sp_executesql causing my query to be very slow

Question

I am having some issues when running sp_executesql on a database table. I am using an ORM (NHibernate) that generates a SQL query that queries one table in this case. This table has about 7 million records in it and is highly indexed.

When I run the query that the ORM spits out without the sp_executesql, it runs very quickly and profiler shows it has 85 reads. When I run the same query using sp_executesql, it has around 201,828 reads.

Is there something I need to do on my SQL Server to improve the performance of running the query without sp_exectuesql? It seems as though its not using my Indexes.

What is the best way to fix this issue? If possible, I would rather not change the way the ORM is generating the SQL but instead fix the problem at the SQL Server/database level because it seems that is where the problem is. I'm guessing I need to do more optimization on the database to fix this problem I just don't know what.

exec sp_executesql N'SELECT top 20 
                            this_.Id as Id0_0_, 
                            this_.Application as Applicat2_0_0_, 
                            this_.[Context] as column3_0_0_, 
                            this_.Logger as Logger0_0_, 
                            this_.Message as Message0_0_, 
                            this_.Exception as Exception0_0_, 
                            this_.Thread as Thread0_0_, 
                            this_.[Level] as column8_0_0_, 
                            this_.LogDate as LogDate0_0_, 
                            this_.SessionId as SessionId0_0_ 
                       FROM LogMessages this_ 
                      WHERE this_.[Context] = @p0',
                   N'@p0 nvarchar(2)',
                   @p0 = N'55'

Context is a varchar(255). This field is very free form. It isn't always an integer and the length may very. In this case, I am querying for a value of '55' but it could just as easily be querying for 'Foooooobaaaarrr'

Answer 1

what is the data type of .[Context] use the same datatype

right now you are using nvarchar(2) but that seems odd for something like 55, if you are not using the same data types you will get conversions which then cause scans

based on your updated question, it looks like it is varchar(255), then do this

WHERE this_.[Context] = @p0',N'@p0 varchar(255)',@p0='55'