Speed Up SQL Queries by Replacing OUTER APPLY with LEFT JOIN

Discover how to optimize your SQL queries by replacing `OUTER APPLY` with `LEFT JOIN` for better performance on low-resource servers.
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Speed Up SQL Queries by Replacing OUTER APPLY with LEFT JOIN

When working with SQL queries, performance is key—especially when your server resources are limited. One common issue arises with complex queries that include OUTER APPLY, which might slow down your process significantly. If you’re experiencing slow performance while executing queries that use OUTER APPLY, this guide will help you understand how to replace it with a LEFT JOIN, making your queries run more efficiently.

The Problem with OUTER APPLY

OUTER APPLY is a powerful feature in SQL that allows you to join a table-valued function or subquery. However, it can be resource-intensive, leading to slower queries and even errors when the SQL Server runs out of resources. This is typically seen in scenarios where:

The query is complex, referencing a large number of tables or partitions.

You’re running the queries on a limited resource server, such as a VPS (Virtual Private Server).

Let’s explore a specific example:

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

In this code snippet, the OUTER APPLY fetches the most recent comment and its details for each event. While this approach may provide the correct results, the performance is lacking, especially on a server with limited resources.

The Solution: Switching to LEFT JOIN

To address the performance issues, you can replace OUTER APPLY with a LEFT JOIN while introducing ROW_NUMBER to manage the results more effectively. This method generates a sequential number based on the order of specified criteria, enabling you to filter results efficiently.

Revised SQL Query

Here’s how you can reformulate your query:

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

Breakdown of the Changes

Using LEFT JOIN: This joins the main Event table with a subquery. Unlike OUTER APPLY, it will fetch all records from the left table even if there are no matching records in the right table.

Incorporating ROW_NUMBER: This function assigns a unique number to each row according to the order of Event_StatusHistory_Date for each corresponding event (grouped by e2.ID). By filtering on Decision.RN = 1, we retrieve only the most recent record per event.

Conclusion

By replacing OUTER APPLY with a LEFT JOIN and utilizing ROW_NUMBER, you can significantly enhance the performance of your SQL queries, particularly on low-resource servers. This change not only optimizes the query execution but also reduces the likelihood of running out of server resources.

If you’re facing issues with SQL query performance, these adjustments might be just what you need to boost efficiency and ensure smoother operations. Give it a try, and see the difference it makes!