RSS 2.0
Sign In
# Thursday, January 19, 2012

While looking at some SQL we have realized that it can be considerably optimized.

Consider a table source like this:

with Data(ID, Type, SubType)
(
  select 1, 'A', 'X'
  union all
  select 2, 'A', 'Y'
  union all
  select 3, 'A', 'Y'
  union all
  select 4, 'B', 'Z'
  union all
  select 5, 'B', 'Z'
  union all
  select 6, 'C', 'X'
  union all
  select 7, 'C', 'X'
  union all
  select 8, 'C', 'Z'
  union all
  select 9, 'C', 'X'
  union all
  select 10, 'C', 'X'
)

Suppose you want to group data by type, to calculate number of elements in each group and to display sub type if all rows in a group are of the same sub type.

Earlier we have written the code like this:

select
  Type,
  case when count(distinct SubType) = 1 then min(SubType) end SubType,
  count(*) C
from
  Data
group by
  Type;

Namely, we select min(SybType) provided that there is a single distinct SubType, otherwise null is shown. That works perfectly, but algorithmically count(distinct SubType) = 1 needs to build a set of distinct values for each group just to ask the size of this set. That is expensive!

What we wanted can be expressed differently: if min(SybType) and max(SybType) are the same then we want to display it, otherwise to show null.

That's the new version:

select
  Type,
  case when min(SubType) = max(SubType) then min(SubType) end SubType,
  count(*) C
from
  Data
group by
  Type;

Such a simple rewrite has cardinally simplified the execution plan:

Execution plans

Another bizarre problem we have discovered is that SQL Server 2008 R2 just does not support the following:

select
  count(distinct SubType) over(partition by Type)
from
  Data

That's really strange, but it's known bug (see Microsoft Connect).

Thursday, January 19, 2012 9:12:11 PM 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
<July 2019>
SunMonTueWedThuFriSat
30123456
78910111213
14151617181920
21222324252627
28293031123
45678910
Statistics
Total Posts: 365
This Year: 1
This Month: 0
This Week: 0
Comments: 221
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)