demonstrating the issue

crates/re_log_types/src/data_table.rs

@@ -1118,3 +1118,69 @@ impl DataTable {
         table
     }
 }
+
+#[test]
+fn data_table_sizes() {
+    use crate::{component_types::InstanceKey, Component as _};
+    use arrow2::array::UInt64Array;
+
+    fn expect(mut cell: DataCell, num_rows: usize, num_bytes: u64) {
+        cell.compute_size_bytes();
+
+        let row = DataRow::from_cells1(
+            RowId::random(),
+            "a/b/c",
+            TimePoint::default(),
+            cell.num_instances(),
+            cell,
+        );
+
+        let table = DataTable::from_rows(
+            TableId::random(),
+            std::iter::repeat_with(|| row.clone()).take(num_rows),
+        );
+        assert_eq!(num_bytes, table.heap_size_bytes());
+
+        let mut table = DataTable::from_arrow_msg(&table.to_arrow_msg().unwrap()).unwrap();
+        table.compute_all_size_bytes();
+        assert_eq!(num_bytes, table.heap_size_bytes());
+    }
+
+    // primitive
+    expect(
+        DataCell::from_arrow(
+            InstanceKey::name(),
+            UInt64Array::from_vec(vec![1, 2, 3]).boxed(),
+        ),
+        10_000,    // num_rows
+        1_800_064, // expected_num_bytes
+    );
+
+    // utf8
+    expect(
+        DataCell::from_native([crate::component_types::Label("hey".into())].as_slice()),
+        10_000,    // num_rows
+        1_670_064, // expected_num_bytes
+    );
+
+    // struct
+    expect(
+        DataCell::from_native([crate::component_types::Point2D::new(42.0, 666.0)].as_slice()),
+        10_000,    // num_rows
+        4_060_064, // expected_num_bytes
+    );
+
+    // struct + fixedsizelist
+    expect(
+        DataCell::from_native([crate::component_types::Vec2D::from([42.0, 666.0])].as_slice()),
+        10_000,    // num_rows
+        2_880_064, // expected_num_bytes
+    );
+
+    // union
+    expect(
+        DataCell::from_native([crate::Transform::Unknown].as_slice()),
+        10_000,     // num_rows
+        15_610_064, // expected_num_bytes
+    );
+}