I was trying to profile some NHibernate sql to see if we could make some improvements.   One thing I noticed very shortly after starting this task was that one of my columns was being duplicated.  Now this is NOT going to make a huge performance difference but was something I wanted to remove this as it felt ‘wrong’.  Below is the original sql segment

resrole1_.PART_B_PROVIDER_NO             as PART16_29_0_,
resrole1_.ADMIN_SET_ID                   as ADMIN17_29_0_,
resrole1_.NATIONAL_PROVIDER_ID           as NATIONAL18_29_0_,
resrole1_.Admin_Set_ID                   as Admin17_29_0_,
resrole1_.Resource_Type                  as Resource19_29_0_,

The first thing I looked at when I started to look for the cause was the mapping for the entity (Role entity) to make sure the ‘Admin_Set_ID’ is not mapped multiple times.  And of course it was not.  After a few minutes of staring at the screen (everyone knows this is how all the hard problems are solved) it dawned on me that Admin_Set_ID was a FK from one of my associations.  Below is my association that was using it.

References( x => x.ResourceType )
	.Access.AsCamelCaseField( Prefix.Underscore )
        .WithColumns( "Admin_Set_ID", "Resource_Type" )
        .FetchType.Select()
        .LazyLoad();

Because Admin_Set_Id is a FK I should NOT have been mapping as part of my entity.  When I removed this column from my entity so that I could get it as part of my association.  When I did this my newly outputted sql looked like below:

resrole1_.PART_B_PROVIDER_NO             as PART16_29_0_,
resrole1_.ADMIN_SET_ID                   as ADMIN17_29_0_,
resrole1_.NATIONAL_PROVIDER_ID           as NATIONAL18_29_0_,
resrole1_.Resource_Type                  as Resource19_29_0_,

Long story short is this.  If you happen to see a column being duplicated in your generated sql make sure that you are not referencing/mapping a FK directly in your mappings. 

Hope this helps.

Till next time,