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JOIN C ON A.ID = C.ID \\  JOIN D ON A.ID = D.ID \\    Two possible execution plans are deep-left tree (shown in Figure-1 ) and Bushy Tree (shown in Figure -2) 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 attributes involved. Therefor the number of possible permutations are N! . But for bushy trees possible number of structures are given by