sunchao commented on a change in pull request #6935:
URL: https://github.com/apache/arrow/pull/6935#discussion_r414235616



##########
File path: rust/parquet/src/column/reader.rs
##########
@@ -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:
       I don't fully get this. In which case this code will yield different 
result than previous one. Could you give an example?




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