[QUOTE][i]Originally posted by ad [/i]
[B]Try count(PK_column), this will make a little boost on the performance compare to count(*).

AD [/B][/QUOTE]
It may boost performance or it may also slow it down considerably. It depends on quite a few factors.

Generaly, if you use cost based optimizer and if you provide it with accurate enough statistics there should be no difference at all because the execution plan will be the same for all cases.