Optimizing SQL Queries: How to Handle UNION with Conditional Logic

Learn effective methods to optimize SQL queries when using `UNION` with conditional logic for better performance.
---
This video is based on the question https://stackoverflow.com/q/64611321/ asked by the user 'FLash' ( https://stackoverflow.com/u/8370346/ ) and on the answer https://stackoverflow.com/a/64611945/ provided by the user 'Joe Shark' ( https://stackoverflow.com/u/11956089/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Optimization in WHERE clause seems to be not working with UNION

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding SQL Query Optimization with UNION

When working with complex SQL queries that involve multiple UNION operations, it’s essential to consider performance implications, especially when dealing with conditional logic. A common issue arises when certain conditions seem to lead to inefficient query execution, leading to longer processing times than expected. This guide explores how to optimize SQL queries that employ UNION statements with conditional checks.

The Problem

Imagine your SQL script is structured as follows:

[[See Video to Reveal this Text or Code Snippet]]

In this structure, you might intuitively think that the last UNION part will be ignored when the variable @ include is set to 0. However, the query still takes a long time to execute. This situation can happen because the database engine doesn't optimize the query as expected, causing it to evaluate the entire set of UNION operations, leading to poor performance.

The Solution

There are multiple ways to address this issue and enhance the performance of your SQL queries despite the presence of conditional logic. Here are two effective methods that can help you optimize your SQL queries:

1. Use OPTION(RECOMPILE)

One way to improve the performance when running queries with dynamic conditions is to use the OPTION(RECOMPILE) hint. This approach forces SQL Server to recompile the execution plan every time the query runs. Here's how to implement it:

[[See Video to Reveal this Text or Code Snippet]]

This option can be particularly effective when the previous plan was not optimal, leading the system to create a more suitable plan aligned with the current parameter values.

2. Use Conditional Logic with IF Statements

If OPTION(RECOMPILE) doesn't yield the expected results, you can restructure your query using conditional logic with IF statements. By separating the execution paths based on the value of @ include, you can avoid evaluating unnecessary parts of the query. Here’s an example:

[[See Video to Reveal this Text or Code Snippet]]

By structuring the query this way, the SQL engine can skip over the unnecessary components, which can significantly enhance performance times.

Conclusion

Optimizing SQL queries that involve UNION statements, particularly when conditional logic is present, is crucial for maintaining efficient database performance. Utilizing strategies like OPTION(RECOMPILE) can yield improved execution plans, while structuring queries with conditional IF statements can prevent unnecessary evaluations. Analyzing how SQL Server processes these commands is key to writing efficient queries. By implementing these approaches, you can ensure your SQL queries run faster and more efficiently, leading to better resource management and user experience.

If you have further questions or need assistance in optimizing your SQL queries, feel free to reach out!