CROSS APPLY vs. INNER JOIN: When Does CROSS APPLY Offer Better Performance?

Performance comparison between CROSS APPLY and INNER JOIN

CROSS APPLY is often considered a more efficient alternative to INNER JOIN when working with large data sets. While both techniques may produce similar execution plans under certain scenarios, CROSS APPLY can provide significant performance advantages in some cases.

Definition of CROSS APPLY

CROSS APPLY is a table-valued function that creates a new row for each input row based on a specified expression. Unlike INNER JOIN, it does not require join conditions between tables.

Applicable scenarios for CROSS APPLY

CROSS APPLY is useful when dealing with complex operations that cannot be easily defined using simple join conditions. For example, selecting a specified number of records from a second table for each row in the first table is a task that CROSS APPLY can handle easily. Here is a query that demonstrates this use case:

<code class="language-sql">SELECT  t1.*, t2o.*
FROM    t1
CROSS APPLY
        (
        SELECT  TOP 3 *
        FROM    t2
        WHERE   t2.t1_id = t1.id
        ORDER BY
                t2.rank DESC
        ) t2o</code>

In the above query, CROSS APPLY selects the three latest records from the 't2' table for each row in the 't1' table. This operation is particularly useful when extracting related data from different tables.

Performance comparison

To illustrate the performance advantages of CROSS APPLY, consider the following scenario:

• A dataset containing 20,000,000 records in the "main" table.
• Query that joins the "main" table with another table containing only two rows.

When executing a query using CTE and window functions to simulate CROSS APPLY, the query takes approximately 30 seconds. However, when using CROSS APPLY directly, the query executes immediately. This demonstrates the efficiency of CROSS APPLY in handling complex operations involving row-by-row iteration.

Conclusion

In summary, while INNER JOIN is the standard way to combine data from multiple tables, CROSS APPLY has advantages in specific situations, especially when dealing with complex operations that require row-by-row processing. By understanding the benefits of CROSS APPLY, developers can optimize the performance of their SQL queries and handle complex data extraction tasks more efficiently.

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