Nested Loop Vs. Hash Match?

I have two queries that seem to be the same, but perform very differently. The first query runs very fast (7000+ records returned in <1 sec.). The execution plan shows that it uses a nested loop with index seeks on both tables.

select *
from t_loadbasic

where ld_nbr in (select ld_nbr from t_tripcombined where comp_date between '11/1/07' and '11/05/07')



The second query is almost the same, save the fact that it uses date variables instead of hard dates. The execution plan shows that it uses a hash match instead of a nested loop with an index scan on the main table (t_loadbasic). This query takes about 12 seconds to run.


declare @startdate datetime


,@enddate datetime


set @startdate = '11/1/07'

set @enddate = '11/5/07'


select *
from t_loadbasic

where ld_nbr in (select ld_nbr from t_tripcombined where comp_date between @startdate and @enddate)

I'm trying to figure out why the database executes these two statements so differently. BTW, I've tried switching the order of the tables. I've tried using joins instead of a subquery. The execution plan seems completely dependant on the use of variables. I can attach the execution plans if necessary.

I apologize if this is too simple a question, but I couldn't find an answer on any forums, web searches or BOL. Thanks in adavance.