Some operations will likely need the plustrace role to be granted to the user performing the operation. A shell script wrap for oracle s explain plan for sqls. The explain plan information is written to an explain plan table. Time in explain plan vs elapsed time in tkrof oracle. Explain plan usage when a sql statement is passed to the server the cost based optimizer cbo uses database statistics to create an execution plan which it uses to navigate through the data. F10 again, you will now have at least two explain plan panels in the worksheet. An ordering of the tables referenced by the statement. A statements execution plan is the sequence of operations oracle performs to run the statement. Format explain plan results easily with oracles dbms. Before issuing an explain plan statement, you must have a table to hold its output. The smallest amount of data read is a single oracle block, the largest is constrained by operating system limits and multiblock io. A discussion i have had any number of times is what the cost column of explain plan means. Please let me know how to keep the explain plan consistent across the environments.
By examining this plan, you can find out if oracle is picking the right indexes and joining your tables in the most efficient manner. Partitions accessed after pruning are shown in the partition start and partition stop columns. Its provides the information how the oracle optimizer is thought of executing the particular statement based on. Now in terms of why would the index change the plan, that is because you have supplied critical information to the cbo that it did not have before. Reading an explain plan is important for many reasons, and oracle sql tuning experts reveal the explain plans to check many things. I noticed the explain plan was different in both these environments. The plan tab is shown if the feature is supported for the database, the execution plan is available for the entered statement and the explain plan button is selected in the query window. Dan goes thru some of the items you will find in an explain plan, what it means, and what you can do to improve it. Oracle explain plan consistent across environments.
Moving an oracle sql execution plan from one environment to another. Jan 29, 2010 while it is important for the query optimizer to reasonably acurately calculate the cost of an execution plan, it makes little difference if the cost of a full table scan is 1, 63,295, or 1,000,000,000 if a full table scan is the only legal method to retrieve the data from the table due to missing indexes, inappropriate indexes such as. Sequence of steps in oracle sql explain plan oracle database tips by donald burlesonjune 15, 2015 when codd and date created the relational data model, the execution plan was an afterthought, largely because the sql optimizer was always supposed to generate the best execution plan, and hence, there was not real need to understand the internal. During the sql plan baseline capture phase, oracle database records information about sql statement execution to detect plan changes and decide whether it is safe to use new plans. Moving an oracle sql execution plan from one environment.
Explain plan which plan is better charles hoopers oracle. If one plan has a cost of 10, and another a cost of 1m, chances are the first will be faster. The the oracle optimizer explain the explain plan pdf document has a. Explain plan is the sequence in which oracle would be going to join the tables to build up the query. The plan table is the table that oracle fills when you have it explain an execution plan for an sql statement. Many people are under the impression that if a plan has a higher cost than another, it is worse. The plan will have both the estimations erows and the actual number of rows arows from the last execution. I found an ask the oracle mentors posting from november 11, 2014. Quick tip comparing explain plans with sql developer. The long awaited accompanying white paper has finally been released on otn.
When you need to troubleshoot inefficient sql, one of the most important diagnostics tools at your disposal is the output of the explain plan command. If the costs are close, chances are theyll have similar runtime behavior, but thats not always the case. Once the oracle explain plan table has been created, suspect sql statements can be evaulated and tuned accordingly. The explain plan statement displays execution plans chosen by the oracle optimizer for select, update, insert, and delete statements. There are a few different ways to utilize explain plan.
At oracle open world last year we delivered a presentation, explain the explain plan session s316955, where we examined the different aspects of an execution plan, from selectivity to parallel execution, and explained what information you should be gleaning from the plan. The deepest will be the driving table to the immediate outer table 3. Once the oracle explain plan table has been created, suspect sql statements can. Syntactic checks the syntax of the query semantic checks that all objects exist and are accessible view merging rewrites query. This statement inserts a row describing each step of the execution plan into a specified table. To successfully understand an explain plan you must be able to know the order that the plan steps are executed. Once youve highlighted a problem query the first thing you should do is explain the statement to check the execution plan that the cbo has created. Plan steps which differ will be highlighted appropriately. This type of method which oracle employs is called the execution plan. How do i display and read the execution plans for a sql.
The following plan shows execution of a select statement. How to check what execution plan the query used yesterday or. I was trying to understand the explain plan in oracle and wanted to know what conditions oracle considers while forming the explain plan. Use explain plan to see how oracle database accesses partitioned objects for specific queries. Explain plan statement provides execution plans chosen by the optimizer during the course of select, update, insert, and delete statements. It doesnt help that the oracle documentation incorrectly states that the execution order in explain plan output begins with the line. You can compare the unconstrained ascp plan recommendations with your mrp plan results and thus can validate the basic ascp setup. 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. 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. A statements execution plan is the sequence of operations oracle performs to execute the statement. How to generate a useful sql execution plan oracle. Its provides the information how the oracle optimizer is thought of executing the particular statement based on the available statistics of the database. When it comes to sql execution plans, if you start from the wrong place, then you probably wont make it to your destination. In the first top query, the index on the state column is used, and the rowid values returned from that index scan are used to select records from the company table.
How to check what execution plan the query used yesterday. 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. Description use the explain plan statement to determine the execution plan oracle database follows to execute a specified sql statement. Examples of the output from this statement are shown in example 74 and example 71. The row source tree is the core of the execution plan. The deepest most indented table in the plan will be executed first starting point 2. May 16, 2006 when you need to troubleshoot inefficient sql, one of the most important diagnostics tools at your disposal is the output of the explain plan command. Ascp unconstrained plan oracle value chain planning blog. Explain plan is available for oracle 9i and later, sql server, db2 and solid and is supported on all db solos os platforms, i.
Because adhoc sql statements do not repeat and thus do not. This temporary table holds the output of explain plan statements for all users. We will focus on using it through sqlplus since most oracle programmers have access to sqlplus. In many of the explain plan reports included below noncritical columns such as time have been removed to fit. An execution plan shows the detailed steps necessary to execute a sql statement. An explain plan is a representation of the access path that is taken when a query is executed within oracle. Ensure that the tables will be joined in optimal order. The explain plan shows that each of the queries is executed separately, and the results of the two queries are returned to the intersection operation. Two important aspects to be focused on while running ascp unconstrained plan should be. If you like, however, you can choose any other name for the plan table, as long.
Once the explain plan table exists you run the explain plan command giving as its argument the sql statement to be explained. Explain plan and execution plan a simple introduction with a single table example. Gleb otochkin, principal consultant and certified oracle expert at pythian, details explain plan on oracle live sql. Best practice for the sending of an oracle execution plan. Explain plan parses a query and records the plan that oracle devises to execute it. The explain plan relies heavily on the cost based optimizer cbo. The table employees is accessed using a full table scan. Explaining the explain plan using pictures so many. The explain plan command is a tool to tune sql statements.
Hence you might use say query block names of q1 and q2, for two queries that both access the emp table, but when you check the execution plan, q2 has disappeared, because oracle has merged q1 and q2 into one because it can do it with one probe of emp. Since you are seeing this, it means that your browser does not support cascading style sheets. A few days ago, developers started complaining, this query is running very slow in. Explain plan table in toad oracle blog by anar godjaev. To do so, the database maintains a history of plans for individual sql statements.
I have a query that executes differenly in different environments. There is an old joke the unreconstructed comic dave allen used to tell, where a traveler asks a passerby for directions to a particular town and the passerby simply says, well i wouldnt start from here if i were you. Ask tom understanding outline data in oracle explain plan. Rightclick in the explain plan tab you have to rightclick in the plan tab, not the plan itself. It is important to note that the explain plan table must be created before running the explain plan statement. 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 can help this process out by gathering statistics on the tables you are querying against. It should be noted that explain plan has been deprecated and it is recommended that. 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.
May 09, 2011 at oracle open world last year we delivered a presentation, explain the explain plan session s316955, where we examined the different aspects of an execution plan, from selectivity to parallel execution, and explained what information you should be gleaning from the plan. Demos, syntax, and example code of oracle explain plan for sql performance tuning. While it is important for the query optimizer to reasonably acurately calculate the cost of an execution plan, it makes little difference if the cost of a full table scan is 1, 63,295, or 1,000,000,000 if a full table scan is the only legal method to retrieve the data from the table due to missing indexes, inappropriate indexes such as. I was testing a simple query in hr schema present in oracle 11g. In many of the explain plan reports included below noncritical columns such as time have been removed to fit the website format. But turn on tracing for your session, run your plsql, run tkprof on the trace file and you should see the exec plan there.
A dictionary view introduced in oracle 9i that shows the execution plan for a sql statement that has been compiled into a cursor in the cursor cache. Horizontal and vertical plan perhaps the most important and most frequently used part of the user interface for a planner is the horizontal plan where planners can see time phased bucketed output of the plan and all of the detailed supply and demand information. The plan history will include relevant information used by the optimizer to reproduce an execution plan, such as the sql text, outline, bind variables, and compilation environment. An oracle explain plan can be terribly hard to understand.
For hash partitions, the row source name is partition hash. Explain plan execution plan to run an sql query, oracle performs various operations in the background i. 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. Create or have your dba create a public synonym to that table and unless somebody has a really good reason to keep them around drop all the rest. The purpose of the oracle optimizer is to determine the most efficient execution plan for your queries.
How to interpret an explain plan oracle database internal. The row source name for the range partition is partition range. Note that if it is a parallel query statement, you must omit the last in the format or you will have statistics only for the coordinator process. Logically oracle finds the data to read by using the following methods. You can use the oracle explain plan to derive userdefined cpu and io costs for domain indexes. Not able to get an explain plan for any sql statements.
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