Github user brkyvz commented on the pull request:

    https://github.com/apache/spark/pull/3200#issuecomment-62982228
  
    @mengxr 
    
    
    > If we have two block matrices, A and B, and A's column block partitioning 
matches B's row block partitioning, can we take advantage of this fact in 
computing A * B? I support having only one block matrix partitioner 
implementation. Then we do the following:
    > 
    > if (A.partitioner.colBlockPartitioner == 
B.partitioner.rowBlockPartitioner) {
    >   // zip ...
    > } else {
    >   ...
    > }
    
    By `partitioner.rowBlockPartitioner` and `partitioner.colBlockPartitioner`, 
are you talking about the number of blocks that form the rows and the number of 
rows per block match?
    
    One problem with zip was that I couldn't guarantee data locality. I tried 
to force it, but the best way to force it turns out to be a join...
    
    



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