How to Efficiently Retrieve Random Rows from a Database Using LINQ to SQL?

LINQ to SQL efficient random row retrieval

When using LINQ to SQL to retrieve random rows from a database, you may encounter performance issues if you rely on client-side filtering and randomization. The following methods provide an efficient way to retrieve random rows matching specific criteria:

Use simulated user-defined functions (UDF)

For small to medium-sized tables, you can use a mock UDF to perform randomization operations at the database level. In the data context, define a partial class and add the following method:

partial class MyDataContext {
     [Function(Name="NEWID", IsComposable=true)] 
     public Guid Random() 
     {
         throw new NotImplementedException(); 
     }
}

This UDF, although not actually implemented in the C# code, will be used by SQL Server to perform random ordering. To retrieve random rows based on a condition, use the following query:

var cust = (from row in ctx.Customers
           where row.IsActive // 您的筛选条件
           orderby ctx.Random()
           select row).FirstOrDefault();

Count-based method

For large tables, a count-based approach may be more efficient. Here’s how it works:

1. Use qry.Count() to calculate the total number of rows.
2. Generate a random index in the range 0 to count -1.
3. Use qry.Skip(index).FirstOrDefault() to retrieve the row at a random index.

This method involves two round trips to the database, but it scales better with table size than using a random sort function.

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