>>Strange. I don't see a difference between these two queries except for explicit amount in the second case. So, could it be that it is implicit conversion of the parameter?
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>>How Amount field is defined in the table and how is @Amount defined? Is it a direct query or an SP? Did you try to measure performance difference between query and SP? Also you can move Amount condition into the JOIN but I doubt it will change anything.
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>Amount was used as an example. In the real code, it is an integer. It is a direct query. As you may have read in this thread, I have used the sub query approach. I will see how it goes.

What is the exact problem here - the inclusion of the second table field with parameter? I don't understand how would subquery help you. SQL Query optimizer should be smart enough by itself.