When you compare two columns of different data types SQL Server will attempt to do implicit conversion on lower precedence.
The following comes from MSDN docs on uniqueidentifier
The following example demonstrates the truncation of data when the value is too long for the data type being converted to. Because the uniqueidentifier type is limited to 36 characters, the characters that exceed that length are truncated.
DECLARE @ID nvarchar(max) = N'0E984725-C51C-4BF4-9960-E1C80E27ABA0wrong';
SELECT @ID, CONVERT(uniqueidentifier, @ID) AS TruncatedValue;
http://msdn.microsoft.com/en-us/library/ms187942.aspx
Documentation is clear that data is truncated
When ever you are unsure about your join operation you can verify Actual Execution Plan
.
Here is test sample that you can run inside SSMS or SQL Sentry Plan Explorer
DECLARE @userbackup TABLE ( _FILENAME VARCHAR(70) )
INSERT INTO @userbackup
VALUES ( '15b993cc-e8be-405d-bb9f-0c58b66dcdfe' ),
( '4cffe724-3f68-4710-b785-30afde5d52f8' ),
( '4cffe724-3f68-4710-b785-30afde5d52f8_Add' )
, ( '7ad22838-ddee-4043-8d1f-6656d2953545' )
DECLARE @Requests TABLE
(
requestID UNIQUEIDENTIFIER
,_Status INT
)
INSERT INTO @Requests
VALUES ( '15b993cc-e8be-405d-bb9f-0c58b66dcdfe', 1 )
, ( '4cffe724-3f68-4710-b785-30afde5d52f8', 1 )
, ( '7ad22838-ddee-4043-8d1f-6656d2953545', 2 )
SELECT *
FROM @userbackup u
JOIN @Requests r
ON u.[_FILENAME] = r.requestID
WHERE r.[_Status] = 1
Instead of regular join
operation SQL Server is doing HASH MATCH
with EXPR 1006
in SSMS it is hard to see what is doing but if you open XML file you will find this
<ColumnReference Column="Expr1006" />
<ScalarOperator ScalarString="CONVERT_IMPLICIT(uniqueidentifier,@userbackup.[_FILENAME] as [u].[_FILENAME],0)">
When ever in doubt check execution plan and always make sure to match data types when comparing.
This is great blog Data Mismatch on WHERE Clause might Cause Serious Performance Problems from Microsoft engineer on exact problem.