A Few Practical Ways to Improve SQL Query Performance
By: Tosska Technologies
Tags: Database, Sql Oracle, Oracle

When you are working with large databases, it is quite imperative to know how you can improve SQL query performance. In this article, we are going to learn more efficient SQL queries to achieve results quicker. The biggest distinction between SQL and other languages is that SQL is a non-procedural language in which you specify the results that you want and not the methods used to get it.

The benefit of a non-procedural language is that it is easier to write codes or programs, hence, it is common for non-programmer business users for generating reports form SQL queries. With the improvement of expertise of users and an increase in the complexity of reports, the performance of queries will quickly deteriorate in the context of speed and memory and use. For the same reason, it is essential to understand some crucial ways required to improve SQL query performance or for improving the performance of reports in SQL. Before we move on to ways, let us understand a few aspects of query optimizers.

Typically, there are two main query optimizers that come with an SQL database. Earlier, the database used syntax-based (rule-based) query optimizers in which the syntax of the SQL query used to determine the query performance. However, in the most recent versions of SQL database use statistics-based (cost-based) query optimizers. In this, the method utilized to execute a query is automatically dependent on the volume of data in the tables used in the SQL query.

Therefore most SQL databases, by default, use cost-based query optimizers. In some cases, when query isn’t performing well, a database like Oracle is still SQL syntax sensitive, providing the developer better control over the way that a query is executed. Writing the query in a certain way can increase query performance. Here are five tips that will be highly effective in database performance tuning.

Every database has a unique way of executing a query, known as a query execution plan. If a complex query is taking an unexpected amount of time to run, you must first find the query execution plan of your specific implementation of SQL. If you get to see a full table scans on a large table, try indexing the attribute through which the table is accessed. Once you have indexed the table, it will help in locating a row quickly, but it can actually deteriorate the performance for smaller tables as it may result in reading both the data table and index table.

The first tip is to use the real column names in the SQL query rather than selecting all columns (using SELECT * ) FROM a table. This will help you in selecting only the necessary columns instead of all the columns in the table.

Secondly, try to avoid correlated subqueries as these can significantly reduce the speed of execution. Rather than using a correlated subquery, you should try using a JOIN operator. Obviously, a decision regarding a choice: a JOIN or a correlated subquery relies on data you wish to extract from a database.

Thirdly, try avoiding the use of IN. Instead, use the EXISTS clause in the main query rather than using a subquery. Also, when you use UNION, try using the UNION ALL clause instead of using UNION alone.

The next tip is the use of the WHERE clause effectively. If you are trying to avoid specific column values, don’t use the NOT condition. Instead, you must try to use the BETWEEN operator.

Likewise, if you are using a LIKE clause with a wild card, don’t forget to design the statement so that the fewest number of rows are selected.

If you still have doubts, always try using the query execution.