When using SetFirstResult(start)
and SetMaxResults(count)
methods to implement paging I've noticed that the generated query only does a select top count * from some_table
and it does not take the start
parameter into account or at least not at the database level. It seems that if I instruct NHibernate to execute the following query:
var users = session.CreateCriteria<User>()
.SetFirstResult(100)
.SetMaxResults(5)
.List<User>();
105 records will transit between the database server and the application which will take care to strip the first 100 records. With tables containing many rows this could be a problem.
I've verified that with an SQLite database NHibernate takes advantage of the OFFSET
and LIMIT
keywords to filter results at the database level. I am aware that there's no equivalent of the OFFSET
keyword and Oracle's ROWNUM
in SQL Server 2000 but is there any workaround? How about SQL Server 2005/2008?
T-SQL, the variant of the SQL language which Microsoft SQL Server uses, does not have a limit
clause. It has a select top {...}
modifier which you see NHibernate taking advantage of with SQL Server 2000.
With SQL Server 2005, Microsoft introduced the Row_Number() over (order by {...})
function which can be used as a replacement to the limit
clause, and you can see NHibernate taking advantage of that with SQL Server 2005/2008.
A query for SQLite might look like
select c.[ID], c.[Name]
from [Codes] c
where c.[Key] = 'abcdef'
order by c.[Order]
limit 20 offset 40
while a similar query for SQL Server 2005 might look like
select c.[ID], c.[Name]
from (
select c.[ID], c.[Name], c.[Order]
, [!RowNum] = Row_Number() over (order by c.[Order])
from [Codes] c
where c.[Key] = 'abcdef'
) c
where c.[!RowNum] > 40 and c.[!RowNum] <= 60
order by c.[Order]
or, using Common Table Expressions, it might look like
with
[Source] as (
select c.[ID], c.[Name], c.[Order]
, [!RowNum] = Row_Number() over (order by c.[Order])
from [Codes] c
where c.[Key] = 'abcdef'
)
select c.[ID], c.[Name]
from [Source] c
where c.[!RowNum] > 40 and c.[!RowNum] <= 60
order by c.[Order]
There is a way to do it in SQL Server 2000 as well
select c.[ID], c.[Name]
from (
select top 20 c.[ID], c.[Name], c.[Order]
from (
select top 60 c.[ID], c.[Name], c.[Order]
from [Codes] c
where c.[Key] = 'abcdef'
order by c.[Order]
) c
order by c.[Order] desc
) c
order by c.[Order]