Additional semantic information that describes data sets is inferred in response to a request for data from the data sets, e.g., in response to a query over the data sets, including analyzing a subset of results extracted based on the request for data to determine the additional semantic information. The additional semantic information can be verified by the publisher as correct, or satisfy correctness probabilistically. Mapping information based on the additional semantic information can be maintained and updated as the system learns additional semantic information (e.g., information about what a given column represents and data types represented), and the form of future data requests (e.g., URL based queries) can be updated to more closely correspond to the updated additional semantic information.