Dynamic Filter and Order By

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Dynamic Filter and Order By

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L' Eomot Inversé

Posted Thursday, October 04, 2012 2:37 AM

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Sean Lange (10/3/2012)

... Isn't LIKE as good as >= for index seeks?!?

NO LIKE is not SARGable so you will get scans. It has to examine every row to determine if it is a match or not.

Actually LIKE is sargable when the pattern expression reduces to matching an initial substring, provided of course the pattern expression is unicode when the column is unicode and not unicode when the column is not unicode; so it doesn't prevent index scans.
(I remember thisone because it's half of a weird inconsistency: using LIKE to match an initial substring is sargable, while using LEFT is not Wink

)
edit:spelling

Tom
Que conclure à la fin de tous mes longs propos? C'est que les préjugés sont la raison des sots. (Voltaire, 1756)

Post #1368232

Eugene Elutin

Posted Thursday, October 04, 2012 3:49 AM

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PiMané (10/3/2012)

Sean Lange (10/3/2012)

NO LIKE is not SARGable so you will get scans. It has to examine every row to determine if it is a match or not.

So a LIKE 'M%' is better replaced with a >= 'M' AND < 'N'..

No, that is wrong! LIKE 'M%' is SARGable, you don't need to replace it with such a mess

However, LIKE '%M' is not SARGable!. But replacing it with comparison operators will be quite problematic BigGrin

.
If you really need the best possible performance for text searches like the above, there is SQL Server feature called Full Text Search. http://msdn.microsoft.com/en-us/library/ms142571.aspx
It's designed for performing effective comprehensive text searches.

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Post #1368279

Sean Lange

Posted Thursday, October 04, 2012 7:45 AM

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L' Eomot Inversé (10/4/2012)

Sean Lange (10/3/2012)

... Isn't LIKE as good as >= for index seeks?!?

NO LIKE is not SARGable so you will get scans. It has to examine every row to determine if it is a match or not.

)
edit:spelling

Thanks for the clarification Tom.

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Post #1368404

PiMané

Posted Thursday, October 04, 2012 8:09 AM

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Hi,

Regarding Dynamic SQL....
Instead of a SELECT suppose I have an UPDATE used on a SP.
The SP receives 6 parameters: @RecordId, @Col1Value, @Col2Update, @Col2Value, @Col3Update, @Col3Value...
Col1 is always updated but Col2 and Col3 are only updated if Col2Update and Col3Update are true (1).
Should dynamic SQL be used here as well?
I could write

UPDATE table SET 
    Col1 = @Col1Value, 
    Col2 = CASE WHEN @Col2Update = 1 THEN @Col2Value ELSE Col2 END,
    Col3 = CASE WHEN @Col3Update = 1 THEN @Col3Value ELSE Col3 END
WHERE Id = @RecordId

but this would make Col2 and Col3 always be updated no matter what, even if the value is themselves...
also it's possible to write:

UPDATE table SET Col1 = @Col1Value WHERE Id = @RecordId
IF @Col2Update = 1
  UPDATE table SET Col2 = @Col2Value WHERE Id = @RecordId
....

This would make another seek and update row and if the table had an UPDATE trigger it would fire the trigger again...
Back to the 1st case if there was also a trigger with the condition IF UPDATED(Col2) the condition would always be true...
There also the long solution:

IF @Col2Update = 1 AND @Col3Update = 1
 ....
ELSE
   IF @Col2Update = 1
   ELSE
 .....
   IF @Col3Update = 1
.....
   ELSE
...

This is long and if necessary to add another column it would even longer... 2^[optional parameters]..

So is dynamic SQL a good option for this case too?!

Thanks,
Pedro

If you need to work better, try working less...

Post #1368438

L' Eomot Inversé

Posted Thursday, October 04, 2012 9:55 AM

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PiMané (10/4/2012)

Regarding Dynamic SQL....
Instead of a SELECT suppose I have an UPDATE used on a SP.
The SP receives 6 parameters: @RecordId, @Col1Value, @Col2Update, @Col2Value, @Col3Update, @Col3Value...
Col1 is always updated but Col2 and Col3 are only updated if Col2Update and Col3Update are true ....
....
So is dynamic SQL a good option for this case too?!

The trigger is going to be fired anyway, teh data engine is going to be invoked anyway, the update is going to be logged anyway, because col1 is alwys updated. So having the 2 case expressions in the update statement to generate updates that either do or don't update col2 and col3 doesn't generate any noticeable overhead, may even be cheaper in performance terms than building dynamic SQL.

Of course if you have an IF UPDATED(col2) condition in the trigger that wants to know whether col2 was actually changed you have to write it differently, but that is trivial for your update statement (and supertrivial if id is a unique key).

Tom
Que conclure à la fin de tous mes longs propos? C'est que les préjugés sont la raison des sots. (Voltaire, 1756)

Post #1368532

PiMané

Posted Thursday, October 04, 2012 10:16 AM

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Thanks,
Pedro

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Post #1368549

Jeff Moden

Posted Thursday, October 04, 2012 12:36 PM

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Sean Lange (10/2/2012)

It will work but referring to columns by ordinal position is fraught with maintenance issues.

Not to mention that (IIRC) it's been deprecated.

--Jeff Moden

"RBAR is pronounced "ree-bar" and is a "Modenism" for "Row-By-Agonizing-Row".

First step towards the paradigm shift of writing Set Based code:
Stop thinking about what you want to do to a row... think, instead, of what you want to do to a column."

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Post #1368633

Sean Lange

Posted Thursday, October 04, 2012 1:03 PM

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Jeff Moden (10/4/2012)

Sean Lange (10/2/2012)

It will work but referring to columns by ordinal position is fraught with maintenance issues.

Not to mention that (IIRC) it's been deprecated.

And something I have avoided like the plague long before it was deprecated.

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Read the article at http://www.sqlservercentral.com/articles/Best+Practices/61537/ for best practices on asking questions.

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Cross Tabs and Pivots, Part 1 – Converting Rows to Columns
Cross Tabs and Pivots, Part 2 - Dynamic Cross Tabs

Post #1368654