8. A method for generating an execution plan for a query in a relational database system, comprising: generating one or more initial logical representations of the query, each initial logical representation including a plurality of operators; performing a bounded exploration process comprising: (a) adding the operators of the one or more initial logical representations to a data structure; (b) identifying zero or more patterns of operators within the data structure that are eligible for the application of a transformation rule, a pattern of operators being eligible for the application of a transformation rule if (i) the pattern of operators matches a pattern associated with the transformation rule and (ii) a search distance associated with one or more of the operators in the pattern of operators, and that is a numerical value, is less than a maximum allowable search distance that is a numerical value corresponding to an upper bound of the search distance associated with the one or more of the operators in the pattern of operators to enable bounding of the exploration process; (c) applying a transformation rule to each eligible pattern of operators identified in step (b) to generate a logically-equivalent pattern of operators within the data structure, thereby generating one or more additional logical representations of the query within the data structure; (d) assigning a search distance to each operator within each logically-equivalent pattern of operators generated during step (c), the assigned search distance being a function of a search distance of one or more operators within the eligible pattern of operators to which the logically-equivalent pattern of operators corresponds; and (e) repeatedly performing steps (b), (c) and (d) until no further patterns of operators that are eligible for the application of a transformation rule can be identified within the data structure; obtaining one or more physical implementation alternatives for each of the logical representations of the query included in the data structure; and selecting one of the physical implementation alternatives as the execution plan.
8. A method for generating an execution plan for a query in a relational database system, comprising: generating one or more initial logical representations of the query, each initial logical representation including a plurality of operators; performing a bounded exploration process comprising: (a) adding the operators of the one or more initial logical representations to a data structure; (b) identifying zero or more patterns of operators within the data structure that are eligible for the application of a transformation rule, a pattern of operators being eligible for the application of a transformation rule if (i) the pattern of operators matches a pattern associated with the transformation rule and (ii) a search distance associated with one or more of the operators in the pattern of operators, and that is a numerical value, is less than a maximum allowable search distance that is a numerical value corresponding to an upper bound of the search distance associated with the one or more of the operators in the pattern of operators to enable bounding of the exploration process; (c) applying a transformation rule to each eligible pattern of operators identified in step (b) to generate a logically-equivalent pattern of operators within the data structure, thereby generating one or more additional logical representations of the query within the data structure; (d) assigning a search distance to each operator within each logically-equivalent pattern of operators generated during step (c), the assigned search distance being a function of a search distance of one or more operators within the eligible pattern of operators to which the logically-equivalent pattern of operators corresponds; and (e) repeatedly performing steps (b), (c) and (d) until no further patterns of operators that are eligible for the application of a transformation rule can be identified within the data structure; obtaining one or more physical implementation alternatives for each of the logical representations of the query included in the data structure; and selecting one of the physical implementation alternatives as the execution plan. 13. The method of claim 8 , wherein selecting one of the physical implementation alternatives as the execution plan comprises: calculating a cost associated with each physical implementation alternative obtained for each of the logical representations of the query included in the data structure; and selecting a physical implementation alternative having the lowest cost associated therewith as the execution plan.