January 13, 2011 at 4:28 am
Recently we replaced cursors with while loop and table variables.
The block is something similar to
[Code]
Declare @tablevar.
insert into @tablevar select stmt
select top 1
while ( @@rowcount <> 0 )
begin
processing..
delete the row
select top 1
end
[/code]
is there any better way, now the sp is performing very slow.
Thanks for your time
January 13, 2011 at 4:33 am
krishnaroopa (1/13/2011)
Recently we replaced cursors with while loop
Ahh , you fell for that one , it is a myth that replacing cursors with a while loop is a good idea.
You need to replace the whole loop with set based logic.
Post the whole code and i'll take a look.
January 13, 2011 at 4:36 am
Dave Ballantyne (1/13/2011)
krishnaroopa (1/13/2011)
Recently we replaced cursors with while loopAhh , you fell for that one , it is a myth that replacing cursors with a while loop is a good idea.
You need to replace the whole loop with set based logic.
Post the whole code and i'll take a look.
He's right Loops are slow
It's better if you can do it one query
not a loop
The database structure should allow it
January 13, 2011 at 4:53 am
[Code]
declare @tablevar table ( tvid int, clause varchar(100), clauserank tinyint, val int )
insert into @tablevar select * from vallookup
select top 1 @tivid=tvid, @clause=clause, @clauserank=clauserank, @val=val from @tablevar
while ( @@rowcount <> 0 )
begin
IF @clause IS NOT NULL AND LTRIM(@clause) <> ''
BEGIN
SET @clauseSID = NULL;
SET @clauseSQL = N'SELECT @@clauseSID=id from tb2 where ' + @clause
EXECUTE dbo.sp_executesql @clauseSQL, N'@clauseSID bigint OUTPUT', @clauseSID OUTPUT
IF @clauseSID = @inputid
BREAK
END
ELSE BREAK
delete from @tablevar where tvid = @tvid
select top 1 @tivid=tvid, @clause=clause, @clauserank=clauserank, @val=val from @tablevar
end
[/code]
The thing is, for each row, get the clause and check whether it satisfies. If yes, break out of the loop else continue. 🙁
January 13, 2011 at 5:04 am
Yuck, that is a horrible idea.
You should be able to join the conditions logically or'ing them by this technique
http://qa.sqlservercentral.com/articles/comma+separated+list/71700/
Then execute the whole statement as a whole with 'top 1 '
January 13, 2011 at 5:11 am
But here the clauses are ordered by clause rank. If the highest rank does not satisfy only, we will move to the next clause.
Also, I should know the row which clause is satisfied.
If this is resolved.. it will really solve a lot of issues for me.
January 13, 2011 at 5:13 am
krishnaroopa (1/13/2011)
Recently we replaced cursors with while loop and table variables.
You didn't really. Looping over a tableset is a cursor in disguise.
Jamie Thomson
http://sqlblog.com/blogs/jamie_thomson
January 13, 2011 at 5:18 am
The issue seems to me to be not the performance of the looping per-se , but the execution speed of the dynamic sql that is being built.
Would that be a fair assumption ?
In that case you need to ensure that all possible condition are supported by indexes etc...
You could do the ranking by using the condition in a case statement.
This is all sounding like a very very bad idea though...
January 13, 2011 at 7:08 am
Dave Ballantyne (1/13/2011)
krishnaroopa (1/13/2011)
Recently we replaced cursors with while loopAhh , you fell for that one , it is a myth that replacing cursors with a while loop is a good idea.
You need to replace the whole loop with set based logic.
Post the whole code and i'll take a look.
Basically, if the code utilizes the keyword "WHILE", then it is a form of a cursor that should be updated. They are both loop-based, Row-By-Agonizing-Row (RBAR) processing that should be avoided.
Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes
January 13, 2011 at 7:10 am
Dave Ballantyne (1/13/2011)
Yuck, that is a horrible idea.You should be able to join the conditions logically or'ing them by this technique
http://qa.sqlservercentral.com/articles/comma+separated+list/71700/
Then execute the whole statement as a whole with 'top 1 '
Thanks for the plug Dave! 😀
Wayne
Microsoft Certified Master: SQL Server 2008
Author - SQL Server T-SQL Recipes
January 14, 2011 at 10:01 am
i feel your pain, i also once worked on a place that had stuff like this.
apart from changing how the system works to get away from that loop, the best you can do is optimize each clause with good indexing
most times changing the system to get away from that situation is the best solution but not the chosen one
--
Thiago Dantas
@DantHimself
January 14, 2011 at 1:12 pm
As my name suggests i dont totally agree. It depends where you work at and who or what team will be supporting it. On occasion these guys tend to provide solutions that do indeed work and are stellar as far as performance however if the guy or team who is supporting it doesnt understand it because it is too complex then it does not good. There is a balance there. Performance and readability and it's relative to the people u work w/ skill set. If their converted front end developers then i find it best to use loop functionality because they can more readily understand it. If your writting the code for banking software or a site who does 20 billion in online sales yearly then you need to trash the loop and go w the most efficient way and be damned w the readability. Dont be that guy who over complexifies a simle problem. It's a balance that needs to be met.
I'm just sayin....
January 14, 2011 at 1:15 pm
sorry. i do agree w the indexing just not the loop haters out there
January 14, 2011 at 1:26 pm
@BaldingLoopMan: Loop code has its place, but when the OP specifically states things are taking too long, it's time for that method to move on. In general: if they're switching systems, they're switching languages. It's part of the reason that coders make bad DBAs and DBAs make bad coders. It's a completely different way of thinking to do either well. Alright, I'll get off my soapbox.
krishnaroopa (1/13/2011)
But here the clauses are ordered by clause rank. If the highest rank does not satisfy only, we will move to the next clause.Also, I should know the row which clause is satisfied.
If this is resolved.. it will really solve a lot of issues for me.
Krishna: Your cursor/loop code above does not have an order by in it, thus, there is no ordering by clause rank. You're grabbing them in whatever order the system presents them to you. I'll shoot one item in the foot before it's stated: The clustered index does not guarantee the order. You need an ORDER BY clause if you want them in a specific ordering.
I assume @inputID is passed from the CREATE PROC parameters? I didn't see it defined anywhere in your code, though you're checking for it. You're also not grabbing a single row, if you are checking for a single ID occurence against your rules from valLookup.
You might change your SQL to SET @clauseSID = CASE WHEN EXISTS( SELECT id FROM tb2 WHERE ' + @clause + ' AND id = ' + CAST( @inputID AS VARCHAR(10)) + ' THEN 1 ELSE 0 END'
That will restrict it to just the ID in question being tested and should get you some further speed.
Without seeing the rules hiding under @clause and the proc in its entirety (I assume tb2 is NOT the actual table name there...), we won't be able to assist you with a rebuild.
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January 14, 2011 at 1:37 pm
BaldingLoopMan (1/14/2011)
sorry. i do agree w the indexing just not the loop haters out there
Not to worry... I only hate slow loops... 😉
--Jeff Moden
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