WHERE in Sql, combining two fast conditions multiplies costs many times - sql

Question

I have a fairly complex sql that returns 2158 rows' id from a table with ~14M rows. I'm using CTEs for simplification.
The WHERE consists of two conditions. If i comment out one of them, the other runs in ~2 second. If i leave them both (separated by OR) the query runs ~100 seconds. The first condition alone needs 1-2 seconds and returns 19 rows, the second condition alone needs 0 seconds and returns 2139 rows.
What can be the reason?
This is the complete SQL:
WITH fpcRepairs AS
(
SELECT FPC_Row = ROW_NUMBER()OVER(PARTITION BY t.SSN_Number ORDER BY t.Received_Date, t.Claim_Creation_Date, t.Repair_Completion_Date, t.Claim_Submitted_Date)
, idData, Repair_Completion_Date, Received_Date, Work_Order, SSN_number, fiMaxActionCode, idModel,ModelName
, SP=(SELECT TOP 1 Reused_Indicator FROM tabDataDetail td INNER JOIN tabSparePart sp ON td.fiSparePart=sp.idSparePart
WHERE td.fiData=t.idData
AND (td.Material_Quantity <> 0)
AND (sp.SparePartName = '1254-3751'))
FROM tabData AS t INNER JOIN
modModel AS m ON t.fiModel = m.idModel
WHERE (m.ModelName = 'LT26i')
AND EXISTS(
SELECT NULL
FROM tabDataDetail AS td
INNER JOIN tabSparePart AS sp ON td.fiSparePart = sp.idSparePart
WHERE (td.fiData = t.idData)
AND (td.Material_Quantity <> 0)
AND (sp.SparePartName = '1254-3751')
)
), needToChange AS
(
SELECT idData FROM tabData AS t INNER JOIN
modModel AS m ON t.fiModel = m.idModel
WHERE (m.ModelName = 'LT26i')
AND EXISTS(
SELECT NULL
FROM tabDataDetail AS td
INNER JOIN tabSparePart AS sp ON td.fiSparePart = sp.idSparePart
WHERE (td.fiData = t.idData)
AND (td.Material_Quantity <> 0)
AND (sp.SparePartName IN ('1257-2741','1257-2742','1248-2338','1254-7035','1248-2345','1254-7042'))
)
)
SELECT t.idData
FROM tabData AS t INNER JOIN modModel AS m ON t.fiModel = m.idModel
INNER JOIN needToChange ON t.idData = needToChange.idData -- needs to change FpcAssy
LEFT OUTER JOIN fpcRepairs rep ON t.idData = rep.idData
WHERE
rep.idData IS NOT NULL -- FpcAssy replaced, check if reused was claimed correctly
AND rep.FPC_Row > 1 -- other FpcAssy repair before
AND (
SELECT SP FROM fpcRepairs lastRep
WHERE lastRep.SSN_Number = rep.SSN_Number
AND lastRep.FPC_Row = rep.FPC_Row - 1
) = rep.SP -- same SP, must be rejected(reused+reused or new+new)
OR
rep.idData IS NOT NULL -- FpcAssy replaced, check if reused was claimed correctly
AND rep.FPC_Row = 1 -- no other FpcAssy repair before
AND rep.SP = 0 -- not reused, must be rejected
order by t.idData
Here's the execution plan:
Download: http://www.filedropper.com/exeplanfpc

Answer 1

Try to use UNION ALL of 2 queries separately instead of OR condition.
I've tried it many times and it really helped. I've read about this issue in Art Of SQL .
Read it, you can find many useful information about performance issues.
UPDATE:
Check related questions
UNION ALL vs OR condition in sql server query
http://www.sql-server-performance.com/2011/union-or-sql-server-queries/
Can UNION ALL be faster than JOINs or do my JOINs just suck?
Check Wes's answer
The usage of the OR is probably causing the query optimizer to no longer use an index in the second query.