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combinations of columns), the fraction of
NULL
values in a column, and spe-
cialized statistics for nonnumeric data values.
2.2.2 Cardinality Estimation
Histograms provide a compressed representation of the underlying column
distribution. When manipulating such compact data structures, we generally
make a number of simplifying assumptions. For instance, we assume that the
tuples represented in a bucket are distributed uniformly in the bucket domain.
11
10
9
7
6
5
5
44
4
4
3
33
2
2
1
1
123456789
(a) Data distribution.
10
11
12
13
14
15
16
17
18
24
17
15
14
9
5
0123456789
(b)
Equi-width
histogram.
10 11 12 13 14 15 16 17 18 19
15
15
14
14
13
13
012345678910
(c)
Equi-depth
histogram.
11 12 13 14 15 16 17 18 19
FIGURE 2.5
Histograms approximate the value distribution of columns in
a database.
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