alamb commented on code in PR #7934:
URL: https://github.com/apache/arrow-rs/pull/7934#discussion_r2211458267


##########
parquet-variant/src/variant/object.rs:
##########
@@ -244,16 +252,22 @@ impl<'m, 'v> VariantObject<'m, 'v> {
                 // to check lexicographical order
                 //
                 // Since we are probing the metadata dictionary by field id, 
this also verifies field ids are in-bounds
-                let are_field_names_sorted = field_ids
-                    .iter()
-                    .map(|&i| self.metadata.get(i))
-                    .collect::<Result<Vec<_>, _>>()?
-                    .is_sorted();
-
-                if !are_field_names_sorted {
-                    return Err(ArrowError::InvalidArgumentError(
-                        "field names not sorted".to_string(),
-                    ));
+                let mut current_field_name = match field_ids_iter.next() {

Review Comment:
   It wasn't added in this PR, but this check for the field names being sorted 
doesn't seem right to me -- I thought the only requirement on an object's 
fields were that the field_ids were sorted (so lookup by field_id can be fast) 
but the corresponding names of the fields don't have to be sorted
   
   Maybe @friendlymatthew can help



##########
parquet-variant/src/variant/object.rs:
##########
@@ -217,23 +217,31 @@ impl<'m, 'v> VariantObject<'m, 'v> {
                 self.header.field_ids_start_byte() as 
_..self.first_field_offset_byte as _,
             )?;
 
-            let field_ids = map_bytes_to_offsets(field_id_buffer, 
self.header.field_id_size)
-                .collect::<Vec<_>>();
-
+            let mut field_ids_iter =
+                map_bytes_to_offsets(field_id_buffer, 
self.header.field_id_size);
             // Validate all field ids exist in the metadata dictionary and the 
corresponding field names are lexicographically sorted
             if self.metadata.is_sorted() {
                 // Since the metadata dictionary has unique and sorted field 
names, we can also guarantee this object's field names
                 // are lexicographically sorted by their field id ordering
-                if !field_ids.is_sorted() {
-                    return Err(ArrowError::InvalidArgumentError(
-                        "field names not sorted".to_string(),
-                    ));
-                }
+                let dictionary_size = self.metadata.dictionary_size();
+
+                if let Some(mut current_id) = field_ids_iter.next() {
+                    for next_id in field_ids_iter {
+                        if current_id >= dictionary_size {
+                            return Err(ArrowError::InvalidArgumentError(
+                                "field id is not valid".to_string(),
+                            ));
+                        }
+
+                        if next_id <= current_id {
+                            return Err(ArrowError::InvalidArgumentError(
+                                "field names not sorted".to_string(),
+                            ));
+                        }
+                        current_id = next_id;
+                    }
 
-                // Since field ids are sorted, if the last field is smaller 
than the dictionary size,
-                // we also know all field ids are smaller than the dictionary 
size and in-bounds.
-                if let Some(&last_field_id) = field_ids.last() {
-                    if last_field_id >= self.metadata.dictionary_size() {
+                    if current_id >= dictionary_size {

Review Comment:
   It took me a few times to figure out why this (redundant) check was still 
needed 
   
   I am not sure if there is some way to refactor the loop to avoid this 
(perhaps by keeping `previous_id: Option<u32>` as you did in the loop above 🤔  
   
   No changes needed, I just figured I would point it out



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