feat: add TimeSeriesRowSelector hint

src/metric-engine/src/metadata_region.rs

@@ -348,6 +348,7 @@ impl MetadataRegion {
             filters: vec![],
             output_ordering: None,
             limit: None,
+            series_row_selector: None,
         };
         let record_batch_stream = self
             .mito
@@ -405,6 +406,7 @@ impl MetadataRegion {
             filters: vec![filter_expr],
             output_ordering: None,
             limit: None,
+            series_row_selector: None,
         }
     }
 
@@ -565,6 +567,7 @@ mod test {
             filters: vec![expected_filter_expr],
             output_ordering: None,
             limit: None,
+            series_row_selector: None,
         };
         let actual_scan_request = MetadataRegion::build_read_request(key);
         assert_eq!(actual_scan_request, expected_scan_request);

src/mito2/src/engine/projection_test.rs

@@ -78,6 +78,7 @@ async fn test_scan_projection() {
         filters: Vec::new(),
         output_ordering: None,
         limit: None,
+        series_row_selector: None,
     };
     let stream = engine.scan_to_stream(region_id, request).await.unwrap();
     let batches = RecordBatches::try_collect(stream).await.unwrap();

src/mito2/src/read/scan_region.rs

@@ -26,7 +26,7 @@ use common_time::Timestamp;
 use datafusion::physical_plan::DisplayFormatType;
 use smallvec::SmallVec;
 use store_api::region_engine::RegionScannerRef;
-use store_api::storage::ScanRequest;
+use store_api::storage::{ScanRequest, TimeSeriesRowSelector};
 use table::predicate::{build_time_range_predicate, Predicate};
 use tokio::sync::{mpsc, Mutex, Semaphore};
 use tokio_stream::wrappers::ReceiverStream;
@@ -297,7 +297,8 @@ impl ScanRegion {
             .with_start_time(self.start_time)
             .with_append_mode(self.version.options.append_mode)
             .with_filter_deleted(filter_deleted)
-            .with_merge_mode(self.version.options.merge_mode());
+            .with_merge_mode(self.version.options.merge_mode())
+            .with_series_row_selector(self.request.series_row_selector.clone());
         Ok(input)
     }
 
@@ -410,6 +411,8 @@ pub(crate) struct ScanInput {
     pub(crate) filter_deleted: bool,
     /// Mode to merge duplicate rows.
     pub(crate) merge_mode: MergeMode,
+    /// Hint to select rows from time series.
+    pub(crate) series_row_selector: Option<TimeSeriesRowSelector>,
 }
 
 impl ScanInput {
@@ -431,6 +434,7 @@ impl ScanInput {
             append_mode: false,
             filter_deleted: true,
             merge_mode: MergeMode::default(),
+            series_row_selector: None,
         }
     }
 
@@ -517,6 +521,16 @@ impl ScanInput {
         self
     }
 
+    /// Sets the time series row selector.
+    #[must_use]
+    pub(crate) fn with_series_row_selector(
+        mut self,
+        series_row_selector: Option<TimeSeriesRowSelector>,
+    ) -> Self {
+        self.series_row_selector = series_row_selector;
+        self
+    }
+
     /// Scans sources in parallel.
     ///
     /// # Panics if the input doesn't allow parallel scan.

src/store-api/src/storage.rs

@@ -25,5 +25,5 @@ pub use datatypes::schema::{
 };
 
 pub use self::descriptors::*;
-pub use self::requests::ScanRequest;
+pub use self::requests::{ScanRequest, TimeSeriesRowSelector};
 pub use self::types::SequenceNumber;

src/store-api/src/storage/requests.rs

@@ -15,6 +15,13 @@
 use common_recordbatch::OrderOption;
 use datafusion_expr::expr::Expr;
 
+/// A hint on how to select rows from a time-series.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub enum TimeSeriesRowSelector {
+    /// Only keep the last row of each time-series.
+    LastRow,
+}
+
 #[derive(Default, Clone, Debug, PartialEq, Eq)]
 pub struct ScanRequest {
     /// Indices of columns to read, `None` to read all columns. This indices is
@@ -29,4 +36,6 @@ pub struct ScanRequest {
     /// If set, it contains the amount of rows needed by the caller,
     /// The data source should return *at least* this number of rows if available.
     pub limit: Option<usize>,
+    /// Optional hint to select rows from time-series.
+    pub series_row_selector: Option<TimeSeriesRowSelector>,
 }