exec sp_executesql nvarchar(max) truncating at 4000 characters

The Problem

Sometimes you may need to write dynamic SQL.

One example is writing query for running over linked server connections. You might write something like this

Create Procedure prcTest
    @start datetime
as
Begin
    Declare @sql nvarchar(max)

    set @sql = 'Insert Into #tblTemp
    select * from OpenQuery(LINKSERVER, ''Select * from dbo.tblRemote r
        inner join tblRemote2 r2 on r.Id = r2.fkId 
    .... lots more sql x 4000+ characters
    Where r.DateCreated > ''''' + Convert(varchar, @start, 106) + '''''
    '')';

    exec sp_executesql @sql

    select * from #tblTemp inner join tblOther on id = otherId

End

Assuming this has over 4000 characters it will truncate at 4000 characters, your SQL will be malformed as it is missing the closing quote ‘ and will crash.

The Resolution

So what is going on!

I’ve read everywhere on the internet about nvarchar(max) having a 2GB limit etc, but it is clearly truncating at 4000 characters.

Add in a

select Len(@sql)

to confirm that it is definitely a 4000 character limit.

So what is going on?

Implicit Type Conversion!!!

The innocent looking code adding a date to the query,

 ''''' + Convert(varchar, @start, 106) + '''''

is causing SQL Server to perform an implicit type conversion and our NVarchar(MAX) is somehow becoming an NVarchar(4000).

Change the addition of the date to the following

''''' + Convert(nvarchar(max), @start, 106) + '''''

The problem is averted and the select Len(@sql) will happily count beyond 4000.

(BTW convert(varchar, @date, 106), creates a ‘1-Jan-2000’ formatted date which resolves any issues that can occur with different regional settings on your sql server causing problems)