liukun4515 commented on code in PR #2968:
URL: https://github.com/apache/arrow-datafusion/pull/2968#discussion_r931725250
##########
datafusion/row/src/writer.rs:
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@@ -349,7 +349,7 @@ pub(crate) fn write_field_utf8(
let from = from.as_any().downcast_ref::<StringArray>().unwrap();
let s = from.value(row_idx);
let new_width = to.current_width() + s.as_bytes().len();
Review Comment:
@comphead you can go through the code of `set_utf8`.
```
fn set_utf8(&mut self, idx: usize, value: &str) {
self.assert_index_valid(idx);
let bytes = value.as_bytes();
let size = bytes.len();
// this line: write the offset and size to the row_buffer
self.set_offset_size(idx, size as u32);
let varlena_offset = self.varlena_offset;
self.data[varlena_offset..varlena_offset +
size].copy_from_slice(bytes);
self.varlena_offset += size;
self.varlena_width += size;
}
```
Not per field, and variable length data type like utf8 and binary.
From the @yjshen codebase, I think the layout of utf8 and binary should be
```
─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
│ offset and size│ utf8 data or binary data │
─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
```
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