How Does Join Order Affect SQL Query Performance?

Effect of Join Order on SQL Performance

In the given SQL query, the order of the joins can indeed impact performance. Traditionally, the order of the joins would have been:

FROM dbo.Reporting_JourneyMaster90 AS jm

followed by joins with Reporting_Journeys and Reporting_WebUsers. However, in some cases, it may be beneficial to alter the join order.

Performance Considerations

In SQL Server 2008R2, the join order can significantly affect query performance, especially in complex queries with multiple joins and where clauses. The optimizer attempts to find an optimal join order, but it may not always consider all possible combinations.

Optimization Technique

To improve performance, it is recommended to start the join order with the tables that apply the most impactful where clauses. This helps the optimizer narrow down the data set early on, reducing the number of rows that need to be processed in subsequent joins.

In the given example, it may make sense to start with the join between Reporting_WebUsers and Reporting_JourneyMaster90, as the where clause on wu.isActive can potentially reduce the data set significantly. Subsequently, the Reporting_Journeys table can be joined to further filter the results based on journey duration and distance.

Note: While force ordering can be used to control the join order, it can be overly restrictive and may not always yield optimal results. It is generally better to let the optimizer determine the optimal join order based on the provided selectivity estimates.

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