Dear Sasini,

Thanks for your interest in our work.

The implementation of "COUNT(miles)" does read the values of miles. If 
you want to avoid this use "COUNT(*)".

In terms of execution time,  I would expect the three versions to go 
from fast to slow as follows:
(1) "SELECT COUNT(*) WHERE gas>1500"
(2) "SELECT miles WHERE gas>1500"
(3) "SELECT COUNT(miles) WHERE gas>1500"

Here is a caveat, if the variable miles contains NULL values, 
COUNT(miles) will skip those NULL values, while "COUNT(*)" would have 
no way of knowing about the NULL values and couldn't skip them.

John
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