RSS 2.0
Sign In
# Friday, 18 November 2011

Recently we have introduced some stored procedure in the production and have found that it performs incredibly slow.

Our reasoning and tests in the development environment did not manifest any problem at all.

In essence that procedure executes some SELECT and returns a status as a signle output variable. Procedure recieves several input parameters, and the SELECT statement uses with(recompile) execution hint to optimize the performance for a specific parameters.

We have analyzed the execution plan of that procedure and have found that it works as if with(recompile) hint was not specified. Without that hint SELECT failed to use index seek but rather used index scan.

What we have lately found is that the same SELECT that produces result set instead of reading result into a variable performs very well.

We think that this is a bug in SQL Server 2008 R2 (and in SQL Server 2008).

To demonstrate the problem you can run this test:

-- Setup
create table dbo.Items
(
  Item int not null primary key
);
go

insert into dbo.Items
select 1
union all
select 2
union all
select 3
union all
select 4
union all
select 5
go

create procedure dbo.GetMaxItem
(
  @odd bit = null,
  @result int output
)
as
begin
  set nocount on;

  with Items as
  (
    select * from dbo.Items where @odd is null
    union all
    select * from dbo.Items where (@odd = 1) and ((Item & 1) = 1)
    union all
    select * from dbo.Items where (@odd = 0) and ((Item & 1) = 0)
  )
  select @result = max(Item) from Items
  option(recompile);
end;
go

create procedure dbo.GetMaxItem2
(
  @odd bit = null,
  @result int output
)
as
begin
  set nocount on;

  declare @results table
  (
    Item int
  );

  with Items as
  (
    select * from dbo.Items where @odd is null
    union all
    select * from dbo.Items where (@odd = 1) and ((Item & 1) = 1)
    union all
    select * from dbo.Items where (@odd = 0) and ((Item & 1) = 0)
  )
  insert into @results
  select max(Item) from Items
  option(recompile);

  select @result = Item from @results;
end;
go

Test with output into a variable:

declare @result1 int;

execute dbo.GetMaxItem @odd = null, @result = @result1 output

Execution plan of dbo.GetMaxItem

Test without output directly into a variable:

declare @result2 int;

execute dbo.GetMaxItem2 @odd = null, @result = @result2 output

Execution plan of dbo.GetMaxItem2

Now, you can see the difference: the first execution plan uses startup expressions, while the second optimizes execution branches, which are not really used. In our case it was crucial, as the execition time difference was minutes (and more in future) vs a split of second.

See also Microsoft Connect Entry.

Friday, 18 November 2011 14:49:50 UTC  #    Comments [0] -
SQL Server puzzle | Tips and tricks
All comments require the approval of the site owner before being displayed.
Name
E-mail
Home page

Comment (Some html is allowed: a@href@title, b, blockquote@cite, em, i, strike, strong, sub, super, u) where the @ means "attribute." For example, you can use <a href="" title=""> or <blockquote cite="Scott">.  

[Captcha]Enter the code shown (prevents robots):

Live Comment Preview
Archive
<2019 March>
SunMonTueWedThuFriSat
242526272812
3456789
10111213141516
17181920212223
24252627282930
31123456
Statistics
Total Posts: 364
This Year: 0
This Month: 0
This Week: 0
Comments: 222
Locations of visitors to this page
Disclaimer
The opinions expressed herein are our own personal opinions and do not represent our employer's view in anyway.

© 2019, Nesterovsky bros
All Content © 2019, Nesterovsky bros
DasBlog theme 'Business' created by Christoph De Baene (delarou)