Drop default constraint on a table function

Question

I need to drop some default constraints that appear to be tied to table functions (and not actual tables).  This means when I try the ALTER TABLE DROP CONSTRAINT command it fails with the error, "unable to drop constraint because object is not
a table" or something similar.
My question is: how do I drop a constraint on a table function?

Answer 1

I suggest you review the documentation for TVFs and how they are (and can be) used.  The table returned by a TVF (and in this case I refer specifically to multistatement TVFs) are defined using a subset of the create table syntax.  They can be
created with constraints of different types - not just defaults.  Why?  Because it suits the logic of the developer and (perhaps) because it assists the database engine or the logic that depends on the output of the function.  
Below is one example that I used (written by Steve Kass) from a LONG time ago.  Notice the primary key.
CREATE FUNCTION [dbo].[uf_sequence] (@N int)
RETURNS @T TABLE (
seq int not null primary key clustered
AS
** 04/21/05.sbm - Bug #306. Initial version.
** Code provided by Steve Kass - MS .programming newsgroup
BEGIN
DECLARE @place int
SET @place = 1
INSERT INTO @T (seq) VALUES (0)
WHILE @place <= @N/2 BEGIN
INSERT INTO @T (seq)
SELECT @place + Seq FROM @T
SET @place = @place + @place
END
INSERT INTO @T (seq)
SELECT @place + Seq FROM @T
WHERE Seq <= @N - @place
RETURN
END
go
For your particular case, the choice of a default constraint is likely due to the implementation of the logic in the function.  Perhaps there are multiple insert statements and it was simpler/easier/more robust to use a default constraint rather than
repeatedly hard-code the value in each statement.  By choosing a default constraint, the developer need only alter the constraint (once) if the value needs to be changed rather than finding and changing each statement that inserts or updates the table.
As you've have already discerned, you can simply ignore any constraints that are defined on the tables returned by a TVF.