How Can I Prevent EF 6 Parameter Sniffing Performance Issues?

EF 6 Parameter Sniffing

When working with large dynamic queries in EF 6, you may encounter performance issues due to parameter sniffing. This occurs when the database engine retains the execution plan of a query with a specific set of parameters and applies it to subsequent executions, even if the parameters have changed.

To address this issue, EF 6 provides an interception feature that allows you to manipulate SQL commands before they are executed. This allows you to embed options such as "OPTION RECOMPILE" into the command.

To utilize this feature, implement the [IDbCommandInterceptor](https://docs.microsoft.com/en-us/dotnet/api/entityframework/system.data.entity.infrastructure.idbcommandinterceptor?view=ef6) interface and override the [ReaderExecuting](https://docs.microsoft.com/en-us/dotnet/api/entityframework/system.data.entity.infrastructure.idbcommandinterceptor.readerexecuting?view=ef6) method. In this method, you can add the desired query hint to the command.

Here is an example implementation of an interceptor that adds the "OPTION RECOMPILE" hint to select queries:

public class OptionRecompileHintDbCommandInterceptor : IDbCommandInterceptor
{
    public void ReaderExecuting(DbCommand command, DbCommandInterceptionContext<DbDataReader> interceptionContext)
    {
        addQueryHint(command);
    }

    private static void addQueryHint(IDbCommand command)
    {
        if (command.CommandType != CommandType.Text || !(command is SqlCommand))
            return;

        if (command.CommandText.StartsWith("select", StringComparison.OrdinalIgnoreCase) &amp;&amp; !command.CommandText.Contains("option(recompile)"))
        {
            command.CommandText = command.CommandText + " option(recompile)";
        }
    }
}

To use the interceptor, add the following line at the beginning of your application:

DbInterception.Add(new OptionRecompileHintDbCommandInterceptor());

This will ensure that the "OPTION RECOMPILE" hint is added to all select queries executed through EF 6, mitigating the impact of parameter sniffing.

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