diff --git a/crates/lance-context-core/src/rollout_store.rs b/crates/lance-context-core/src/rollout_store.rs
index e8095a1..fc9f3cb 100644
--- a/crates/lance-context-core/src/rollout_store.rs
+++ b/crates/lance-context-core/src/rollout_store.rs
@@ -4066,6 +4066,37 @@ mod tests {
         });
     }
 
+    /// A merge into a base table that has rows but no id BTree builds the
+    /// BTree first: without it the delete-only `merge_insert` is a hash
+    /// join over the whole base table. The very first merge (empty base)
+    /// has nothing to join and builds nothing.
+    #[test]
+    fn merge_builds_the_id_btree_when_the_base_has_rows_but_no_index() {
+        let dir = TempDir::new().unwrap();
+        let uri = dir.path().to_string_lossy().to_string();
+        let runtime = tokio::runtime::Runtime::new().unwrap();
+        runtime.block_on(async {
+            let mut store = RolloutStore::open(&uri).await.unwrap();
+            store.add(&[assistant_record("a-0")]).await.unwrap();
+            store.flush().await.unwrap();
+            store.cleanup_own_shard().await.unwrap();
+            assert_eq!(store.base.dataset.count_fragments(), 1);
+            assert!(
+                !store.has_id_btree_index().await.unwrap(),
+                "empty base: nothing to index"
+            );
+
+            store.add(&[assistant_record("a-1")]).await.unwrap();
+            store.flush().await.unwrap();
+            store.cleanup_own_shard().await.unwrap();
+            assert!(
+                store.has_id_btree_index().await.unwrap(),
+                "second merge joins against a populated base and builds the BTree first"
+            );
+            assert_eq!(store.list(None, None).await.unwrap().len(), 2);
+        });
+    }
+
     /// The WAL merge is a delete-only `merge_insert` on the key followed by
     /// an append: an upsert would take the matched target rows with every
     /// column (multi-megabyte inline blobs included) to rewrite them.
diff --git a/crates/lance-context-core/src/store_base.rs b/crates/lance-context-core/src/store_base.rs
index 6f1e32c..b3f10f2 100644
--- a/crates/lance-context-core/src/store_base.rs
+++ b/crates/lance-context-core/src/store_base.rs
@@ -1226,6 +1226,15 @@ impl StorageBase {
         batches: Vec<RecordBatch>,
         merge_schema: Arc<Schema>,
     ) -> LanceResult<()> {
+        // Without an exact-answer key index the delete-only merge_insert is
+        // a hash join over the whole base table. On a 174 GB / 67k-row store
+        // that no master had ever indexed, that join OOMKilled every worker
+        // that tried to merge it; building the BTree first took 19 s. The
+        // master builds it before fan-out (#277), but a worker's own timer
+        // or a manual merge must not depend on the master having been here.
+        if self.dataset.count_fragments() > 0 && !self.has_key_btree_index().await? {
+            self.create_key_btree_index().await?;
+        }
         let key_index = merge_schema.index_of(&self.key_column)?;
         let key_schema = Arc::new(merge_schema.project(&[key_index])?);
         let keys = batches
diff --git a/crates/lance-context-master/src/scheduler.rs b/crates/lance-context-master/src/scheduler.rs
index c9c028f..0587206 100644
--- a/crates/lance-context-master/src/scheduler.rs
+++ b/crates/lance-context-master/src/scheduler.rs
@@ -1068,12 +1068,22 @@ mod tests {
         assert_eq!(status.state, TaskState::Done, "got {status:?}");
         assert_eq!(status.detail.as_deref(), Some("built btree index on id"));
 
-        // A second compaction with nothing to rewrite does not enqueue another.
-        let again = enqueue(&state, TaskKind::Compact, name).await.unwrap();
-        assert_eq!(
-            await_terminal(&state, &again.id).await.state,
-            TaskState::Done
-        );
+        // Compact until nothing is left to rewrite. The first merge built the
+        // id BTree over the first fragment, and Lance compacts indexed and
+        // unindexed fragments in separate groups, so reaching one fragment
+        // can take more than one pass. Only passes that rewrote fragments
+        // enqueue an IndexId; the final no-op pass must not.
+        let mut rewriting_compactions = 1;
+        loop {
+            let again = enqueue(&state, TaskKind::Compact, name).await.unwrap();
+            let status = await_terminal(&state, &again.id).await;
+            assert_eq!(status.state, TaskState::Done, "got {status:?}");
+            if status.detail.as_deref() == Some("removed 0 / added 0 fragments") {
+                break;
+            }
+            rewriting_compactions += 1;
+            assert!(rewriting_compactions <= 4, "compaction never converged");
+        }
         let indexes = state
             .task_store
             .list()
@@ -1082,7 +1092,7 @@ mod tests {
             .into_iter()
             .filter(|task| task.kind == TaskKind::IndexId)
             .count();
-        assert_eq!(indexes, 1);
+        assert_eq!(indexes, rewriting_compactions);
 
         worker.abort();
     }