private ValueExtractor buildDateValueExtractor(final String columnName, final int fieldIndex) {
        return (rs, arr, cache) ->
                arr[fieldIndex] = getResultSetDateMs(rs, columnName);
    }
Build a mapper function that will only extract the columns of interest from the resultSet row.
Assumes something external to the returned function will advance the resultSet
stroom-statistics/stroom-statistics-impl-sql/src/main/java/stroom/statistics/impl/sql/search/StatisticsSearchServiceImpl.java