How to Create the Most Efficient User-Defined Function for Generating Number Lists in SQL?

Build efficient SQL number list user-defined functions

Introduction

Auxiliary number tables can improve the efficiency of certain SQL queries. These tables can be fixed size or dynamically generated. This article focuses on how to create an optimal user-defined function for dynamically generating lists of numbers.

Why recursive CTE is not the best choice

The popular recursive CTE method, while "fairly efficient" in generating numbers, is actually not the best choice for large and frequently used lists. Performance tests show that it has higher duration, CPU usage, and memory pressure compared to other methods.

Best method: traditional cross join table

For large lists of numbers, the traditional cross-join table approach always outperforms other techniques. It can generate a list in a single pass with minimal memory overhead.

<code class="language-sql">SELECT TOP (1000000)
        ROW_NUMBER() OVER (ORDER BY (SELECT 1)) AS N
   INTO #Tally3
   FROM Master.sys.All_Columns ac1
  CROSS JOIN Master.sys.ALL_Columns ac2;</code>

Benefits of Permanent Counting Table

While creating a permanent count table requires forethought, it provides the best performance if the list is used frequently. By sizing the table appropriately, it can be cached for optimal performance without impacting memory.

Conclusion

Choosing the right way to create a list of numbers depends on your specific usage pattern. For large and frequently used lists, the traditional cross-join table approach provides the best performance. For one-time or infrequently used lists, it is recommended to create a permanent count sheet.

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