[CI] About uncorrelated cost model

[Hideaki Kimura]

I understand the cost model of uncorrelated case is correct, but it 
might give reviewers a wrong impression that we are claiming that B+Tree 
does something stupid in the case and our CT doesn't.

As the CT base cost model shifts to sequential scan if c_per_u is high 
(which is almost accurate in figure 3), I think we don't need a 
different cost model for uncorrelated case.
-- 
Hideaki Kimura <hkimura at cs.brown.edu>