[GitHub] [arrow] westonpace commented on a diff in pull request #35522: GH-35521: [C++] Hash null bitmap only if null count is 0

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on code in PR #35522:
URL: https://github.com/apache/arrow/pull/35522#discussion_r1190235212


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cpp/src/arrow/scalar.cc:
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@@ -153,9 +153,10 @@ struct ScalarHashImpl {
 
   Status ArrayHash(const ArrayData& a) {
     RETURN_NOT_OK(StdHash(a.length) & StdHash(a.GetNullCount()));
-    if (a.buffers[0] != nullptr) {
+    if (a.GetNullCount() != 0 && a.buffers[0] != nullptr) {

Review Comment:
   IIRC, it's also possible that `a.buffers[0] == nullptr` if all the elements are valid.  Is it possible that we still get differing hashes in this case?
   
   * All elements valid and equal and validity bitmap present (would hash the validity bitmap)
   * All elements valid and equal and validity bitmap missing (would not hash the validity bitmap)



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