[GitHub] [arrow] wgtmac commented on a diff in pull request #35098: GH-35097: [C++] ArrayData support for child_data slice.

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

wgtmac commented on code in PR #35098:
URL: https://github.com/apache/arrow/pull/35098#discussion_r1166248633


##########
cpp/src/arrow/array/data.cc:
##########
@@ -144,6 +144,8 @@ std::shared_ptr<ArrayData> ArrayData::Slice(int64_t off, int64_t len) const {
   } else {
     copy->null_count = null_count != 0 ? kUnknownNullCount : 0;
   }
+  for (auto& child : copy->child_data) 
+    child = child->Slice(copy->offset, copy->length);

Review Comment:
   I think this breaks the contract of a sliced array. Please check the docstring of `ArrayData`
   ```
     // The logical start point into the physical buffers (in values, not bytes).
     // Note that, for child data, this must be *added* to the child data's own offset.
     int64_t offset = 0;
   ```



##########
cpp/src/arrow/array/data.cc:
##########
@@ -144,6 +144,8 @@ std::shared_ptr<ArrayData> ArrayData::Slice(int64_t off, int64_t len) const {
   } else {
     copy->null_count = null_count != 0 ? kUnknownNullCount : 0;
   }
+  for (auto& child : copy->child_data) 
+    child = child->Slice(copy->offset, copy->length);

Review Comment:
   cc @lidavidm @westonpace 



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