Click here to monitor SSC
SQLServerCentral is supported by Redgate
 
Log in  ::  Register  ::  Not logged in
 
 
 


How to expand this variable for the query to work


How to expand this variable for the query to work

Author
Message
GSquared
GSquared
SSChampion
SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)

Group: General Forum Members
Points: 14375 Visits: 9729
Two points:

1. Splitting a list in a While loop is more expensive than splitting one using a Numbers table. (See http://www.simple-talk.com/sql/t-sql-programming/the-helper-table-workbench/ for a lot of good data on this subject.)

2. The "Like" solution fails the moment you start using multi-digit IDs. For example, try it out with "12,13,175", and you'll get IDs 1, 2, 3, 7, 5, 12, 13, 175. Becase "%1%" is like "12", and like "13" and like "175".

The function I use for parsing is:

ALTER function [dbo].[StringParser]
(@String_in varchar(max),
@Delimiter_in char(1))
returns table
as
return   (
      SELECT top 100 percent
      SUBSTRING(@String_in+@Delimiter_in, number,
         CHARINDEX(@Delimiter_in, @String_in+@Delimiter_in, number) - number) as Parsed
      FROM dbo.numbers
      WHERE number <= LEN(@String_in)
      AND SUBSTRING(@Delimiter_in + @String_in, number, 1) = @Delimiter_in
      ORDER BY number
      Wink

(I have this in my Common database, along with a Numbers table that goes from 0 to 9999.)

I picked a medium sized table, dbo.Names, with 149,000 rows of data, and ran:

select *
from dbo.names
where '12,13,175' like '%' + cast(nameid as varchar(10)) + '%'

select *
from dbo.names
inner join common.dbo.stringparser('12,13,175', ',')
   on names.nameid = stringparser.parsed

The first one returned values I didn't really want (IDs 1, 2, 3) as well as the IDs I want, and had an execution cost of .838. The second one returned the exact values I want, and an execution cost of .799.

The cost is only slightly different, but the first one included a Clustered Index Scan, while the second had a Clustered Index Seek. On a larger table, the difference in cost would matter more.

So the difference in cost is an illusion, caused by using a While loop to take apart the string instead of using a more efficient solution, and it can result in wrong data.

I'd take a very close look at this before including it in "best practices".

- Gus "GSquared", RSVP, OODA, MAP, NMVP, FAQ, SAT, SQL, DNA, RNA, UOI, IOU, AM, PM, AD, BC, BCE, USA, UN, CF, ROFL, LOL, ETC
Property of The Thread

"Nobody knows the age of the human race, but everyone agrees it's old enough to know better." - Anon
Grant Fritchey
Grant Fritchey
SSCoach
SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)

Group: General Forum Members
Points: 17569 Visits: 32254
Very good points. I was just going to start looking up similar solutions in Itzik's books when I saw your post. I figured there was a way to bring a table of numbers into this somehow.

----------------------------------------------------
The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood...
Theodore Roosevelt

The Scary DBA
Author of: SQL Server Query Performance Tuning and SQL Server Execution Plans
Product Evangelist for Red Gate Software
Kenneth.Fisher
Kenneth.Fisher
Hall of Fame
Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)Hall of Fame (3.6K reputation)

Group: General Forum Members
Points: 3592 Visits: 2021
Actually there is a flaw in your logic. I agree if I am compairing '1' to '2,3,12' I'm going to get a false hit. However I'm compairing ',1,' to ',2,3,12,'. Thats why you add the extra comma at the beginning and the end. Then you add a comma at the beginning and end of your value. That eliminates your false hits.

Also while using a number table may help in this case the other method works on any delimited list. Not just a list of numbers.
For example ',John,Joe,Jim,' LIKE '%,'+First_Name+',%'

I will certainly be running some tests using the number table. I'll be interested to see how it compares out when you have 100 or more items in the list and are returning back several thousand rows.

Kenneth Fisher
I strive to live in a world where a chicken can cross the road without being questioned about its motives.
--------------------------------------------------------------------------------
For better, quicker answers on T-SQL questions, click on the following...
http://www.sqlservercentral.com/articles/Best+Practices/61537/
For better answers on performance questions, click on the following...
http://www.sqlservercentral.com/articles/SQLServerCentral/66909/

Link to my Blog Post --> www.SQLStudies.com
antonio.collins
antonio.collins
SSC-Addicted
SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)

Group: General Forum Members
Points: 454 Visits: 921
I think you're assuming that a simpler execution plan is faster. I just ran this on a load table which as 126K records. seq is a unique key.

select seq from [qhd]
where ',22,99,1031,2050,99999,84000,9900,12333,2,' like '%,'+ cast(seq as varchar(8))+ ',%'

select seq from [qhd]
where seq in (select val from dbo.fListToValues( "22,99,1031,2050,99999,84000,9900,12333,2"))



yes the execution plan for the second query is more complex, but if you look at it closely, the data is retreived using an index seek of [qhd]. the first plan uses an index scan so at least 99999 items are considered.

here are the timings:

1> set statistics time on
2> go
1> select seq from [qhd]
2> where ',22,99,1031,2050,99999,84000,9900,12333,2,' like '%,'+ cast(seq as var
char(8))+ ',%'
5> select seq from [qhd]
6> where seq in (select val from dbo.fListToValues( "22,99,1031,2050,99999,84000
,9900,12333,2"))
8> go
SQL Server parse and compile time:
CPU time = 0 ms, elapsed time = 1 ms.
seq
-----------
2
22
99
1031
2050
9900
12333
84000
99999

(9 rows affected)

SQL Server Execution Times:
CPU time = 1422 ms, elapsed time = 1422 ms.
seq
-----------
2
22
99
1031
2050
9900
12333
84000
99999

(9 rows affected)

SQL Server Execution Times:
CPU time = 0 ms, elapsed time = 2 ms.



1422ms for the like vs 2ms for the UDF. For small tables, the overhead of the function may cause option #2 to be slower but that difference will be negligible since both queries will run near instantaneously. However, once you progress to decent sized tables (10000 entries or more), option #2 is significantly faster. Also, the UDF can be re-used for any integer key and enhanced to support string keys. And UDF makes the SQL clearer and easier to understand. So, in my book option #2 is far superior.
GSquared
GSquared
SSChampion
SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)SSChampion (14K reputation)

Group: General Forum Members
Points: 14375 Visits: 9729
Kenneth Fisher (1/7/2008)
Actually there is a flaw in your logic. I agree if I am compairing '1' to '2,3,12' I'm going to get a false hit. However I'm compairing ',1,' to ',2,3,12,'. Thats why you add the extra comma at the beginning and the end. Then you add a comma at the beginning and end of your value. That eliminates your false hits.

Also while using a number table may help in this case the other method works on any delimited list. Not just a list of numbers.
For example ',John,Joe,Jim,' LIKE '%,'+First_Name+',%'

I will certainly be running some tests using the number table. I'll be interested to see how it compares out when you have 100 or more items in the list and are returning back several thousand rows.

My StringParser function will accept any string with any single-character delimiter. It works on numbers, names, street addresses, whatever. I've run it with up to 3,000 values and it works quite well. (For long lists, it needs more numbers. My production servers have a Numbers table in a Common database with values from 0 to 100-million.)

Adding commas at the beginning and end of the like statement will eliminate false results. But it doesn't help with the cost and the speed. That's why I mentioned that the "Like" solution requires an index scan, while my join solution uses an index seek. Another post in this thread had the same results I did.

The "Like" solution is only better if your string parsing UDF is built poorly, using a cursor or a while loop. If it uses a simple numbers table, the "Like" solution is slower and more expensive.

Again, take a look at the data from Robyn Page and Phil Factor that I referenced in my first post. It will make it much more clear. (They're better writers and have more data than I.)

- Gus "GSquared", RSVP, OODA, MAP, NMVP, FAQ, SAT, SQL, DNA, RNA, UOI, IOU, AM, PM, AD, BC, BCE, USA, UN, CF, ROFL, LOL, ETC
Property of The Thread

"Nobody knows the age of the human race, but everyone agrees it's old enough to know better." - Anon
antonio.collins
antonio.collins
SSC-Addicted
SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)

Group: General Forum Members
Points: 454 Visits: 921
GSquared: Thanks for helping me out.

I think some people are missing the point. Assuming [key] is indexed, cast([key] as anything) will mean that [key]'s index will never be used thus resulting in a table/index scan.

It's just like converting a datetime to a string. dateTimeKey's index is useless for selectivity if it's converted like so:
select ... from [table] T where convert(char(8),T.dateTimeKey,12) = "080107"

So searching with '%'+ cast([key] as varchar) + '%' is bad. It works fine for small tables because even bad SQL runs acceptably on small tables.
Grant Fritchey
Grant Fritchey
SSCoach
SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)

Group: General Forum Members
Points: 17569 Visits: 32254
You just have to keep the tests honest. I wasn't counting icons and going "Oooh, it's faster." In the simple tests, both processes had a table scan. The one that did not also have to perform a join against data missing statistics was faster.

But those were simple tests. I'm setting up something a bit more comprehensive and including a table of numbers in the test.

----------------------------------------------------
The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood...
Theodore Roosevelt

The Scary DBA
Author of: SQL Server Query Performance Tuning and SQL Server Execution Plans
Product Evangelist for Red Gate Software
antonio.collins
antonio.collins
SSC-Addicted
SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)SSC-Addicted (454 reputation)

Group: General Forum Members
Points: 454 Visits: 921
Grant Fritchey (1/7/2008)
You just have to keep the tests honest. I wasn't counting icons and going "Oooh, it's faster." In the simple tests, both processes had a table scan. The one that did not also have to perform a join against data missing statistics was faster.

But those were simple tests. I'm setting up something a bit more comprehensive and including a table of numbers in the test.


For the IN( UDF ) alternative, the table being queried (hosts/udf) did not have a table scan if the identifying column (hostid/seq) was indexed. The table the UDF created had a table scan but it only has a handful of rows.
Mark Cowne
Mark Cowne
SSCrazy
SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)SSCrazy (2.1K reputation)

Group: General Forum Members
Points: 2090 Visits: 22778
There's several techniques and timings described here

http://www.sommarskog.se/arrays-in-sql-2005.html

____________________________________________________

Deja View - The strange feeling that somewhere, sometime you've optimised this query before

How to get the best help on a forum

http://www.sqlservercentral.com/articles/Best+Practices/61537




Grant Fritchey
Grant Fritchey
SSCoach
SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)SSCoach (17K reputation)

Group: General Forum Members
Points: 17569 Visits: 32254
I did some tests. For good or for ill, I found that the XML & Table of Numbers worked better than anything else when dealing with large tables and complex queries. The function just didn't work that well. Yes, the LIKE clause always lead to table scans, so while it was sometimes faster than the function, depending on how well the function was working with the clustered index in the tables for the queries I was testing with, overall, it just didn't cut the mustard once I went to really complex queries.

----------------------------------------------------
The credit belongs to the man who is actually in the arena, whose face is marred by dust and sweat and blood...
Theodore Roosevelt

The Scary DBA
Author of: SQL Server Query Performance Tuning and SQL Server Execution Plans
Product Evangelist for Red Gate Software
Go


Permissions

You can't post new topics.
You can't post topic replies.
You can't post new polls.
You can't post replies to polls.
You can't edit your own topics.
You can't delete your own topics.
You can't edit other topics.
You can't delete other topics.
You can't edit your own posts.
You can't edit other posts.
You can't delete your own posts.
You can't delete other posts.
You can't post events.
You can't edit your own events.
You can't edit other events.
You can't delete your own events.
You can't delete other events.
You can't send private messages.
You can't send emails.
You can read topics.
You can't vote in polls.
You can't upload attachments.
You can download attachments.
You can't post HTML code.
You can't edit HTML code.
You can't post IFCode.
You can't post JavaScript.
You can post emoticons.
You can't post or upload images.

Select a forum

































































































































































SQLServerCentral


Search