Joining on varchar(50) foreign key slows query - database

Question

I have this query which is pretty long, but adding a where clause to it, or joining on a string makes it take an extra 2 seconds to run. I can't figure out why.
Here's the query in full:
ALTER PROCEDURE [dbo].[RespondersByPracticeID]
#practiceID int = null,
#activeOnly bit = 1
AS
BEGIN
SET NOCOUNT ON;
select
isnull(sum(isResponder),0) as [Responders]
,isnull(count(*) - sum(isResponder),0) as [NonResponders]
,isnull((select
count(p.patientID)
from patient p
inner join practice on practice.practiceid = p.practiceid
inner join [lookup] l on p.dosing = l.lookupid and l.lookupid = 'da_ncd'
where
p.practiceID = isnull(#practiceID, p.practiceID)
and p.active = case #activeOnly when 1 then 1 else p.active end
) - (isnull(sum(isResponder),0) + isnull(count(*) - sum(isResponder),0)),0)
as [Undetermined]
from (
select
v.patientID
,firstVisit.hbLevel as startHb
,maxHbVisit.hblevel as maxHb
, case when (maxHbVisit.hblevel - firstVisit.hbLevel >= 1) then 1 else 0 end as isResponder
,count(v.patientID) as patientCount
from patient p
inner join visit v on v.patientid = v.patientid
inner join practice on practice.practiceid = p.practiceid
inner join [lookup] l on p.dosing = l.lookupid and l.lookupid = 'da_ncd'
inner join (
SELECT
p.PatientID
,v.VisitID
,v.hblevel
,v.VisitDate
FROM Patient p
INNER JOIN Visit v ON p.PatientID = v.PatientID
WHERE
v.VisitDate = (
SELECT MIN(VisitDate)
FROM Visit
WHERE PatientId = p.PatientId
)
) firstVisit on firstVisit.patientID = v.patientID
inner join (
select
p.patientID
,max(v.hbLevel) as hblevel
from Patient p
INNER JOIN Visit v ON p.PatientID = v.PatientID
group by
p.patientID
) MaxHbVisit on maxHbVisit.patientid = v.patientId
where
p.practiceID = isnull(#practiceID, p.practiceID)
and p.active = case #activeOnly when 1 then 1 else p.active end
group by
v.patientID
,firstVisit.hbLevel
,maxHbVisit.hblevel
having
datediff(
d,
dateadd(
day
,-DatePart(
dw
,min(v.visitDate)
) + 1
,min(v.visitDate)
)
, max(v.visitDate)
) >= (7 * 8) -- Eight weeks.
) responders
END
The line that slows it down is:
inner join [lookup] l on p.dosing = l.lookupid and l.lookupid = 'da_ncd'
Also, moving it to the where clause has the same effect:
where p.dosing = 'da_ncd'
Otherwise, the query runs almost instantly. >.<

Answer 1

Ah, sorry I figured it out. Patient.Dosing was set as allow nulls. I guess that made it a different sort of index.

Answer 2

For the record, even though the question is answered.
Usually things like this happen because the execution plan is changed. Compare the plans in query analyzer.

Answer 3

Another gotcha is data types - if p.dosing and l.lookupid differ - nvarchar vs. varchar, for example, can have a huge impact.

Answer 4

Try creating an index on that table, being sure to properly include that VARCHAR field in the list of fields.