Search query result set count estimation

Search query result set count estimation is described. A system parses data set query that includes first query attribute and second query attribute. The system identifies first hierarchy of connected nodes including a first node representing a first query attribute, and a second hierarchy of other connected nodes including a second node representing a second query attribute. The system identifies a directed arc connecting first correlated node in first hierarchy to second correlated node in second hierarchy. The system identifies cross-hierarchy probabilities of correlations between values of a first attribute represented by the first correlated node and values of a second attribute represented by the second correlated node. The system outputs query result set estimated count generated from cross-hierarchy probabilities, probabilities that values of first attribute are associated with values corresponding to first node, and probabilities that values of second attribute are associated with values corresponding to second node.