USE CASE: on scan only; want to have a "summing combiner" that rolls up by (rowId, colfam, colqual) on all row keys where the client has visibility.
below is a simple example that expresses the use case. accumulo table holding student to professor relationship by departments +----------+------------------+-----------+--------------+-----+ | rowId | colfam | colqual | colvis | val | +----------+------------------+-----------+--------------+-----+ | student1 | TAKES_CLASS_WITH | prof1 | MATH_DEPT | 1 | | student1 | TAKES_CLASS_WITH | prof1 | MATH_DEPT | 1 | | student1 | TAKES_CLASS_WITH | prof1 | COM_SCI_DEPT | 1 | | student1 | TAKES_CLASS_WITH | prof1 | COM_SCI_DEPT | 1 | | student2 | TAKES_CLASS_WITH | prof1 | MATH_DEPT | 1 | | student2 | TAKES_CLASS_WITH | prof1 | COM_SCI_DEPT | 1 | +----------+------------------+-----------+--------------+-----+ with the summing combiner the results would be +----------+------------------+-----------+--------------+-----+ | rowId | colfam | colqual | colvis | val | +----------+------------------+-----------+--------------+-----+ | student1 | TAKES_CLASS_WITH | prof1 | MATH_DEPT | 2 | | student1 | TAKES_CLASS_WITH | prof1 | COM_SCI_DEPT | 2 | | student2 | TAKES_CLASS_WITH | prof1 | MATH_DEPT | 1 | | student2 | TAKES_CLASS_WITH | prof1 | COM_SCI_DEPT | 1 | +----------+------------------+-----------+--------------+-----+ - the math department can only see math department totals - the com sci department can only see the com sci department total - the office of the dean has both access therefore when scanning (it wouldnt work for compaction), how can you sum over colvis? assuming you had both colvis access the desired results would be: +----------+------------------+-----------+-----+ | rowId | colfam | colqual | val | +----------+------------------+-----------------+ | student1 | TAKES_CLASS_WITH | prof1 | 4 | | student2 | TAKES_CLASS_WITH | prof1 | 2 | +----------+------------------+-----------+-----+
