[GitHub] [arrow] westonpace commented on a diff in pull request #13138: ARROW-16562: Avoid slicing array inputs in ExecBatchIterator that would result in one slice

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

westonpace commented on code in PR #13138:
URL: https://github.com/apache/arrow/pull/13138#discussion_r886285595


##########
cpp/src/arrow/compute/exec.cc:
##########
@@ -308,16 +308,30 @@ bool ExecBatchIterator::Next(ExecBatch* batch) {
   // Now, fill the batch
   batch->values.resize(args_.size());
   batch->length = iteration_size;
-  for (size_t i = 0; i < args_.size(); ++i) {
-    if (args_[i].is_scalar()) {
-      batch->values[i] = args_[i].scalar();
-    } else if (args_[i].is_array()) {
-      batch->values[i] = args_[i].array()->Slice(position_, iteration_size);
-    } else {
-      const ChunkedArray& carr = *args_[i].chunked_array();
-      const auto& chunk = carr.chunk(chunk_indexes_[i]);
-      batch->values[i] = chunk->data()->Slice(chunk_positions_[i], iteration_size);
-      chunk_positions_[i] += iteration_size;
+
+  if (iteration_size == length_) {
+    ARROW_DCHECK_EQ(position_, 0);
+    for (size_t i = 0; i < args_.size(); ++i) {
+      if (args_[i].kind() == Datum::CHUNKED_ARRAY) {
+        const ChunkedArray& carr = *args_[i].chunked_array();
+        batch->values[i] = Datum(carr.chunk(chunk_indexes_[i])->data());
+        chunk_positions_[i] += iteration_size;
+      } else {
+        batch->values[i] = std::move(args_[i]);
+      }

Review Comment:
   ```suggestion
         if (args_[i].kind() == Datum::CHUNKED_ARRAY) {
           chunk_positions_[i] += iteration_size;
         }
         batch->values[i] = std::move(args_[i]);
   ```
   
   I'm not entirely certain this is correct but if `iteration_size == length_` I think that means you are guaranteed that any chunked arrays are a single chunk (or at least, there is only one non-zero size chunk) and so you are consuming it all at once.  I'm not even sure you need to update `chunk_positions_[i]`.



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