COUNT(*) Versus COUNT(1)

Lukas Eder takes on the myth that COUNT(*) differs from COUNT(1):

Now that we know the theory behind these COUNT expressions, what’s the difference between COUNT(*) and COUNT(1). There is none, effectively. The 1 expression in COUNT(1) evaluates a constant expression for each row in the group, and it can be proven that this constant expression will never evaluate to NULL, so effectively, we’re running COUNT(*), counting ALL the rows in the group again.

There should be no difference, and parsers / optimisers should be able to recognise this and not do the extra work of checking every expression evaluation for NULL-ness.

Interestingly, in one of the four major RDBMS platforms (not including DB2), there is a performance difference of about 10%.