A user interface that includes a data flow pane and a profile pane. The data flow pane displays a flow diagram that identifies a data source. For each of multiple queries against the data source, the process issues the query against the data source asynchronously with an initial block size. Upon retrieval of the initial set of rows, the process repeats the query asynchronously with an updated block size until all of the rows have been retrieved. Periodically the process determines a high water mark for rows from the data source that have been retrieved for all of the queries. When the water mark changes, the process updates the profile pane to display data value histograms for multiple data fields in the data source. Each bar in each data value histogram counts the rows below the water mark that have a single specific data value or range of data values.