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Bhathiya edited section_Challenegs_in_Bushy_Trees__.tex
about 8 years ago
Commit id: a2179e23804384ad09c8822955facb4530f144e2
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diff --git a/section_Challenegs_in_Bushy_Trees__.tex b/section_Challenegs_in_Bushy_Trees__.tex
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--- a/section_Challenegs_in_Bushy_Trees__.tex
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Moving from left-deep tree to a Bushy tree is a challenge as the number of possible structures in bushy trees are much larger. Left-deep trees have only one structure regardless of the number of attributes involved. Therefor the number of possible permutations are N! . But for bushy trees possible number of structures are given by
$$S(N) = \begin{cases}
1
& \text{if $ N = 1$}; \\
$\displaystyle \sum_{i=1}^{N} \frac{1}{n}$
& \text{if $N \ne 0$};
2
& \text{if $ N = 1$};.\end{cases}$$
Therefore the number of possible permutations are $S(N)*N!$. Unlike left-deep tree case, estimating the cost for all the possible bushy trees is computationally infeasible for moderately large N. Therefore it is required to come with a heuristic to select set of permutations for cost computations.Some heuristics considered are