laskoviymishka commented on code in PR #3058:
URL: https://github.com/apache/iceberg-rust/pull/3058#discussion_r3842877062


##########
crates/iceberg/src/arrow/reader/pipeline.rs:
##########
@@ -1453,6 +1510,523 @@ mod tests {
         );
     }
 
+    /// A scan task projecting `id` + `_row_id`, with the given `first_row_id`.
+    fn row_id_task(file_path: String, first_row_id: Option<i64>) -> 
FileScanTask {
+        let schema = Arc::new(
+            Schema::builder()
+                .with_schema_id(1)
+                .with_fields(vec![
+                    NestedField::required(1, "id", 
Type::Primitive(PrimitiveType::Int)).into(),
+                ])
+                .build()
+                .unwrap(),
+        );
+
+        FileScanTask::builder()
+            
.with_file_size_in_bytes(std::fs::metadata(&file_path).unwrap().len())
+            .with_start(0)
+            .with_length(0)
+            .with_data_file_path(file_path)
+            .with_data_file_format(DataFileFormat::Parquet)
+            .with_schema(schema)
+            .with_project_field_ids(vec![1, RESERVED_FIELD_ID_ROW_ID])
+            .with_first_row_id(first_row_id)
+            .with_case_sensitive(false)
+            .build()
+    }
+
+    /// Asserts the logical per-row values of the `_row_id` column across all 
batches,
+    /// independent of the physical (run-end) encoding.
+    fn assert_row_id_column(batches: &[RecordBatch], expected: &[Option<i64>]) 
{
+        use arrow_array::cast::AsArray;
+        use arrow_cast::cast;
+        use arrow_schema::DataType;
+
+        let mut actual = Vec::new();
+        for batch in batches {
+            let col = batch
+                .column_by_name(RESERVED_COL_NAME_ROW_ID)
+                .expect("_row_id column should be present");
+            let logical = cast(col, &DataType::Int64).unwrap();
+            let values = 
logical.as_primitive::<arrow_array::types::Int64Type>();
+            for i in 0..values.len() {
+                actual.push((!values.is_null(i)).then(|| values.value(i)));
+            }
+        }
+        assert_eq!(actual, expected);
+    }
+
+    /// A parquet field carrying the embedded `_row_id` field id.
+    fn physical_row_id_field() -> Field {
+        Field::new(RESERVED_COL_NAME_ROW_ID, DataType::Int64, 
true).with_metadata(HashMap::from([
+            (
+                PARQUET_FIELD_ID_META_KEY.to_string(),
+                RESERVED_FIELD_ID_ROW_ID.to_string(),
+            ),
+        ]))
+    }
+
+    #[tokio::test]
+    async fn test_row_id_synthesized_from_first_row_id_and_pos() {
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        let file_path = write_plain_parquet(dir, "row_id_synth.parquet", 
vec![], vec![]);
+
+        // No physical column: every row is first_row_id + pos.
+        let task = row_id_task(file_path, Some(100));
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        assert_row_id_column(&batches, &[Some(100), Some(101), Some(102)]);

Review Comment:
   Every `_row_id` test here writes a single row group, so nothing exercises 
the case that actually makes `first_row_id + pos` correct: `_pos` has to be the 
global file position, not a per-row-group offset. If parquet-rs's RowNumber 
ever resets per row group (or does so under some row-selection config), a 2+ 
row-group file would hand group 1 the same ids as group 0 and we'd silently 
emit duplicate row ids — the hardest guarantee `_row_id` has to make, and every 
test here would still pass.
   
   I'd add a test that writes with a small `max_row_group_size` (say 2) over 5 
rows, `first_row_id = Some(0)`, and asserts `[0,1,2,3,4]` — specifically that 
group 1 continues the count rather than restarting. wdyt?



##########
crates/iceberg/src/arrow/record_batch_transformer.rs:
##########
@@ -263,6 +276,11 @@ pub(crate) enum ColumnConstant {
     /// per-row value is null (e.g. `_last_updated_sequence_number` on a file 
that
     /// carries the column, falling back to the data sequence number).
     CoalesceLastUpdatedSeq(Datum),
+    /// The `_row_id` metadata column. `Some(first_row_id)` synthesizes it 
(the per-row
+    /// value where the file physically carries `_row_id`, else `first_row_id` 
plus the
+    /// row's position); `None` produces an all-null column (a file with a null
+    /// `first_row_id`).
+    RowId(Option<i64>),

Review Comment:
   `RowId(Some(_))` isn't really a constant — every other `ColumnConstant` 
variant is row-invariant, but this one triggers `RowIdSynthesis`, which reads 
two other columns and produces a per-row result. Since `with_row_id_column` 
stashes it in `constant_fields`, anyone auditing `constant_fields` or 
pattern-matching on `ColumnConstant` later is going to be misled about what's 
actually constant.
   
   I'd pull the synthesis rules into their own map — something like 
`synthesis_rules: HashMap<i32, SynthesisRule>` with `SynthesisRule::RowId { 
first_row_id }` — or rename `ColumnConstant` to something like `ColumnBinding` 
and document that it covers per-row computation. Not a correctness issue, but 
it's the kind of abstraction leak that gets expensive later. wdyt?



##########
crates/iceberg/src/arrow/reader/pipeline.rs:
##########
@@ -289,6 +298,28 @@ impl FileScanTaskReader {
         let coalesce_last_updated_seq_leaf = phys_last_updated_seq_leaf
             .filter(|_| task.first_row_id.is_some() && 
task.data_sequence_number.is_some());
 
+        let phys_row_id_leaf = if project_row_id {
+            find_leaf_by_field_id(
+                record_batch_stream_builder.parquet_schema(),
+                RESERVED_FIELD_ID_ROW_ID,
+            )
+        } else {
+            None
+        };
+
+        // Present by name but without the embedded id: unthreadable, rejected 
below.
+        let row_id_present_by_name_only = project_row_id

Review Comment:
   This keys the name-only rejection purely on the column name. A file read 
through positional fallback gets synthetic field ids from 1 upward that never 
match the reserved `_row_id` id, so `phys_row_id_leaf` is `None` — but if that 
file happens to have a legitimate user column named `_row_id`, this flags 
`row_id_present_by_name_only` and we reject valid user data with 
`FeatureUnsupported`. Java and PyIceberg don't hit this because they key on the 
reserved field id, not the name.
   
   Could we scope this so a name collision on a fallback file is treated as a 
user column rather than a rejection? wdyt?



##########
crates/iceberg/src/arrow/reader/pipeline.rs:
##########
@@ -421,6 +456,27 @@ impl FileScanTaskReader {
             };
         }
 
+        if project_row_id {
+            // Synthesize the column, gated on `first_row_id`. Java gates it 
the same way
+            // (`ValueReaders.rowIds` returns nulls when the base row id is 
null); unlike
+            // `_last_updated_sequence_number` there is no 
data-sequence-number dependency.
+            // Reject a name-only physical column, but only when we would read 
it.
+            if task.first_row_id.is_some() && row_id_present_by_name_only {

Review Comment:
   The `task.first_row_id.is_some() &&` here means a file with a 
`_row_id`-named column that's missing the embedded field id gets silently 
turned into an all-null column when `first_row_id` is absent, instead of the 
error we raise when it's present. That name-only shape is evidence of a 
write-side bug either way, and someone debugging it would see all-null and 
conclude the feature just isn't supported.
   
   I'd at least `tracing::warn!` on the name-only + null-`first_row_id` path so 
it's not silent — or reject unconditionally, since a name-only `_row_id` never 
threads regardless of `first_row_id`. wdyt?



##########
crates/iceberg/src/arrow/reader/pipeline.rs:
##########
@@ -1453,6 +1510,523 @@ mod tests {
         );
     }
 
+    /// A scan task projecting `id` + `_row_id`, with the given `first_row_id`.
+    fn row_id_task(file_path: String, first_row_id: Option<i64>) -> 
FileScanTask {
+        let schema = Arc::new(
+            Schema::builder()
+                .with_schema_id(1)
+                .with_fields(vec![
+                    NestedField::required(1, "id", 
Type::Primitive(PrimitiveType::Int)).into(),
+                ])
+                .build()
+                .unwrap(),
+        );
+
+        FileScanTask::builder()
+            
.with_file_size_in_bytes(std::fs::metadata(&file_path).unwrap().len())
+            .with_start(0)
+            .with_length(0)
+            .with_data_file_path(file_path)
+            .with_data_file_format(DataFileFormat::Parquet)
+            .with_schema(schema)
+            .with_project_field_ids(vec![1, RESERVED_FIELD_ID_ROW_ID])
+            .with_first_row_id(first_row_id)
+            .with_case_sensitive(false)
+            .build()
+    }
+
+    /// Asserts the logical per-row values of the `_row_id` column across all 
batches,
+    /// independent of the physical (run-end) encoding.
+    fn assert_row_id_column(batches: &[RecordBatch], expected: &[Option<i64>]) 
{
+        use arrow_array::cast::AsArray;
+        use arrow_cast::cast;
+        use arrow_schema::DataType;
+
+        let mut actual = Vec::new();
+        for batch in batches {
+            let col = batch
+                .column_by_name(RESERVED_COL_NAME_ROW_ID)
+                .expect("_row_id column should be present");
+            let logical = cast(col, &DataType::Int64).unwrap();
+            let values = 
logical.as_primitive::<arrow_array::types::Int64Type>();
+            for i in 0..values.len() {
+                actual.push((!values.is_null(i)).then(|| values.value(i)));
+            }
+        }
+        assert_eq!(actual, expected);
+    }
+
+    /// A parquet field carrying the embedded `_row_id` field id.
+    fn physical_row_id_field() -> Field {
+        Field::new(RESERVED_COL_NAME_ROW_ID, DataType::Int64, 
true).with_metadata(HashMap::from([
+            (
+                PARQUET_FIELD_ID_META_KEY.to_string(),
+                RESERVED_FIELD_ID_ROW_ID.to_string(),
+            ),
+        ]))
+    }
+
+    #[tokio::test]
+    async fn test_row_id_synthesized_from_first_row_id_and_pos() {
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        let file_path = write_plain_parquet(dir, "row_id_synth.parquet", 
vec![], vec![]);
+
+        // No physical column: every row is first_row_id + pos.
+        let task = row_id_task(file_path, Some(100));
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        assert_row_id_column(&batches, &[Some(100), Some(101), Some(102)]);
+    }
+
+    #[tokio::test]
+    async fn test_row_id_physical_column_coalesced() {
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        // A file that physically carries `_row_id`, as written when carrying 
rows forward
+        // across a rewrite: some rows have a stored value, some are null.
+        let id_col = Arc::new(Int64Array::from(vec![Some(5), None, Some(8)])) 
as ArrayRef;
+        let file_path = write_plain_parquet(
+            dir,
+            "row_id_phys.parquet",
+            vec![physical_row_id_field()],
+            vec![id_col],
+        );
+
+        let task = row_id_task(file_path, Some(100));
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        // Per-row value where non-null; first_row_id + pos (101) where null.
+        assert_row_id_column(&batches, &[Some(5), Some(101), Some(8)]);
+    }
+
+    #[tokio::test]
+    async fn test_row_id_only_synthesis_reads_no_data_columns() {
+        // The common v3 case: a new-row file with `first_row_id` set and NO 
physically
+        // stored `_row_id`, projecting only `_row_id`. `_row_id` synthesis 
installs the
+        // RowNumber virtual column (via `need_row_number`), so the row count 
comes from it
+        // -- the scan must read no data columns, not fall back to reading 
everything.
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+
+        let mut meta_only = metadata_projection_task(
+            write_parquet_with_wide_column(dir, "row_id_only.parquet", vec![], 
vec![]),
+            id_and_wide_schema(),
+            vec![RESERVED_FIELD_ID_ROW_ID],
+        );
+        meta_only.first_row_id = Some(100);
+        let (batches, meta_only_bytes) = scan_task(meta_only).await;
+
+        assert_eq!(batches[0].num_columns(), 1);
+        assert_row_id_column(&batches, &[Some(100), Some(101), Some(102)]);
+
+        // A scan that also projects the wide data column must read materially 
more.
+        let mut with_data = metadata_projection_task(
+            write_parquet_with_wide_column(dir, "row_id_only_ref.parquet", 
vec![], vec![]),
+            id_and_wide_schema(),
+            vec![2, RESERVED_FIELD_ID_ROW_ID],
+        );
+        with_data.first_row_id = Some(100);
+        let (_, with_data_bytes) = scan_task(with_data).await;
+
+        assert!(
+            meta_only_bytes < with_data_bytes,
+            "_row_id-only synthesis should read fewer bytes than a scan of the 
wide column: \
+             {meta_only_bytes} vs {with_data_bytes}"
+        );
+    }
+
+    #[tokio::test]
+    async fn test_row_id_resolves_alongside_id_less_leaf() {
+        // A file with an id-less leaf (mimicking a Variant column's internal 
metadata/value
+        // leaves, which the spec requires to have no field id) plus a 
physical `_row_id`
+        // that carries its embedded id. The reserved id must still resolve -- 
an
+        // all-or-nothing field map would bail on the id-less leaf and wrongly 
reject the file.
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        let idless_field = Field::new("variant_internal", DataType::Utf8, 
true);
+        let idless_col = Arc::new(StringArray::from(vec!["a", "b", "c"])) as 
ArrayRef;
+        let row_id_col = Arc::new(Int64Array::from(vec![Some(5), None, 
Some(8)])) as ArrayRef;
+        let file_path = write_plain_parquet(
+            dir,
+            "row_id_with_idless_leaf.parquet",
+            vec![idless_field, physical_row_id_field()],
+            vec![idless_col, row_id_col],
+        );
+
+        let task = row_id_task(file_path, Some(100));
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        // Physical value where non-null; first_row_id + pos (101) where null.
+        assert_row_id_column(&batches, &[Some(5), Some(101), Some(8)]);
+    }
+
+    #[tokio::test]
+    async fn test_row_id_and_last_updated_seq_co_projected() {
+        use 
crate::metadata_columns::RESERVED_FIELD_ID_LAST_UPDATED_SEQUENCE_NUMBER;
+
+        // Both lineage columns projected together over a file carrying both 
physical
+        // leaves. Each must materialize independently -- neither leaf's mask 
clobbers the
+        // other, and the two synthesized columns keep their own values.
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        let row_id_col = Arc::new(Int64Array::from(vec![Some(5), None, 
Some(8)])) as ArrayRef;
+        let seq_col = Arc::new(Int64Array::from(vec![Some(50), None, 
Some(70)])) as ArrayRef;
+        let file_path = write_plain_parquet(
+            dir,
+            "row_id_and_seq.parquet",
+            vec![physical_row_id_field(), physical_last_updated_seq_field()],
+            vec![row_id_col, seq_col],
+        );
+
+        let schema = Arc::new(
+            Schema::builder()
+                .with_schema_id(1)
+                .with_fields(vec![
+                    NestedField::required(1, "id", 
Type::Primitive(PrimitiveType::Int)).into(),
+                ])
+                .build()
+                .unwrap(),
+        );
+        let task = FileScanTask::builder()
+            
.with_file_size_in_bytes(std::fs::metadata(&file_path).unwrap().len())
+            .with_start(0)
+            .with_length(0)
+            .with_data_file_path(file_path)
+            .with_data_file_format(DataFileFormat::Parquet)
+            .with_schema(schema)
+            .with_project_field_ids(vec![
+                1,
+                RESERVED_FIELD_ID_ROW_ID,
+                RESERVED_FIELD_ID_LAST_UPDATED_SEQUENCE_NUMBER,
+            ])
+            .with_first_row_id(Some(100))
+            .with_data_sequence_number(Some(9))
+            .with_case_sensitive(false)
+            .build();
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        // _row_id: physical value where non-null, else first_row_id + pos 
(101).
+        assert_row_id_column(&batches, &[Some(5), Some(101), Some(8)]);
+        // _last_updated_sequence_number: physical value where non-null, else 
data seq (9).
+        assert_last_updated_seq_column(&batches, &[Some(50), Some(9), 
Some(70)]);
+    }
+
+    #[tokio::test]
+    async fn test_row_id_null_when_no_first_row_id() {
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        // Physically carries `_row_id`, but the file has a null first_row_id.
+        let id_col = Arc::new(Int64Array::from(vec![Some(5), Some(6), 
Some(7)])) as ArrayRef;
+        let file_path = write_plain_parquet(
+            dir,
+            "row_id_no_first.parquet",
+            vec![physical_row_id_field()],
+            vec![id_col],
+        );
+
+        // Null first_row_id: the whole column is null; the physical values 
are not read.
+        let task = row_id_task(file_path, None);
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        assert_row_id_column(&batches, &[None, None, None]);
+    }
+
+    #[tokio::test]
+    async fn test_row_id_with_pos_column() {
+        use crate::metadata_columns::RESERVED_COL_NAME_POS;
+
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        let id_col = Arc::new(Int64Array::from(vec![Some(5), None, Some(8)])) 
as ArrayRef;
+        let file_path = write_plain_parquet(
+            dir,
+            "row_id_and_pos.parquet",
+            vec![physical_row_id_field()],
+            vec![id_col],
+        );
+
+        // Co-project `_pos` and `_row_id`. `_row_id` synthesis consumes the 
position, and
+        // `_pos` is also emitted -- the RowNumber column must be added once 
and the two
+        // must not interfere.
+        let schema = Arc::new(
+            Schema::builder()
+                .with_schema_id(1)
+                .with_fields(vec![
+                    NestedField::required(1, "id", 
Type::Primitive(PrimitiveType::Int)).into(),
+                ])
+                .build()
+                .unwrap(),
+        );
+        let task = FileScanTask::builder()
+            
.with_file_size_in_bytes(std::fs::metadata(&file_path).unwrap().len())
+            .with_start(0)
+            .with_length(0)
+            .with_data_file_path(file_path)
+            .with_data_file_format(DataFileFormat::Parquet)
+            .with_schema(schema)
+            .with_project_field_ids(vec![1, RESERVED_FIELD_ID_POS, 
RESERVED_FIELD_ID_ROW_ID])
+            .with_first_row_id(Some(100))
+            .with_case_sensitive(false)
+            .build();
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        // `_row_id` coalesces correctly...
+        assert_row_id_column(&batches, &[Some(5), Some(101), Some(8)]);
+        // ...and `_pos` is the row position, not double-counted.
+        let pos_col = batches[0]
+            .column_by_name(RESERVED_COL_NAME_POS)
+            .expect("_pos column should be present")
+            .as_primitive::<arrow_array::types::Int64Type>();
+        assert_eq!(pos_col.values(), &[0, 1, 2]);
+    }
+
+    #[tokio::test]
+    async fn test_row_id_mixed_files_share_schema() {
+        use arrow_select::concat::concat_batches;
+
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+
+        // Three files in one scan exercising all three column paths, which 
must all
+        // produce the SAME Arrow type (plain Int64) or concatenation fails:
+        //   - synthesis: first_row_id set, no physical column -> first_row_id 
+ pos
+        //   - null gate: no first_row_id -> null column
+        //   - coalesce: first_row_id set, physical column present -> per-row 
+ fallback
+        let synth = row_id_task(
+            write_plain_parquet(dir, "row_id_synth2.parquet", vec![], vec![]),
+            Some(42),
+        );
+        let nulled = row_id_task(
+            write_plain_parquet(dir, "row_id_null2.parquet", vec![], vec![]),
+            None,
+        );
+        let coalesced = row_id_task(
+            write_plain_parquet(
+                dir,
+                "row_id_coalesced2.parquet",
+                vec![physical_row_id_field()],
+                vec![Arc::new(Int64Array::from(vec![Some(5), None, Some(8)])) 
as ArrayRef],
+            ),
+            Some(50),
+        );
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![
+            Ok(synth),
+            Ok(nulled),
+            Ok(coalesced),
+        ])) as FileScanTaskStream;
+        let batches: Vec<RecordBatch> = reader
+            .read(tasks)
+            .unwrap()
+            .stream()
+            .try_collect()
+            .await
+            .unwrap();
+
+        assert_eq!(batches.len(), 3);
+        let schema = batches[0].schema();
+        concat_batches(&schema, &batches)
+            .expect("synthesis, null and coalesce files must share one column 
type");
+    }
+
+    #[tokio::test]
+    async fn test_row_id_present_by_name_without_id_unsupported() {
+        let tmp_dir = TempDir::new().unwrap();
+        let dir = tmp_dir.path().to_str().unwrap();
+        // Column present by name but WITHOUT the embedded field id. The 
transformer keys
+        // the source column by field id, so this shape can't be threaded and 
is rejected.
+        let id_field = Field::new(RESERVED_COL_NAME_ROW_ID, DataType::Int64, 
true);
+        let id_col = Arc::new(Int64Array::from(vec![Some(5), None, Some(8)])) 
as ArrayRef;
+        let file_path =
+            write_plain_parquet(dir, "row_id_by_name.parquet", vec![id_field], 
vec![id_col]);
+
+        let task = row_id_task(file_path, Some(100));
+
+        let reader = ArrowReaderBuilder::new(FileIO::new_with_fs(), 
Runtime::current()).build();
+        let tasks = Box::pin(futures::stream::iter(vec![Ok(task)])) as 
FileScanTaskStream;
+        let result: Result<Vec<RecordBatch>, _> =
+            reader.read(tasks).unwrap().stream().try_collect().await;
+
+        let err = result.unwrap_err();
+        assert_eq!(err.kind(), crate::ErrorKind::FeatureUnsupported);
+        assert!(
+            format!("{err}").contains("without an embedded field id"),
+            "unexpected error: {err}"
+        );
+    }
+
+    /// Builds a `row_id_task` (see above) that additionally carries a bound 
predicate,
+    /// so a `RowSelection` is applied when the reader has row selection 
enabled.
+    fn row_id_task_with_predicate(
+        file_path: String,
+        first_row_id: Option<i64>,
+        extra_project_field_ids: Vec<i32>,
+        predicate: crate::expr::Predicate,
+    ) -> FileScanTask {
+        use crate::expr::Bind;
+
+        let schema = Arc::new(
+            Schema::builder()
+                .with_schema_id(1)
+                .with_fields(vec![
+                    NestedField::required(1, "id", 
Type::Primitive(PrimitiveType::Int)).into(),
+                ])
+                .build()
+                .unwrap(),
+        );
+        let bound = predicate.bind(Arc::clone(&schema), false).unwrap();
+
+        let mut project_field_ids = vec![1];
+        project_field_ids.extend(extra_project_field_ids);
+        project_field_ids.push(RESERVED_FIELD_ID_ROW_ID);
+
+        FileScanTask::builder()
+            
.with_file_size_in_bytes(std::fs::metadata(&file_path).unwrap().len())
+            .with_start(0)
+            .with_length(0)
+            .with_data_file_path(file_path)
+            .with_data_file_format(DataFileFormat::Parquet)
+            .with_schema(schema)
+            .with_project_field_ids(project_field_ids)
+            .with_predicate(Some(bound))
+            .with_first_row_id(first_row_id)
+            .with_case_sensitive(false)
+            .build()
+    }
+
+    #[tokio::test]
+    async fn test_row_id_stable_under_row_selection() {

Review Comment:
   Nice that this asserts physical positions `[100, 102]` rather than dense 
`[100, 101]` — that's the right invariant. The one path it doesn't cover is 
delete-file-based selection: predicate row selection 
(`get_row_selection_for_filter_predicate`) and positional-delete selection 
(`build_deletes_row_selection` + intersection) reach the reader through 
different code, so a positional-delete case would close that explicitly.
   
   I'd add a test with an actual positional delete file dropping the middle row 
of 3 and assert `_row_id == [first_row_id, first_row_id + 2]`. wdyt?



##########
crates/iceberg/src/arrow/reader/pipeline.rs:
##########
@@ -314,25 +345,29 @@ impl FileScanTaskReader {
         // which `get_arrow_projection_mask` maps to "read all columns" (so 
`COUNT(*)` still
         // gets a row count). Downgrade that to "read no data columns" when a 
row-count
         // source exists independently of the data columns: the RowNumber 
virtual column
-        // (installed above under `project_pos`) or a physical metadata leaf 
unioned in
-        // below. Pure-constant / `COUNT(*)` projections have neither and must 
keep reading
-        // all columns to preserve the row count. Any future physical metadata 
leaf (e.g. a
-        // `_row_id` read path) is likewise a row source.
+        // (installed above under `need_row_number`, which covers `_pos` and 
`_row_id`
+        // synthesis) or a physical `_last_updated_sequence_number` leaf 
unioned in below
+        // (that column does not install RowNumber, so it is a separate 
source). Pure-constant
+        // / `COUNT(*)` projections have neither and must keep reading all 
columns to preserve
+        // the row count.
         //
-        // This runs BEFORE the union so the physical leaf is added onto a 
`none` base,
-        // pruning the read to just that leaf (`union` with an `all` base 
stays `all`).
+        // This runs BEFORE the union so the physical leaves are added onto a 
`none` base,
+        // pruning the read to just those leaves (`union` with an `all` base 
stays `all`).
         if project_field_ids_without_metadata.is_empty()
-            && (project_pos || coalesce_last_updated_seq_leaf.is_some())
+            && (need_row_number || coalesce_last_updated_seq_leaf.is_some())

Review Comment:
   When only `_row_id` is projected and `first_row_id` is `None`, 
`need_row_number` is false and there's no coalesce leaf, so this guard stays 
false and the mask falls back to `all()` — we read every data column just to 
emit an all-null `_row_id`. `SELECT _row_id ...` over any pre-V3 file pays full 
column I/O for a null output.
   
   Not a correctness bug, and the same gap already exists for 
`_last_updated_sequence_number` with a null `first_row_id`, so I'm fine leaving 
it — but I'd extend the guard with something like `|| (project_row_id && 
task.first_row_id.is_none())` to prune to a row-count source, or drop a comment 
marking it as a known follow-up. wdyt?



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