>>Well, one option is to specify the index to use on the JOIN condition. However, utilizing optimizer's hints should be the last resort option. In any case, it would not hurt to try.
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>It is possible that in those cases that it will never use the primary key?

Do you mean if you don't specify the index hint? SQL Server Optimizer is a complex product and how exactly does it decide what to use may not be immediately clear. So, I'd say it's possible.