How does one make NHibernate stop using nvarchar(4000) for insert parameter strings? - sql-server

Question

I need to optimize a query that is being produced by a save (insert query) on a domain entity. I've configured NHibernate using Fluent NHibernate.
Here's the query generated by NHibernate during the insertion of a user's response to a poll:
exec sp_executesql N'INSERT INTO dbo.Response (ModifiedDate, IpAddress, CountryCode,
IsRemoteAddr, PollId) VALUES (#p0, #p1, #p2, #p3, #p4); select SCOPE_IDENTITY()',N'#p0
datetime,#p1 nvarchar(4000),#p2 nvarchar(4000),#p3 bit,#p4 int',
#p0='2001-07-08 03:59:05',#p1=N'127.0.0.1',#p2=N'US',#p3=1,#p4=2
If one looks at the input parameters for IpAddress and CountryCode, one will notice that NHibernate is using nvarchar(4000). The problem is that nvarchar(4000) is far larger than I need for either IpAddress or CountryCode and due to high traffic and hosting requirements I need to optimize the database for memory usage.
Here's the Fluent NHibernate auto-mapping overrides for those columns:
mapping.Map(x => x.IpAddress).CustomSqlType("varchar(15)");
mapping.Map(x => x.CountryCode).CustomSqlType("varchar(6)");
This isn't the only place that I see unnecessary nvarchar(4000)'s popping up.
How do I control NHibernate's usage of nvarchar(4000) for string representation?
How do I change this insert statement to use the proper sized input parameters?

Answer 1

Specify the Type as NHibernateUtil.AnsiString with a Length instead of using a CustomSqlType.

Answer 2

This issue can cause a huge performance problem in queries if it forces SQL Server to perform a table scan instead of using an index. We use varchar throughout our database so I created a convention to set the type globally:
/// <summary>
/// Convert all string properties to AnsiString (varchar). This does not work with SQL CE.
/// </summary>
public class AnsiStringConvention : IPropertyConventionAcceptance, IPropertyConvention
{
public void Accept(IAcceptanceCriteria<IPropertyInspector> criteria)
{
criteria.Expect(x => x.Property.PropertyType.Equals(typeof(string)));
}
public void Apply(IPropertyInstance instance)
{
instance.CustomType("AnsiString");
}
}

Answer 3

Okay this is what we have to do, the SQLClientDriver ignores the length property of the SqlType. So we created a our own driverclass inheriting from SQLClientDriver and override the method GenerateCommand...Something like this:
public override IDbCommand GenerateCommand(CommandType type, NHibernate.SqlCommand.SqlString sqlString, SqlType[] parameterTypes)
{
var dbCommand = base.GenerateCommand(type, sqlString, parameterTypes);
SetParameterSizes(dbCommand.Parameters, parameterTypes);
return dbCommand;
}
private static void SetParameterSizes(IDataParameterCollection parameters, SqlType[] parameterTypes)
{
for (int index = 0; index < parameters.Count; ++index)
SetVariableLengthParameterSize((IDbDataParameter)parameters[index], parameterTypes[index]);
}
private static void SetVariableLengthParameterSize(IDbDataParameter dbParam, SqlType sqlType)
{
SetDefaultParameterSize(dbParam, sqlType);
if (sqlType.LengthDefined && !IsText(dbParam, sqlType) && !IsBlob(dbParam, sqlType))
dbParam.Size = sqlType.Length;
if (!sqlType.PrecisionDefined)
return;
dbParam.Precision = sqlType.Precision;
dbParam.Scale = sqlType.Scale;
}

Answer 4

Here is a work around, if you want to replace all nvarchar with varchar
public class Sql2008NoNVarCharDriver : Sql2008ClientDriver
{
public override void AdjustCommand(IDbCommand command)
{
foreach (System.Data.SqlClient.SqlParameter x in command.Parameters)
{
if (x.SqlDbType == SqlDbType.NVarChar)
{
x.SqlDbType = SqlDbType.VarChar;
}
}
base.AdjustCommand(command);
}
}
Then plug it into your config
var cfg = Fluently.Configure()
.Database(MsSqlConfiguration.MsSql2008.ConnectionString(connectionString)
.Driver<Sql2008NoNVarCharDriver>())
...