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Message Posted: Mon, 19 Jan 2004 @ 15:20:52 GMT
Mathematically these are two quite different things. Let me explain the difference using the simplest example.
Table T had two columns, A and B, each of which can only take values 0 and 1. T has 100 rows, and 0 and 1 are distributed evenly in A and B, i.e., there exactly 50 0 and 50 1 in both A and B.
Now, take all possible values of the pair (A,B):
I can have quite different tables of this kind:
T1 has only (0,0) and (1,1), they occur 50 times each;
T2 has all 4 combinations (0,0), (0,1), (1,0), (1,1), 25 times each.
Obviously, T1 and T2 are quite different tables but their one-dimensional projections are the same (50 0s and 50 1s in 100 rows). When you collect multi-column statistics you should get more accurate (multidimensional) information (at least let's hope so!). Single-column statistics describe projections only and they might be missing something important.
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