Interesting Bug/Feature In SQL Server 2008 RTM

Someone had a problem with 8 year old procs which started to fail after moving to SQL Server 2008
Of course he should have used ints, but let's see what happens

Run this code on SQL Server 2005 and 2000

DECLARE @num_Passed Numeric(2, 0);
SET @num_Passed = -1;
SELECT @num_Passed


IF (@num_Passed = 0)
PRINT 'True';



No problem right?

Run just this part on SQL 2008


DECLARE @num_Passed Numeric(2, 0);
SET @num_Passed = -1;

SELECT @num_Passed


No problem either
Now run this whole thing

DECLARE @num_Passed Numeric(2, 0);
SET @num_Passed = -1;
SELECT @num_Passed


IF (@num_Passed = 0)
PRINT 'True';




Oops, this is what we get
Server: Msg 8115, Level 16, State 2, Line 7
Arithmetic overflow error converting expression to data type tinyint.


Change the -1 to 1

DECLARE @num_Passed Numeric(2, 0);
SET @num_Passed = 1;
SELECT @num_Passed



IF (@num_Passed = 0)
PRINT 'True';




No problem either.

Run this

IF (convert(Numeric(2, 0),-1) = 0)
PRINT 'True';

That fails
Let's make it numeric(3,0)

IF (convert(Numeric(3, 0),-1) = 0)
PRINT 'True';



No problem, that runs fine. So is this a bug because of implicit conversion to tinyint which can't hold negative values?

Only In A Database Can You Get 1000% + Improvement By Changing A Few Lines Of Code

Take a look at this query.

SELECT * FROM

(

SELECT customer_id, ‘MTD’ AS record_type, COUNT(*), SUM(…), AVG(…)

FROM payment_table

WHERE YEAR(payment_dt) = YEAR(GETDATE())

and MONTH(payment_dt) = MONTH(GETDATE())

GROUP BY customer_id) MTD_payments

UNION ALL

(

SELECT customer_id, ‘YTD’ AS record_type, COUNT(*), SUM(…), AVG(…)

FROM payment_table

WHERE

WHERE YEAR(payment_dt) = YEAR(GETDATE())

GROUP BY customer_id) YTD_payments

UNION ALL

(

SELECT customer_id, ‘LTD’ AS record_type, COUNT(*), SUM(…), AVG(…)

FROM payment_table) LTD_payments

) payments_report

ORDER BY customer_id, record_type

Can you see the problem?
A person had this query, it would run for over 24 hours. Wow, that is pretty bad, I don't think I had ever written something that ran over an hour, and the ones I did were mostly defragmentation and update statistics jobs.

The problem is that the following piece of code

where year(payment_dt) = year(getDate())
and month(payment_dt) = month(getDate())

is not sargable. First what does it mean to be sargable? A query is said to be sargable if the DBMS engine can take advantage of an index to speed up the execution of the query (using index seeks, not covering indexes). The term is derived from a contraction of Search ARGument Able.

This query is not sargable because there is a function on the column, whenever you use a function on the column you will not get an index seek but an index scan. The difference between an index seek and an index scan can be explained like this: when searching for something in a book, you go to the index in the back find the page number and go to the page, that is an index seek. When looking for something in a book you go from page one until the last page, read all the words on all the ages and get what you need, that was an index scan. Do you see how much more expensive in terms of performance that was?

Let's get back to the query, what can we do to make this piece of code use an index seek?
where year(payment_dt) = year(getDate())
and month(payment_dt) = month(getDate())

You would change it to this:
where payment_dt >= dateadd(mm, datediff(mm, 0, getdate())+0, 0)
and payment_dt < dateadd(mm, datediff(mm, 0, getdate())+1, 0)

You can see the complete question on the MSDN forum site here:
http://forums.microsoft.com/msdn/ShowPost.aspx?PostID=3746751&SiteID=1
The Person said that his query went from over 24 hours to 36 seconds. Wow!! That is very significant. hardware cannot help you out if you have bad queries like that.

The same exact day I answered a very similar question, take a look here: http://forums.microsoft.com/msdn/ShowPost.aspx?PostID=3752248&SiteID=1

The person had this

AND DATEDIFF(d, '08/10/2008', DateCreated) >= 0

AND DATEDIFF(d, DateCreated, '08/15/2008') >= 0

I told him to change it to this

AND DateCreated >= '08/10/2008'
and DateCreated <= '08/15/2008'

And that solved that query. If you are interested in some more performance, I have written some Query Optimization items on the LessThanDot Wiki. Below are some direct links

Case Sensitive Search
No Functions on Left Side of Operator
Query Optimizations With Dates
Optimization: Set Nocount On
No Math In Where Clause
Don't Use (select *), but List Columns

If you are interested in some blogposts about dates, take a look at these two which I wrote earlier
How Are Dates Stored In SQL Server?
Do You Know How Between Works With Dates?

SQL Tip, Compiling Your SQL Without Running It to See If It Would Run

Let's say you have a big SQL script with a ton of code and you want to make sure it runs but you don't want to execute it because it updates tables, deletes data etc etc.
Take this simple example

SELECT GETDATE()
GO
SELECT 1/asasasas
GO


You can probably guess that the second statement is not valid, when you have a lot of code it is more difficult to spot these things.
Execeute the code above and you will get this


(1 row(s) affected)

Server: Msg 207, Level 16, State 1, Line 1
Invalid column name 'asasasas'.

SQL server has the SET NOEXEC statement. From BOL:

When SET NOEXEC is ON, Microsoft® SQL Server™ compiles each batch of
Transact-SQL statements but does not execute them. When SET NOEXEC is OFF, all
batches are executed after compilation.

The execution of statements in
SQL Server consists of two phases: compilation and execution. This setting is
useful for having SQL Server validate the syntax and object names in
Transact-SQL code when executing. It is also useful for debugging statements
that would usually be part of a larger batch of statements.

The setting
of SET NOEXEC is set at execute or run time and not at parse time.

So execute the code below

SET NOEXEC ON
GO

SELECT GETDATE()
GO
SELECT 1/asasasas
GO
SET NOEXEC OFF
GO



As you can see the output is the following:
Server: Msg 207, Level 16, State 1, Line 1
Invalid column name 'asasasas'.


You never see the getdate. Parsing that code will not throw an error and because of deferred name resolution you can fat-finger table names and it will parse without a problem.

Microsoft Visual Studio 2008 Service Pack 1 Available For Download

Get it here: http://www.microsoft.com/downloads/thankyou.aspx?familyId=fbee1648-7106-44a7-9649-6d9f6d58056e&displayLang=en

Here is the link to the 831MB ISO
http://www.microsoft.com/downloads/details.aspx?FamilyId=27673C47-B3B5-4C67-BD99-84E525B5CE61&displaylang=en


Finally, Now I can also install SQL Server 2008 :-)

SQL Server 2008 is RTM

that is right and you can already download it from MSDN