What is 'User LookUp' and how it affects performance.

‘User Lookup’ means query is looking for more data than it is present in the NON CLUSTERED index. And this index(NON CLUSTERED) uses clustered index as look ups.Now whether it is good or bad we cannot come up with sold answers as we usually don’t know the other users systems. To avoid we can add the columns to NON CLUSTERED index which is being used.

Bu the good part is we can reduce the ‘User Lookup’, bad part is NON CLUSTERED index will be heavy

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