SQL server - Conversion failed error but not really - sql-server-2008

Question

I've got a user-defined function to split lists of integers into a table of values. I'm using this to parse input to select a set of records for a given set of types or statuses.
This works:
select * from RequestStatus
where RequestStatusUID in (select [value] from dbo.SplitIDs('1,2,3', ','))
This does not:
select * from Requests
where RequestStatusUID in (select [value] from dbo.SplitIDs('1,2,3', ','))
The Requests query returns the error "Conversion failed when converting the varchar value '1,2,3' to data type int." RequestStatusUID on both tables are int columns. Both Explain plans look the same to me. The function is working perfectly the same way in unrelated queries. So far as I can tell it's only the Requests table that has the problem.
CREATE TABLE [dbo].[Requests] (
[RequestUID] int IDENTITY(1,1) NOT NULL,
[UserUID] int NOT NULL,
[LocationUID] int NOT NULL,
[DateOpened] date NULL,
[DateClosed] date NULL,
[RequestStatusUID] int NOT NULL,
[DiscussionUID] int NULL,
[RequestTypeUID] int NOT NULL,
[RequestNo] varchar(16) NOT NULL,
[LastUpdateUID] int NOT NULL,
[LastUpdated] date NOT NULL,
CONSTRAINT [PK_Requests] PRIMARY KEY NONCLUSTERED([RequestUID])
It does work if I use a different function that returns varchars and I convert the RequestStatusUID column to a varchar as well:
select * from Requests
where cast(RequestStatusUID as varchar(4)) in (select [value] from dbo.Split('1,2,3', ','))
For reference, the SplitIDs function I'm using (a modified version of Arnold Fribble's solution). The Split function is the same without the cast as int at the end:
ALTER FUNCTION [dbo].[SplitIDs] ( #str VARCHAR(MAX), #delim char(1)=',' )
RETURNS TABLE
AS
RETURN
(
with cte as (
select 0 a, 1 b
union all
select b, cast(charindex(#delim, #str, b) + 1 as int)
from cte
where b > a
)
select cast(substring(#str,a,
case when b > 1 then b-a-1 else len(#str) - a + 1 end) as int) [value]
from cte where a >0
)
I can use the convert-to-strings solution but I'd really like to know why this is failing in the first place.

Answer 1

I think you will find that this syntax performs a lot better:
SELECT r.* FROM dbo.Requests AS r
INNER JOIN dbo.SplitIDs('1,2,3', ',') AS s
ON r.RequestStatusUID = s.value;
The predicate still has a bunch of implicit converts, due to your function choice, but the join eliminates an expensive table spool. You may see even slightly better performance if you use a proper column list, limited to the actual columns you need, instead of using SELECT *. You should change this even if you do need all of the columns.
Your IN () query, with an expensive table spool (click to enlarge):
My JOIN version, where the cost is transferred to the scan you're doing anyway (click to enlarge):
And here are runtime metrics (based on a small number of rows of course) - (click to enlarge):
The conversion errors seem to be stemming from the function. So I substituted my own (below). Even after adding the foreign key we didn't initially know about, I was unable to reproduce the error. I am not sure exactly what the problem is with the original function, but all those implicit converts it creates seem to cause an issue to the optimizer at some point. So I suggest this one instead:
CREATE FUNCTION dbo.SplitInts
(
#List VARCHAR(MAX),
#Delimiter VARCHAR(255) = ','
)
RETURNS TABLE
WITH SCHEMABINDING
AS
RETURN
(
SELECT [value] = y.i.value('(./text())[1]', 'int')
FROM
(
SELECT x = CONVERT(XML, '<i>'
+ REPLACE(#List, #Delimiter, '</i><i>')
+ '</i>').query('.')
) AS a CROSS APPLY x.nodes('i') AS y(i)
);
GO
So, seems to me like you want to get rid of the function.
Also, here is one way to use a join and still make the param optional:
DECLARE #param VARCHAR(MAX) = NULL;-- also try = '1,2,3';
SELECT r.*
FROM dbo.Requests AS r
LEFT OUTER JOIN dbo.SplitInts(#param, default) AS s
ON r.RequestStatusUID = s.value
WHERE (r.RequestStatusUID = s.value OR #param IS NULL);