Query optimization using of a query that is compiled into a query tree. The optimization is efficiently performed by using a distinct value estimation data structure (e.g., a KMV synopsis) that represents within an interval distinctness of values that are generated based on data within an interval, even if the resultant data from a subinterval is considered. Various candidate query trees are evaluated, with distinct value generation data structures being propagated for parent nodes based on the distinct value generation data structures of its child node(s). Propagation operations correlate to the operation represented by the parent node in the query tree. The optimizer uses the propagated distinct value estimation structure in order to evaluate the number of distinct values of data that would result from execution of the candidate query tree at least at the corresponding operations (and not just based on the distinct values of the input data).