An cost based optimizer optimizes access to at least a portion of hierarchically-organized documents, such as those formatted using eXtensible Markup Language (XML), by estimating a number of results produced by the access of the hierarchically-organized documents. Estimating the number of results comprises computing the cardinality of each operator executing query language expressions and further computing a sequence size of sequences of hierarchically-organized nodes produced by the query language expressions. Access to the hierarchically-organized documents is optimized using the structure of the query expression and/or path statistics involving the hierarchically-organized data. The cardinality and the sequence size are used to calculate a cost estimation for execution of alternate query execution plans. Based on the cost estimation, an optimal query execution plan is selected from among the alternate query execution plans.