[GitHub] [arrow] rdettai commented on a change in pull request #6935: ARROW-8455: [Rust] Parquet Arrow column read on partially compatible files

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rdettai commented on a change in pull request #6935:
URL: https://github.com/apache/arrow/pull/6935#discussion_r414417688



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File path: rust/parquet/src/column/reader.rs
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@@ -190,15 +190,12 @@ impl<T: DataType> ColumnReaderImpl<T> {
                     (self.num_buffered_values - self.num_decoded_values) as usize,
                 );
 
-                // Adjust batch size by taking into account how much space is left in
-                // values slice or levels slices (if available)
-                adjusted_size = min(adjusted_size, values.len() - values_read);
-                if let Some(ref levels) = def_levels {
-                    adjusted_size = min(adjusted_size, levels.len() - levels_read);
-                }
-                if let Some(ref levels) = rep_levels {
-                    adjusted_size = min(adjusted_size, levels.len() - levels_read);
-                }
+                // Adjust batch size by taking into account how much data there
+                // to read. As batch_size is also smaller than value and level
+                // slices (if available), this ensures that available space is not
+                // exceeded.
+                adjusted_size = min(adjusted_size, batch_size - values_read);

Review comment:
       As stated in my PR comment, the `read_batch` function can receive any combination of `batch_size`, `def_levels.len()`, `rep_levels.len()` and `values.len()`. If the `batch_size` is the limiting factor your `iter_batch_size` might end up larger than the `batch_size`. This happened to me when called from `record_reader.rs` on a parquet file with relatively small row groups (500k rows). I did not manage to reproduce the phenomenon with a mock data file though ... :-/




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