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Using COALESCE forces SQL to timeout

Medium Priority
913 Views
Last Modified: 2012-06-22
When I run

select * from company where comp_companyid = 2619

the query comes back immediately.  However, when I run

select * from company where coalesce(comp_companyid,0) = 2619

then SQL times out.   Why would it be doing this?
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Guy Hengel [angelIII / a3]Billing Engineer
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Most Valuable Expert 2014
Top Expert 2009

Commented:
it's possible, because the coalesce() function on the field makes that the index on the field cannot be used.
AneeshDatabase Consultant
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Top Expert 2009

Commented:
when u use a coalesce, sql has to perform another check operation to see whether companyid is null, if so set it as 0 ,

select * from company where comp_companyid = 2619
UNION ALL
select * from company where comp_companyid is null
Guy Hengel [angelIII / a3]Billing Engineer
CERTIFIED EXPERT
Most Valuable Expert 2014
Top Expert 2009

Commented:
ie, I presume you run the query test-wise in your management studio, query window.
=> use the Query Menu => include actual execution plan

and rerun both queries.
you will get a nice graph of the 2 queries execution plans.
one with index lookup
the second one with full table (or full clustered index) scan

Author

Commented:
Also, I just noticed I'm getting the following error:

"SQL Execution Error"
Error Source: .NET SQLClient Data Provider
Error Message:  Lock request time out period exceeded.

I'm perplexed.  I have no problems querying any of the other tables in the database.
Guy Hengel [angelIII / a3]Billing Engineer
CERTIFIED EXPERT
Most Valuable Expert 2014
Top Expert 2009

Commented:
anyhow, note that:


select * from company where coalesce(comp_companyid,0) = 2619

will not produce any results different from

select * from company where comp_companyid = 2619

unless you wanted to find the null values:

select * from company where comp_companyid IS NULL


Author

Commented:
I do realize that; however, I'm troubleshooting an application that builds that query out and I don't have any control over the query...

Also, I just noticed that I can't even rebuild the indexes because of that error...Can individual tables be locked?   How do I release those locks if so?  
Commented:
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