manytoone or manytomany. 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. 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. Having to do a select count query for every aspect of a complex plan can be extremely painful. 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. Jun 21, 2017 high cardinality means that the column contains a large percentage of totally unique values. Theoretically equivalent queries should return same cardinality.
This statement inserts a row describing each step of the execution plan into a specified table. Cost is the estimated amount of work the plan will do. Generate explain plan using autotrace in sql developer 3. The code download for this book is packed with readytorun scripts to monitor and identify even the most challenging performance issues. Optimization, such as the cost and cardinality of each operation. Why low cardinality indexes negatively impact performance. 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. How do i display and read the execution plans for a sql. Explain plan statement provides execution plans chosen by the optimizer during the course of select, update, insert, and delete statements. Oracle explain plan estimates incorrect cardinality for an index. 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.
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. The explain plan statement displays execution plans chosen by the. 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. High cardinality means that the column contains a large percentage of totally unique values. The explain plan statement displays execution plans chosen by the oracle optimizer for select, update, insert, and delete statements. It is available on linux, windows, solaris, hpux and aix platforms as well as the oracle cloud. The explain statement requires the presence of this rem table in order to store the descriptions of the row sources. Paleolithic explain plan demo now that youve seen it.
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. The optimizer selects the execution plan with the lowest cost, where cost represents the estimated resource usage for that plan. Moving an oracle sql execution plan from one environment. It is important to note that the explain plan table must be created before running the explain plan statement. I know there is more than one way to move a sql plan hash value from one environment to another but this way was simple and worked very efficiently for me. 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. Oracle database features such as hash joins, parallel query, partitioning, etc. A statements execution plan is the sequence of operations oracle performs to execute the statement. Explaining the explain plan using pictures so many. Oracle sql tuning how to read execution plans explain plan. The explain plan statement displays execution plans chosen by the optimizer for select, update, insert, and delete statements. This article will empower the dba to understand the logic behind why low cardinality indexes can be bad for performance or cause erratic performance.
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. A higher cardinality youre going to fetch more rows youre going to do more work the query will take longer. A statements execution plan is the sequence of operations oracle performs to run the statement. Hi, i am using oracle database 11g enterprise edition release 11. In the context of execution plans, the cardinality shows the number of rows. Sep 19, 2014 cardinality is the maximum number of connections a row of one table can have with rows of another table. It doesnt help that the oracle documentation incorrectly states that the execution order in explain plan output begins with the line. The explain plan is just a prediction of the join methods that will be used when executing a query.
There are a few different ways to utilize explain plan. 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. By examining this plan, you can find out if oracle is picking the right indexes and joining your tables in the most efficient manner. Sqlmaria oracle database product manager with a passion. Explain plan parses a query and records the plan that oracle devises to execute it. It need not be necessary that in future cardinality will be same too. We will focus on using it through sqlplus since most oracle programmers have access to sqlplus. In this free tutorial you will learn how to generate and read interpret an execution plan in oracle databases. 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. Once the access path has been decided upon it is stored in the library cache together with the statement itself.
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