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Bhathiya edited section_Challenegs_in_Bushy_Trees__.tex
about 8 years ago
Commit id: a0b19e52c7c05caaf85994962a4358982f1f75d2
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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
$$S(N) = \begin{cases}
$1$ 1 & \text{if $N = 1$}; \\
$\sum_{i=1}^{N}$ \sum_{i=1}^{N}$ & \text{if $ N \ne 0$}; \\ $$
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