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How to optimize SQL query?

· 4 min read
Aytan Jalilova

Sometimes, we inherit old SQL queries from previous developers, and these queries can be long, complex, and difficult to understand, even for experienced developers. To make matters worse, these queries often have no documentation, making it even harder to figure out what they've done.

How to optimize an old long SQL query?​

Start by understanding the query. What is it trying to accomplish? What data is it trying to return? What are the relationships between the tables? Once you have a good understanding of the query, you can start to break it down into smaller, more manageable pieces. Reformat the query will make it easier to read and understand. You can use an online SQL formatter or do it yourself.

Identify the different parts of the query. This includes the tables being joined, the columns being selected, and the filtering and aggregation criteria. Break the query down into Common Table Expressions (CTEs). CTEs are temporary tables that can be used within a query. This can be helpful for optimizing complex queries into smaller, more manageable steps.

Test the optimized query thoroughly. Once you have made any changes to the query, it is important to test it thoroughly to make sure that it is still working correctly. Here are a few additional tips:

  • Use a query profiler A query profiler can help you to identify which parts of your query are taking the most time to execute. This can be helpful for optimizing the performance of your queries.
  • Ask for help from others If you're struggling to understand or modify an old SQL query, don't be afraid to ask for help from a more experienced developer or database administrator.
  • Use the right tools There are a number of tools available that can help you to write, format, and debug SQL queries. For example, the IOMETE provides a visual interface for building and executing SQL queries, making it easy to get started even if you're not a SQL expert. If you want to discover with sample account here is the link to IOMETE sandbox.

Step-by-Step Guide for optimizing Lengthy SQL Queries:​

  1. Understanding the Query:

    • Begin by comprehending the query's objectives. What is it aiming to achieve? What data is it attempting to retrieve? How do the various tables interconnect? As you gain clarity on the query's intent, you can start dissecting it into more manageable components.
    • Identify all the referenced tables, shedding light on their relationships and data manipulation. If you notice recurring table references, it can be helpful to tally how often each table appears. This assists in pinpointing the most crucial tables and enhancing your overall grasp of the query's logic.

  2. Reformatting the Query:

    • Reformulating the query enhances its readability and comprehensibility. Whether you utilize an online SQL formatter or manually format it, consistency is key. Proper indentation and correct formatting of keywords and operators are crucial. Here's an example of a well-formatted SQL query:

    SELECT *
    FROM table
    WHERE column_1 = 'x'
    AND column_2 = 'y'
    AND column_3 = 'z';

  3. Identifying Query Components:

    • Distinguish the query's elements, including the tables being joined, the selected columns, and the filtering and aggregation criteria.

  4. Breaking Down with CTEs (Common Table Expressions):

  5. Rigorous Testing:

    • After any alterations to the query, it's vital to rigorously test it to ensure it still functions correctly. Utilizing CHECKSUM can be valuable during this stage.