Query got timeout error in NHibernate but not In SQL Server

Question

I got a problem with NHibernate in C#.

When it wants to execute a query the application face with a ADO timeout error, but when I use SQL Profiler to capture the query,

Answer 1

When you capture the query from SQL Profiler and run it in SSMS, are you running it as an sp_executesql query? I ran into a similar problem using NHibernate 2.1GA and this answer applies to that version, I haven't converted to NH3 yet. NH Profiler is a great tool but it helpfully extracts the SQL into a formatted query that doesn't represent the actual query sent to the server.

The problem is the way NHibernate supplies string parameters to sp_executesql. String parameters are typed as nvarchar with a length equal to the value's length. For example, this query restricts two columns that are varchar(4) and varchar(20) respectively:

exec sp_executesql N'SELECT this_.Column0, this_.Column1 FROM MySchema.MyTable this_ WHERE this_.Column0 = @p0 and this_.Column1 = @p1',N'@p0 nvarchar(4),@p1 nvarchar(7)',@p0='Val0',@p1='Value01'

The query plan for this used an index scan and took 17 sec. Changing the nvarchar to varchar generated a plan that used an index seek and executed in < 2 sec. This was reproducible in SSMS.

The root cause was the NHibnerate uses DbType.String instead of DbType.AnsiString for varchar columns by default. The solution for me was to add a Fluent NHibernate convention to change all string mappings to AnsiString which caused NHibernate to create queries that supplied parameters as varchar.