SA0181 : The query joins too many table sources

When writing T-SQL queries, joining a large number of tables can result in complex execution plans and slow query performance. This issue is particularly relevant in SQL Server, where optimal performance depends on efficient query execution strategies.

For example:

1-- Example of a problematic query with too many joins
2SELECT *
3FROM Orders
4JOIN Customers ON Orders.CustomerID = Customers.CustomerID
5JOIN Products ON Orders.ProductID = Products.ProductID
6JOIN Suppliers ON Products.SupplierID = Suppliers.SupplierID
7JOIN Categories ON Products.CategoryID = Categories.CategoryID;

The above query joins multiple tables, potentially leading to a complicated execution plan that may degrade performance. Such queries can also become difficult to read and maintain.

• Complex execution plans may cause longer query execution times, impacting database performance.

• Increased difficulty in understanding and maintaining queries, leading to potential errors or inefficiencies in future modifications.

To resolve the issue of queries joining too many tables, follow a series of steps to optimize and simplify the SQL query.

Follow these steps to address the issue:

1. Identify the necessity of each joined table. Determine if all joined tables are needed for the desired result. Remove any unnecessary joins.

2. Refactor the query to use common table expressions (CTEs) or derived tables to break the problem into smaller, more manageable components. This can simplify the query structure.

3. Ensure that relevant indexes exist on join columns to enhance join performance.

4. Consider using indexed views if certain complex joins are reused frequently and necessary for performance.

For example:

 1-- Simplified query using a CTE
 2WITH OrderDetails AS (
 3    SELECT Orders.OrderID, Customers.CustomerID, Products.ProductID, Suppliers.SupplierID
 4    FROM Orders
 5    JOIN Customers ON Orders.CustomerID = Customers.CustomerID
 6    JOIN Products ON Orders.ProductID = Products.ProductID
 7),
 8CategoryInfo AS (
 9    SELECT ProductID, CategoryID
10    FROM Products
11    JOIN Categories ON Products.CategoryID = Categories.CategoryID
12)
13SELECT OrderDetails.OrderID, OrderDetails.CustomerID, OrderDetails.ProductID, CategoryInfo.CategoryID
14FROM OrderDetails
15JOIN CategoryInfo ON OrderDetails.ProductID = CategoryInfo.ProductID;

The rule has a Batch scope and is applied only on the SQL script.

The rule does not need Analysis Context or SQL Connection.

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