[GitHub] [arrow] emkornfield commented on a change in pull request #8589: ARROW-10493: [C++][Parquet] Fix offset lost in MaybeReplaceValidity

[GitBox <gi...@apache.org>]

emkornfield commented on a change in pull request #8589:
URL: https://github.com/apache/arrow/pull/8589#discussion_r517512787



##########
File path: cpp/src/parquet/column_writer.cc
##########
@@ -1230,8 +1230,9 @@ class TypedColumnWriterImpl : public ColumnWriterImpl, public TypedColumnWriter<
     buffers[0] = bits_buffer_;
     // Should be a leaf array.
     DCHECK_EQ(array->num_fields(), 0);
-    return arrow::MakeArray(std::make_shared<ArrayData>(
-        array->type(), array->length(), std::move(buffers), new_null_count));
+    return arrow::MakeArray(std::make_shared<ArrayData>(array->type(), array->length(),
+                                                        std::move(buffers),

Review comment:
       Nice catch.
   
   I think there is an edge case here.  I think offset applies to the null buffer also, so this would cause an illegal access if any values are actually null.  unfortunately, I think we need to slice all the buffers according to offset.




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