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I have a query as below:

SELECT * FROM Members (NOLOCK) 
 WHERE Phone= dbo.FormatPhone(@Phone)

Now here I understand that formatting has to be applied on the variable on column. But should I apply it on variable to assign to some other local variable then use it (as below).

Set @SomeVar = dbo.FormatPhone(@Phone) 

SELECT * 
  FROM Members (NOLOCK) WHERE Phone= @SomeVar

Which way is better or both are good?

EDIT: And how is first query different from 

SELECT * FROM Members (NOLOCK) 
 WHERE dbo.FormatPhone(Phone) = @Phone
noob.spt
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4 Answers4

8

As usual with SQL, the query is largely irelevant without knowing the actual schema is used against.

Do you have an index on Members.Phone? If no, then it makes no difference how you write the query, they all gonna scan the whole table and performe the same (ie. perform badly). If you do have an index then the way you write the query makes all the difference:

SELECT * FROM Members WHERE Phone= @Phone;
SELECT * FROM Members WHERE Phone= dbo.FormatPhone(@Phone);
SELECT * FROM Members WHERE  dbo.FormatPhone(Phone)=@Phone;

First query is guaranteed optimal, will seek the phone on the index.
Second query depends on the characteristics of the dbo.FormatPhone. It may or may not use an optimal seek.
Last query is guaranteed to be bad. Will scan the table.

Also, I removed the NOLOCK hint, it seem the theme of the day... See syntax for nolock in sql. NOLOCK is always the wrong answer. Use snapshot isolation.

Community
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Remus Rusanu
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5

The second is definitely preferred. The first one will evaluate the function for each row in the table, whilst the other one will do the calculation only once.

Dan Sydner
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  • In addition the first option most likely won't be able to use an index. – Kuberchaun Nov 12 '09 at 18:36
  • The function evaluation for every row will be there if we apply function on table column or variable? Or both ways are same? – noob.spt Nov 12 '09 at 18:38
  • No, if you perform the calculation up front, you are essentially handing the query a constant. – Aaron Bertrand Nov 12 '09 at 18:40
  • You mean these two queries are same? SELECT * FROM Members (NOLOCK) WHERE Phone= dbo.FormatPhone(@Phone) SELECT * FROM Members (NOLOCK) WHERE dbo.FormatPhone(Phone) = @Phone – noob.spt Nov 12 '09 at 18:43
  • No, those two queries are not the same. Doing "WHERE Phone = dbo.FormatPhone(@Phone)" and "WHERE Phone = @FormattedPhone" should be the same though. – Tom H Nov 12 '09 at 18:48
  • Dan - the optimizer should not evaluate the function for each row in the first query - "WHERE Phone = dbo.FormatPhone(@Phone)" – Tom H Nov 12 '09 at 18:50
  • Thanks Tom. I agree with you, but not 100% sure. – noob.spt Nov 12 '09 at 18:51
  • Tom could you post your comment as a solution. Thanks. – noob.spt Nov 12 '09 at 18:54
5

You'll almost certainly get better predictability if you assign to a variable first, lots of dependency in the optimizer around determinism vs. non-determinism.

boydc7
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SELECT * FROM Members (NOLOCK) 
WHERE Phone= dbo.FormatPhone(@Phone)

in the query above, function dbo.FormatPhone will be executed for every row in Members table.

when second query 

Set @SomeVar = dbo.FormatPhone(@Phone) 

SELECT * 
FROM Members (NOLOCK) WHERE Phone= @SomeVar

it'll execute the function only once. So I think second option will be faster in case you have large data in member table.

Koopakiller
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Radhi
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  • Thanks vrunda. I am getting different responses and opinion here, though I don't believe that function will execute for every row in first query. DO you have any references which prove it? – noob.spt Nov 13 '09 at 15:54