Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/20561#discussion_r167387073
  
    --- Diff: 
sql/core/src/main/java/org/apache/spark/sql/execution/UnsafeKVExternalSorter.java
 ---
    @@ -98,10 +99,20 @@ public UnsafeKVExternalSorter(
             numElementsForSpillThreshold,
             canUseRadixSort);
         } else {
    -      // The array will be used to do in-place sort, which require half of 
the space to be empty.
    -      // Note: each record in the map takes two entries in the array, one 
is record pointer,
    -      // another is the key prefix.
    -      assert(map.numKeys() * 2 <= map.getArray().size() / 2);
    +      // `BytesToBytesMap`'s point array is only guaranteed to hold all 
the distinct keys, but
    +      // `UnsafeInMemorySorter`'s point array need to hold all the 
entries. Since `BytesToBytesMap`
    --- End diff --
    
    yea, but it's not trivial, I'd like to do it later. The required change I 
can think of: `BytesToBytesMap` is actually a key -> list<value>, and we need 
to provide a way to iterate key -> list<value> instead of key -> value.


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