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How do you optimize a query in Oracle using the EXPLAIN plan?

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If you work with Oracle databases, you know how important it is to optimize your queries for performance and efficiency. But how do you know if your query is running as fast as possible, or if there is room for improvement? One of the tools you can use to analyze and tune your queries is the EXPLAIN plan, which shows you how Oracle executes your query and what resources it uses. In this article, you will learn how to use the EXPLAIN plan to optimize your queries in Oracle, and what to look for in the output.

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1 What is the EXPLAIN plan?

The EXPLAIN plan is a feature of Oracle that allows you to see the execution plan of a query, which is the sequence of steps and operations that Oracle performs to retrieve the data you requested. The EXPLAIN plan also shows you the estimated cost and cardinality of each step, which are measures of the resources and rows involved in the execution. The cost represents the relative amount of CPU and I/O resources required for each step, while the cardinality represents the number of rows returned or processed by each step. By using the EXPLAIN plan, you can identify the bottlenecks and inefficiencies in your query, and apply appropriate optimizations.

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Piyush Mishra

Database Admin Professional at Teva Pharmaceuticals
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To understand how Oracle plans to execute the query. Pay attention to the following aspects: The order in which tables are accessed (join order). The access methods used (full table scan, index scan, etc.). Any sorting or aggregation steps. Cardinality estimates (the number of rows Oracle expects at each step). Cost estimates (the estimated resource usage for each step) Identify any steps in the execution plan that have high estimated costs or unexpected access methods. These are potential performance bottlenecks. Evaluate whether adding or modifying indexes on the involved tables can improve query performance. Indexes can help Oracle access data more efficiently If Index is already there, then have to check and not using by query

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2 How to generate the EXPLAIN plan?

Generating an EXPLAIN plan for a query in Oracle is possible through the use of the SQL*Plus command line tool. To access this environment and the PLAN_TABLE table, where Oracle stores the EXPLAIN plan output, you need to run the script utlxplan.sql, located in the $ORACLE_HOME/rdbms/admin directory. To generate the EXPLAIN plan for a query, connect to the SQL*Plus environment using your username and password, enter the command SET AUTOTRACE TRACEONLY EXPLAIN, then enter the query with a semicolon at the end. The EXPLAIN plan output will be displayed in a tabular format, with each row representing a step in the execution plan.

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3 How to read the EXPLAIN plan?

The EXPLAIN plan output consists of several columns that provide information about each step in the execution plan. The most important of these are the Id, which is a numeric identifier that indicates the order and hierarchy of the execution, and the Operation, which is the type of operation performed by the step. Additionally, there is an Options column that displays any modifiers or variations of the operation, an Object Name column that shows the name of any database objects involved in the step, a Cost (%CPU) column which estimates the cost of the step based on statistics and parameters, and a Cardinality column that estimates the number of rows returned or processed by the step. The indentation of the Id column displays parent-child relationships between steps. The percentage of CPU time used by a step is also shown in parentheses. However, these estimates may not reflect actual runtime performance if statistics are outdated or inaccurate.

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4 How to optimize the query using the EXPLAIN plan?

The EXPLAIN plan can help you optimize your query by showing you where the execution plan is inefficient or suboptimal, and suggesting possible improvements. Common signs of a poor execution plan include high cost or cardinality, full table or index scans, nested loops or hash joins, and sort operations. High cost or cardinality can be reduced with indexes, partitions, hints, or rewritten query logic. Full table or index scans can be avoided with selective predicates, indexes, partitions, or hints. Nested loops and hash joins may be costly for large data sets and can be replaced with sort-merge joins or star joins using hints, indexes, or rewritten query logic. Sort operations can be expensive and memory-intensive; they may be reduced with indexes, partitions, hints, or rewritten query logic. Reporting and documenting the incident will help you inform and reassure the stakeholders, as well as provide valuable insights for improving your cloud security posture.

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Anandaganesh Balakrishnan

AI and Data Engineering Influencer | Inventor | Author | Speaker | Carnegie Mellon Alum
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Some of the ways to optimize the query and reduce the run times are 1) Check the cardinality and verify whether the scan operation by the query optimizer is efficient. Gathering incremental or full statistics on tables recreates the histogram and the optimizer might change the access path based on the new metadata. 2) If the query mimics the lookup on a huge table, then converting the table to Index index-organized table (IOT) might help to scan the needed rows quickly. 3) Most of the databases come up with the option of query tuning advisor. Running the same will help identify the bottlenecks and costs can be compared between the old plan and the new plan. 4) Creating appropriate partitions, sub-partitions, and local indexes helps.

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5 What are some best practices for using the EXPLAIN plan?

The EXPLAIN plan is a powerful and useful tool for optimizing your queries in Oracle, but there are some limitations and caveats that you should be aware of. When using the EXPLAIN plan, it's important to remember that it shows estimated execution plans and costs, not the actual ones. The actual performance may vary depending on runtime conditions like data distribution, concurrency, memory, etc. Therefore, it's important to test and validate your query results and timings. Additionally, regular updates to the database object statistics should be done using the DBMS_STATS package or the automatic statistics gathering feature to ensure accuracy and consistency of the EXPLAIN plan. Lastly, hints can be useful for overriding the optimizer's default behavior, but they can also have unintended consequences. Therefore, use hints with caution and only after trying other optimization techniques and verifying the results.

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6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you optimize a query in Oracle using the EXPLAIN plan?

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1 What is the EXPLAIN plan?

2 How to generate the EXPLAIN plan?

3 How to read the EXPLAIN plan?

4 How to optimize the query using the EXPLAIN plan?

5 What are some best practices for using the EXPLAIN plan?

6 Here’s what else to consider

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How do you optimize a query in Oracle using the EXPLAIN plan?

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1 What is the EXPLAIN plan?

2 How to generate the EXPLAIN plan?

3 How to read the EXPLAIN plan?

4 How to optimize the query using the EXPLAIN plan?

5 What are some best practices for using the EXPLAIN plan?

6 Here’s what else to consider

Database Administration

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