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Two possible execution plans are deep-left tree (shown in Figure-1 ) and Bushy Tree (shown in Figure-2). Other possible structures include right-deep trees zig-zag trees. Similar to deep-left trees right-deep and zig-zag are linear tree structure. But bushy trees are different and more complicated compared to others.   \subsection{Challenegs in Bushy Tree Implementation}  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 tables involved. Therefor the number of possible permutations are N! . But for bushy trees possible number of structures are is  given by\\  $$ S(N) = 1 if $N \ = \ 1$ $$  \\   $$ $S(N) = \displaystyle \sum_{N=1}^{i} (N)(N-i)$ \ $ if for  N > 1$ $$\\ 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 (~7). Therefore it is required to come with a heuristic to select set of permutations for cost computations.  \subsection{Heuristics to Select Bushy Trees}