fix: Always return a PartitionRecord which maybe streams record batches

Connects to #6421.

Even if the ingester doesn't have data in memory for a query, we need to
send back metadata about the ingester UUID and the number of files
persisted so that the querier can decide whether it needs to refresh the
cache.

ingester2/src/buffer_tree/root.rs

@@ -1041,8 +1041,9 @@ mod tests {
         let partition = partitions.pop().unwrap();
 
         // Perform the partition read
-        let batches =
-            datafusion::physical_plan::common::collect(partition.into_record_batch_stream())
+        let batches = datafusion::physical_plan::common::collect(
+            partition.into_record_batch_stream().unwrap(),
+        )
         .await
         .expect("failed to collate query results");
 

ingester2/src/buffer_tree/table.rs

@@ -267,7 +267,7 @@ where
         );
 
         // Gather the partition data from all of the partitions in this table.
-        let partitions = self.partitions().into_iter().filter_map(move |p| {
+        let partitions = self.partitions().into_iter().map(move |p| {
             let mut span = SpanRecorder::new(span.clone().map(|s| s.child("partition read")));
 
             let (id, completed_persistence_count, data) = {
@@ -275,9 +275,12 @@ where
                 (
                     p.partition_id(),
                     p.completed_persistence_count(),
-                    p.get_query_data()?,
+                    p.get_query_data(),
                 )
             };
+
+            let ret = match data {
+                Some(data) => {
                     assert_eq!(id, data.partition_id());
 
                     // Project the data if necessary
@@ -288,17 +291,16 @@ where
                         Projection::Some(columns.as_ref())
                     };
 
-            let ret = PartitionResponse::new(
-                Box::pin(MemoryStream::new(
+                    let data = Box::pin(MemoryStream::new(
                         data.project_selection(selection).into_iter().collect(),
-                )),
-                id,
-                None,
-                completed_persistence_count,
-            );
+                    ));
+                    PartitionResponse::new(data, id, None, completed_persistence_count)
+                }
+                None => PartitionResponse::new_no_batches(id, None, completed_persistence_count),
+            };
 
             span.ok("read partition data");
-            Some(ret)
+            ret
         });
 
         Ok(PartitionStream::new(futures::stream::iter(partitions)))

ingester2/src/query/partition_response.rs

@@ -8,7 +8,7 @@ use datafusion::physical_plan::SendableRecordBatchStream;
 /// Response data for a single partition.
 pub(crate) struct PartitionResponse {
     /// Stream of snapshots.
-    batches: SendableRecordBatchStream,
+    batches: Option<SendableRecordBatchStream>,
 
     /// Partition ID.
     id: PartitionId,
@@ -42,7 +42,20 @@ impl PartitionResponse {
         completed_persistence_count: u64,
     ) -> Self {
         Self {
-            batches,
+            batches: Some(batches),
+            id,
+            max_persisted_sequence_number,
+            completed_persistence_count,
+        }
+    }
+
+    pub(crate) fn new_no_batches(
+        id: PartitionId,
+        max_persisted_sequence_number: Option<SequenceNumber>,
+        completed_persistence_count: u64,
+    ) -> Self {
+        Self {
+            batches: None,
             id,
             max_persisted_sequence_number,
             completed_persistence_count,
@@ -61,7 +74,7 @@ impl PartitionResponse {
         self.completed_persistence_count
     }
 
-    pub(crate) fn into_record_batch_stream(self) -> SendableRecordBatchStream {
+    pub(crate) fn into_record_batch_stream(self) -> Option<SendableRecordBatchStream> {
         self.batches
     }
 }

ingester2/src/query/response.rs

@@ -51,6 +51,7 @@ impl QueryResponse {
     /// Reduce the [`QueryResponse`] to a stream of [`RecordBatch`].
     pub(crate) fn into_record_batches(self) -> impl Stream<Item = Result<RecordBatch, ArrowError>> {
         self.into_partition_stream()
-            .flat_map(|partition| partition.into_record_batch_stream())
+            .flat_map(|partition| futures::stream::iter(partition.into_record_batch_stream()))
+            .flatten()
     }
 }

ingester2/src/server/grpc/query.rs

@@ -322,11 +322,13 @@ impl From<QueryResponse> for FlatIngesterQueryResponseStream {
                         completed_persistence_count,
                     })
                 });
-                let tail = partition
-                    .into_record_batch_stream()
-                    .flat_map(|snapshot_res| match snapshot_res {
+
+                match partition.into_record_batch_stream() {
+                    Some(stream) => {
+                        let tail = stream.flat_map(|snapshot_res| match snapshot_res {
                             Ok(snapshot) => {
-                            let schema = Arc::new(prepare_schema_for_flight(&snapshot.schema()));
+                                let schema =
+                                    Arc::new(prepare_schema_for_flight(&snapshot.schema()));
 
                                 let schema_captured = Arc::clone(&schema);
                                 let head = futures::stream::once(async {
@@ -335,8 +337,10 @@ impl From<QueryResponse> for FlatIngesterQueryResponseStream {
                                     })
                                 });
 
-                            let tail =
-                                match prepare_batch_for_flight(&snapshot, Arc::clone(&schema)) {
+                                let tail = match prepare_batch_for_flight(
+                                    &snapshot,
+                                    Arc::clone(&schema),
+                                ) {
                                     Ok(batch) => {
                                         futures::stream::iter(split_batch_for_grpc_response(batch))
                                             .map(|batch| {
@@ -353,6 +357,9 @@ impl From<QueryResponse> for FlatIngesterQueryResponseStream {
                         });
 
                         head.chain(tail).boxed()
+                    }
+                    None => head.boxed(),
+                }
             })
             .boxed()
     }