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From paul-rogers <...@git.apache.org>
Subject [GitHub] drill pull request #778: DRILL-5344: Priority queue copier fails with an emp...
Date Sat, 11 Mar 2017 02:17:16 GMT
Github user paul-rogers commented on a diff in the pull request:

    https://github.com/apache/drill/pull/778#discussion_r105522650
  
    --- Diff: exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/xsort/managed/PriorityQueueCopierTemplate.java
---
    @@ -57,9 +57,12 @@ public void setup(FragmentContext context, BufferAllocator allocator,
VectorAcce
     
         queueSize = 0;
         for (int i = 0; i < size; i++) {
    -      vector4.set(i, i, batchGroups.get(i).getNextIndex());
    -      siftUp();
    -      queueSize++;
    +      int index = batchGroups.get(i).getNextIndex();
    +      vector4.set(i, i, index);
    --- End diff --
    
    This is tricky, and likely to due conflicting design decisions.
    
    It seems that the vector container retrieved by `get()` has a built-in iterator. Not a
good design as it will cause havoc if two bits of code try to iterate at the same time. Sigh...
    
    The iterator decides to return -1 when it reaches the end. That is OK, as long as code
checks.
    
    The original code did not check, it just used -1 (truncated to 16 bits, stored as a char,
so 65535) as a row index.
    
    The solution is to not add the batch to the "queue" when the iterator returns -1 in the
first position.
    
    Note that if the iterator has simply returned the count at the end, and we used the classic
"index >= count" as the terminal condition, then none of this would be necessary.


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