fix: Project last column if count query (#16569)

crates/polars-plan/src/logical_plan/optimizer/projection_pushdown/projection.rs

@@ -62,8 +62,9 @@ pub(super) fn process_projection(
     // the whole file while we only want the count
     if exprs.len() == 1 && is_count(exprs[0].node(), expr_arena) {
         let input_schema = lp_arena.get(input).schema(lp_arena);
-        // simply select the first column
-        let (first_name, _) = input_schema.try_get_at_index(0)?;
+        // simply select the last column
+        // NOTE: the first can be the inserted index column, so that might not work
+        let (first_name, _) = input_schema.try_get_at_index(input_schema.len() - 1)?;
         let expr = expr_arena.add(AExpr::Column(ColumnName::from(first_name.as_str())));
         if !acc_projections.is_empty() {
             check_double_projection(

py-polars/tests/unit/io/test_lazy_parquet.py

@@ -59,6 +59,11 @@ def test_row_index(foods_parquet_path: Path) -> None:
     assert df["foo"].to_list() == [10, 16, 21, 23, 24, 30, 35]
 
 
+def test_row_index_len_16543(foods_parquet_path: Path) -> None:
+    q = pl.scan_parquet(foods_parquet_path).with_row_index()
+    assert q.select(pl.all()).select(pl.len()).collect().item() == 27
+
+
 @pytest.mark.write_disk()
 def test_categorical_parquet_statistics(tmp_path: Path) -> None:
     tmp_path.mkdir(exist_ok=True)