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\section{Challenegs \section{Query Optimization and Execution Plans}  Query optimization is one of the most important role in relational databases. Almost all the relational database systems such as MySQL, Postgres, Oracle DB, Apache Derby consist of query optimizers.   \subsubsection{Join Query Execution Plans}  Consider query   SELECT * FROM A  JOIN B ON A.ID = B.ID \\  JOIN C ON A.ID = C.ID \\  JOIN D ON A.ID = D.ID \\    Possible execution plans are shown in Figure-1 and Figure-2      \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   $$ S(N) = 1 if $N = 1 $ \\