TMUDF: Add an interface that makes it easier to detect partition boundaries

[Hans Zeller]

Right now, a TMUDF needs to implement its own code to detect when one partition (defined by the PARTITION BY clause of a table-valued input) ends and the next one begins. Another problem is that it knows that only after it has read the first row of the next partition or encountered EOD. That makes it hard to use passthrough columns in a reducer, because those passthrough columns are not longer available when we detect the end of the partition.

Ideally, we should add two new methods UDF::getNextPartition() and UDF::getNextRowInPartition() as an alternative to UDF::getNextRow(). These methods should read only rows within the partition and keep the passthrough columns available when we reach the end of the partition. A TMUDF utilizing partitions could then look like this:

while (getNextPartition(info))
  {
    while (getNextRowInPartition(info))
      {
        // process the row, could emit rows or accumulate information
      } // loop over rows in partition

    // finalize information

    // copy passthru columns, those are still available in this model
    info.copyPassThruData();
    // emit final row for this partition
    emitRow();
  } // loop over partitions