1. A method comprising: estimating a number of rows that would be processed by each of one or more execution plans for a query based at least in part on a particular density model that approximates a weighted combination of at least a first density model for a first portion of a set of data and a second density model for a second portion of the set of data; based at least in part on the number of rows estimated for said each of one or more execution plans for the query, selecting an execution plan for the query; wherein the method is performed by one or more computing devices.