perf: acclerate scatter query (GreptimeTeam#4607)

Signed-off-by: Ruihang Xia <waynestxia@gmail.com>

src/query/src/promql/planner.rs

@@ -90,6 +90,12 @@ const FIELD_COLUMN_MATCHER: &str = "__field__";
 /// Special modifier for cross schema query
 const SCHEMA_COLUMN_MATCHER: &str = "__schema__";
 
+/// Threshold for scatter scan mode
+const MAX_SCATTER_POINTS: i64 = 400;
+
+/// Interval 1 hour in millisecond
+const INTERVAL_1H: i64 = 60 * 60 * 1000;
+
 #[derive(Default, Debug, Clone)]
 struct PromPlannerContext {
     // query parameters
@@ -726,21 +732,10 @@ impl PromPlanner {
             Some(Offset::Neg(duration)) => -(duration.as_millis() as Millisecond),
             None => 0,
         };
-        let range_ms = self.ctx.range.unwrap_or_default();
         let mut scan_filters = self.matchers_to_expr(label_matchers.clone())?;
-        scan_filters.push(self.create_time_index_column_expr()?.gt_eq(DfExpr::Literal(
-            ScalarValue::TimestampMillisecond(
-                Some(self.ctx.start - offset_duration - self.ctx.lookback_delta - range_ms),
-                None,
-            ),
-        )));
-        scan_filters.push(self.create_time_index_column_expr()?.lt_eq(DfExpr::Literal(
-            ScalarValue::TimestampMillisecond(
-                Some(self.ctx.end - offset_duration + self.ctx.lookback_delta),
-                None,
-            ),
-        )));
-
+        if let Some(time_index_filter) = self.build_time_index_filter(offset_duration)? {
+            scan_filters.push(time_index_filter);
+        }
         // make table scan with filter exprs
         let table_ref = self.table_ref()?;
         let mut table_scan = self
@@ -967,6 +962,54 @@ impl PromPlanner {
         Ok(table_ref)
     }
 
+    fn build_time_index_filter(&self, offset_duration: i64) -> Result<Option<DfExpr>> {
+        let start = self.ctx.start;
+        let end = self.ctx.end;
+        let lookback_delta = self.ctx.lookback_delta;
+        let range = self.ctx.range.unwrap_or_default();
+        let interval = self.ctx.interval;
+        let time_index_expr = self.create_time_index_column_expr()?;
+        let num_points = (end - start) / interval;
+
+        // Scan a continuous time range
+        if (end - start) / interval > MAX_SCATTER_POINTS || interval <= INTERVAL_1H {
+            let single_time_range = time_index_expr
+                .clone()
+                .gt_eq(DfExpr::Literal(ScalarValue::TimestampMillisecond(
+                    Some(self.ctx.start - offset_duration - self.ctx.lookback_delta - range),
+                    None,
+                )))
+                .and(
+                    time_index_expr.lt_eq(DfExpr::Literal(ScalarValue::TimestampMillisecond(
+                        Some(self.ctx.end - offset_duration + self.ctx.lookback_delta),
+                        None,
+                    ))),
+                );
+            return Ok(Some(single_time_range));
+        }
+
+        // Otherwise scan scatter ranges separately
+        let mut filters = Vec::with_capacity(num_points as usize);
+        for timestamp in (start..end).step_by(interval as usize) {
+            filters.push(
+                time_index_expr
+                    .clone()
+                    .gt_eq(DfExpr::Literal(ScalarValue::TimestampMillisecond(
+                        Some(timestamp - offset_duration - lookback_delta - range),
+                        None,
+                    )))
+                    .and(time_index_expr.clone().lt_eq(DfExpr::Literal(
+                        ScalarValue::TimestampMillisecond(
+                            Some(timestamp - offset_duration + lookback_delta),
+                            None,
+                        ),
+                    ))),
+            )
+        }
+
+        Ok(filters.into_iter().reduce(DfExpr::or))
+    }
+
     /// Create a table scan plan and a filter plan with given filter.
     ///
     /// # Panic