refactor!: drop unused loss field from IndexStatistics (#3496)

BREAKING CHANGE: direct Rust users lose the `IndexStatistics::loss`
field. Python and Node.js consumers are unaffected in practice for
remote tables (the value was always `None`/absent), but the attribute is
gone for local tables too.

`IndexStatistics::loss` was local-only — LanceDB Cloud never returned
it, so
`RemoteTable::index_stats` always set `loss: None`. It's vestigial; this
removes it.

- Remove `loss` from `IndexStatistics` and the internal `IndexMetadata`
in `rust/lancedb/src/index.rs`, plus the summing logic in
`NativeTable::index_stats`.
- Drop `loss` from the Python and Node.js bindings (and their
tests/docs).

Fixes #3493

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/src/js/interfaces/IndexStatistics.md

@@ -30,17 +30,6 @@ The type of the index
 
 ***
 
-### loss?
-
-```ts
-optional loss: number;
-```
-
-The KMeans loss value of the index,
-it is only present for vector indices.
-
-***
-
 ### numIndexedRows
 
 ```ts

nodejs/__test__/table.test.ts

@@ -721,7 +721,7 @@ describe("When creating an index", () => {
       columns: ["vec"],
     });
     const stats = await tbl.indexStats("vec_idx");
-    expect(stats?.loss).toBeDefined();
+    expect(stats).toBeDefined();
 
     // Search without specifying the column
     let rst = await tbl
@@ -1150,7 +1150,6 @@ describe("When creating an index", () => {
     expect(stats?.distanceType).toBeUndefined();
     expect(stats?.indexType).toEqual("BTREE");
     expect(stats?.numIndices).toEqual(1);
-    expect(stats?.loss).toBeUndefined();
   });
 
   test("when getting stats on non-existent index", async () => {

nodejs/src/table.rs

@@ -838,9 +838,6 @@ pub struct IndexStatistics {
     pub distance_type: Option<String>,
     /// The number of parts this index is split into.
     pub num_indices: Option<u32>,
-    /// The KMeans loss value of the index,
-    /// it is only present for vector indices.
-    pub loss: Option<f64>,
 }
 impl From<lancedb::index::IndexStatistics> for IndexStatistics {
     fn from(value: lancedb::index::IndexStatistics) -> Self {
@@ -850,7 +847,6 @@ impl From<lancedb::index::IndexStatistics> for IndexStatistics {
             index_type: value.index_type.to_string(),
             distance_type: value.distance_type.map(|d| d.to_string()),
             num_indices: value.num_indices,
-            loss: value.loss,
         }
     }
 }

python/python/lancedb/table.py

@@ -5637,8 +5637,6 @@ class IndexStatistics:
         The distance type used by the index.
     num_indices: Optional[int]
         The number of parts the index is split into.
-    loss: Optional[float]
-        The KMeans loss for the index, for only vector indices.
     """
 
     num_indexed_rows: int
@@ -5658,7 +5656,6 @@ class IndexStatistics:
     ]
     distance_type: Optional[Literal["l2", "cosine", "dot"]] = None
     num_indices: Optional[int] = None
-    loss: Optional[float] = None
 
     # This exists for backwards compatibility with an older API, which returned
     # a dictionary instead of a class.

python/python/tests/test_index.py

@@ -226,7 +226,6 @@ async def test_create_vector_index(some_table: AsyncTable):
     assert stats.num_indexed_rows == await some_table.count_rows()
     assert stats.num_unindexed_rows == 0
     assert stats.num_indices == 1
-    assert stats.loss >= 0.0
 
 
 @pytest.mark.asyncio
@@ -250,7 +249,6 @@ async def test_create_4bit_ivfpq_index(some_table: AsyncTable):
     assert stats.num_indexed_rows == await some_table.count_rows()
     assert stats.num_unindexed_rows == 0
     assert stats.num_indices == 1
-    assert stats.loss >= 0.0
 
 
 @pytest.mark.asyncio

python/src/table.rs

@@ -711,10 +711,6 @@ impl Table {
                         dict.set_item("num_indices", num_indices)?;
                     }
 
-                    if let Some(loss) = stats.loss {
-                        dict.set_item("loss", loss)?;
-                    }
-
                     Ok(Some(dict.unbind()))
                 })
             } else {

rust/lancedb/src/index.rs

@@ -372,7 +372,6 @@ pub(crate) struct IndexMetadata {
     pub metric_type: Option<DistanceType>,
     // Sometimes the index type is provided at this level.
     pub index_type: Option<IndexType>,
-    pub loss: Option<f64>,
 }
 
 // This struct is used to deserialize the JSON data returned from the Lance API
@@ -404,6 +403,4 @@ pub struct IndexStatistics {
     pub distance_type: Option<DistanceType>,
     /// The number of parts this index is split into.
     pub num_indices: Option<u32>,
-    /// The loss value used by the index.
-    pub loss: Option<f64>,
 }

rust/lancedb/src/remote/table.rs

@@ -4121,7 +4121,6 @@ mod tests {
             index_type: IndexType::IvfPq,
             distance_type: Some(DistanceType::L2),
             num_indices: None,
-            loss: None,
         };
         assert_eq!(indices, expected);
 

rust/lancedb/src/table.rs

@@ -3019,20 +3019,12 @@ impl BaseTable for NativeTable {
                 .ok_or_else(|| Error::InvalidInput {
                     message: "index statistics was missing index type".to_string(),
                 })?;
-        let loss = stats
-            .indices
-            .iter()
-            .map(|index| index.loss.unwrap_or_default())
-            .sum::<f64>();
-
-        let loss = first_index.loss.map(|first_loss| first_loss + loss);
         Ok(Some(IndexStatistics {
             num_indexed_rows: stats.num_indexed_rows,
             num_unindexed_rows: stats.num_unindexed_rows,
             index_type,
             distance_type: first_index.metric_type,
             num_indices: stats.num_indices,
-            loss,
         }))
     }
 
@@ -3435,7 +3427,6 @@ mod tests {
         assert_eq!(stats.num_unindexed_rows, 0);
         assert_eq!(stats.index_type, crate::index::IndexType::IvfPq);
         assert_eq!(stats.distance_type, Some(crate::DistanceType::L2));
-        assert!(stats.loss.is_some());
 
         table.drop_index(index_name).await.unwrap();
         assert_eq!(table.list_indices().await.unwrap().len(), 0);