Query optimization techniques in Oracle

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Posted December 17, 2018

Databases play a vital role in the performance of online operations which makes it very crucial to keep them optimized for an overall higher performance. Diligent Database Administrators make sure that the databases are performance tuned and optimized from time to time for seamless operations.

 

Oracle is one of the most used databases where Query optimization plays an important role in the performance tuning of the database. Before we get into the details of Query optimization, let us first understand what does Query Optimization means.

What is Query Optimization in Oracle?

 

Query optimization is the process of choosing the most efficient means of executing a SQL statement. Since SQL is non-procedural in nature, hence the optimizer is free to merge, reorganize, and process the SQL statements in any order for the utmost efficiency.

 

The database optimizes each SQL statement based on statistics collected about the accessed data. The optimizer determines the optimal plan for a SQL statement.

 

This is done by examining multiple access methods, such as full table scan or index scans, different join methods such as nested loops and hash joins, different join orders, and possible transformations.

 

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Let’s explore the various techniques of query optimization in Oracle and Oracle query optimization tool.

 

How to Optimize SQL query in Oracle?

 

Here are the query optimization techniques in oracle which you could use to performance tune your Oracle database:

 

1.   Start with the System Level SQL tuning

To get started with the SQL query optimization it is important that you perform SQL tuning at the System level first else other changes might get undo automatically leading to rework in performance tuning.

2.   Rewrite complex subqueries with temporary tables

You can easily rewrite complex subqueries in Oracle SQL with Global Temporary Table (GTT) and SQL WITH operator. This is helpful in complex subqueries such as WHERE clause subqueries, SELECT clause scalar subqueries and FROM clause in-line views.

 

Upon tuning with the queries with temporary tables and WITH clause,the performance of the SQL query in Oracle increases by many folds.

3.   Index all predicates

For a high performance and scalability, Index all predicates of JOIN, WHERE, ORDER BY and GROUP BY clauses in SQL queries. In absence of the proper indexing, it causes table scanning of SQL queries which consequently results in performance or locking issues.

 

Thus, it is good to have all predicates indexed except where the column data has low cardinality.

4.   Use Inner Joins instead of Outer Joins

Use outer join only if it is very necessary, otherwise, try replacing it with inner joins for a better SQL query performance. As outer joins slower the SQL execution, thereby affecting optimization, thus using inner joins is highly advisable for better performance.

5.   Use CLOB/BLOB columns at end statements

Oracle 10g and Oracle 11g require CLOB/BLOB columns at the end statements, else it might result in an execution failure when the input value size is more than 1000 characters.

 

 

 

 

Oracle Query Optimization Tool

Oracle Query Optimization Tool (The Query Optimizer), is a built-in database software which helps in the optimization of SQL queries in Oracle, by finding out the most efficient method in which a SQL query can access data from an Oracle database.

 

It generates multiple possible Execution plans along with costs involve for a query and compares them to find out the most feasible plan to boost up overall database performance. It takes Query blocks, which is a parsed representation of the SQL statement as the input and based on the statistics tell the most feasible execution plan as a result.

 

This way Oracle Query Optimization Tool helps in getting the most feasible and cost-effective execution plan for the SQL queries in your Oracle database.

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In a nutshell

Query Optimization is very important for a healthy and high performing Oracle database. Using the above techniques could be very useful in Optimizing SQL queries Oracle by fastening the execution. At the same, Oracle Query Optimization Tool could largely help to figure out the most feasible execution plan for the queries boosting up the performance and making it more cost effective.

 

A diligent database administrator could very well use the above techniques the Query Optimizer to enable a high functioning database. At OptimizDBA, we are dedicated to making your database highly optimized by using the advanced techniques and due diligence with our expert and experienced team of Database Administrators. Feel free to contact us and we will be obliged to assist you in optimizing your database.

 


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