implement cost computation for limit

Author: xx01cyx

optd-cost-model/src/cost/limit.rs

@@ -0,0 +1,29 @@
+use optd_persistent::CostModelStorageLayer;
+
+use crate::{
+    common::{
+        nodes::{ArcPredicateNode, ReprPredicateNode},
+        predicates::constant_pred::ConstantPred,
+    },
+    cost_model::CostModelImpl,
+    CostModelResult, EstimatedStatistic,
+};
+
+impl<S: CostModelStorageLayer> CostModelImpl<S> {
+    pub(crate) fn get_limit_row_cnt(
+        &self,
+        child_row_cnt: EstimatedStatistic,
+        fetch_expr: ArcPredicateNode,
+    ) -> CostModelResult<EstimatedStatistic> {
+        let fetch = ConstantPred::from_pred_node(fetch_expr)
+            .unwrap()
+            .value()
+            .as_u64();
+        // u64::MAX represents None
+        if fetch == u64::MAX {
+            Ok(child_row_cnt)
+        } else {
+            Ok(EstimatedStatistic(child_row_cnt.0.min(fetch)))
+        }
+    }
+}

optd-cost-model/src/cost/mod.rs

@@ -1,3 +1,6 @@
+#![allow(unused)]
+
 pub mod agg;
 pub mod filter;
 pub mod join;
+pub mod limit;