Clustered Index Cost is high using Sql Query - tsql

Question

I have given a task to optimize the below sql query. It is currently performing badly. I ran the execution plan for it, and i see it is doing Clustered Index Scan, and it's cost is very high. How can i reduce the cost or it is possible to change it to use Index seek instead?
SELECT TOP 20 CustomerPrimaryExtID,
Max(POSTimeStamp) AS TransactionDate,
ExtLocationCode,
0 AS RedemptionAmount,
0 AS RedemptionCount,
TerminalNum,
LogixTransNum,
POSTransNum AS TransNum,
0 AS DetailRecords,
CustomerTypeID,
PresentedCustomerID,
PresentedCardTypeID,
HHID,
Replayed,
0 AS TransContext,
isnull(TransTotal, 0) AS TransTotal
FROM TransHist AS TH WITH(nolock)
WHERE ( ( ( CustomerPrimaryExtID IN ( '' )
AND HHID IS NULL )
OR HHID = '0000000250000013408'
AND CustomerTypeID <> 1 )
OR ( CustomerPrimaryExtID = '0000000250000013408'
AND CustomerTypeID = 1 ) )
AND NOT EXISTS (SELECT LogixTransNum
FROM TransRedemption AS TR2 with(nolock)
WHERE ( ( ( CustomerPrimaryExtID IN ( '' )
AND HHID IS NULL )
OR HHID = '0000000250000013408'
AND CustomerTypeID <> 1 )
OR ( CustomerPrimaryExtID = '0000000250000013408'
AND CustomerTypeID = 1 ) )
AND TH.LogixTransNum = TR2.LogixTransNum)
GROUP BY CustomerPrimaryExtID,
HHID,
CustomerTypeID,
PresentedCustomerID,
PresentedCardTypeID,
LogixTransNum,
POSTransNum,
TerminalNum,
ExtLocationCode,
Replayed,
TransTotal
ORDER BY TransactionDate DESC

Answer 1

One thing to note: CustomerTypeID <> 1 will not use indexes as effectively as CustomerTypeID > 1, see the below test example. Also, WHERE NOT EXISTS or NOT IN are also performance killers.
CREATE TABLE NumberCrazy(
IntegersAreCool INT
)
-- Run this 20+ times
INSERT INTO NumberCrazy
VALUES (1)
-- Run this twice
DECLARE #begin INT = 1, #max INT = 2000, #r INT
WHILE #begin <= #max
BEGIN
SET #r = (RAND()*(#begin))
INSERT INTO NumberCrazy
SELECT #r
SET #begin = #begin + 1
END
CREATE CLUSTERED INDEX [IX_DaInts] ON [dbo].[NumberCrazy]
(
[IntegersAreCool] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
GO
SET STATISTICS IO ON
SELECT *
FROM NumberCrazy
WHERE IntegersAreCool > 1
SELECT *
FROM NumberCrazy
WHERE IntegersAreCool <> 1
Results:
Table 'NumberCrazy'. Scan count 1, logical reads 12, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'NumberCrazy'. Scan count 2, logical reads 15, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
NOTE THE EXECUTION PLAN (Looks like the mess you have above this).