SQL Server 2008: Why table scaning when another logical condition is satisfied first?

Question

Consider following piece of code:

declare @var bit = 0

select * from tableA as A
where
1=
(case when @var = 0 then 1
      when exists(select null from tableB as B where A.id=B.id) 
      then 1
      else 0
end)

Since variable @var is set to 0, then the result of evaluating searched case operator is 1. In the documentation of case it is written that it is evaluated until first WHEN is TRUE. But when I look at execution plan, I see that tableB is scanned as well.

Does anybody know why this happens? Probably there are ways how one can avoid second table scan when another logical condition is evaluated to TRUE?

Answer 1

Because the plan that is compiled and cached needs to work for all possible values of @var

You would need to use something like

if (@var = 0)
select * from tableA 
else
select * from tableA as A
where exists(select * from tableB as B where A.id=B.id) 

Even OPTION RECOMPILE doesn't look like it would help actually. It still doesn't give you the plan you would have got with a literal 0=0

declare @var bit = 0

select * from 
master.dbo.spt_values  as A
where
1=
(case when 0 = @var then 1
      when exists(select null from master.dbo.spt_values as B where A.number=B.number) 
      then 1
      else 0
end)
option(recompile)

select * from 
master.dbo.spt_values  as A
where
1=
(case when 0 = 0 then 1
      when exists(select null from master.dbo.spt_values as B where A.number=B.number) 
      then 1
      else 0
end)

RE: Question in comments. Try the following with the "Include Actual Execution Plan" option enabled.

declare @var bit = datepart(second,GETDATE())%2

print @var

if (@var = 0)
select * from 
master.dbo.spt_values  --8BA71BA5-3025-4967-A0C8-38B9FBEF8BAD
else
select * from 
master.dbo.spt_values  as A --8BA71BA5-3025-4967-A0C8-38B9FBEF8BAD
where exists(select null from master.dbo.spt_values as B where A.number=B.number) 

Then try

SELECT usecounts, cacheobjtype, objtype, text, query_plan
FROM sys.dm_exec_cached_plans 
CROSS APPLY sys.dm_exec_sql_text(plan_handle) 
CROSS APPLY sys.dm_exec_query_plan(plan_handle) 
where text like '%8BA71BA5-3025-4967-A0C8-38B9FBEF8BAD%'

The Compiled Plan will look like

The Actual Execution Plan will show only one path was actually executed though.

Answer 2

if tableB has few rows, a table scan is the fastest way to go.

best source for dynamic search conditions:

Dynamic Search Conditions in T-SQL by Erland Sommarskog

there are a lot of subtle implications on how you do this as to if an index can be used or not. If you are on the proper release of SQL Server 2008 you can just add OPTION (RECOMPILE) to the query and the local variable's value at run time is used for the optimizations.

Consider this, OPTION (RECOMPILE) will take this code (where no index can be used with this mess of ORs):

WHERE
    (@search1 IS NULL or Column1=@Search1)
    AND (@search2 IS NULL or Column2=@Search2)
    AND (@search3 IS NULL or Column3=@Search3)

and optimize it at run time to be (provided that only @Search2 was passed in with a value):

WHERE
    Column2=@Search2

and an index can be used (if you have one defined on Column2)