join-reordering

Author: JanKaul

datafusion/optimizer/src/reorder_join/cost.rs

@@ -0,0 +1,74 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use datafusion_common::{Result, plan_err};
+use datafusion_expr::{JoinType, LogicalPlan};
+
+use super::query_graph::Edge;
+
+pub trait JoinCostEstimator: std::fmt::Debug {
+    fn cardinality(&self, plan: &LogicalPlan) -> Option<f64> {
+        estimate_cardinality(plan).ok()
+    }
+
+    fn selectivity(&self, edge: &Edge) -> f64 {
+        match edge.join_type {
+            JoinType::Inner => 0.1,
+            _ => 1.0,
+        }
+    }
+
+    fn cost(&self, selectivity: f64, cardinality: f64) -> f64 {
+        selectivity * cardinality
+    }
+}
+
+/// Default implementation of JoinCostEstimator
+#[derive(Debug, Clone, Copy)]
+pub struct DefaultCostEstimator;
+
+impl JoinCostEstimator for DefaultCostEstimator {}
+
+fn estimate_cardinality(plan: &LogicalPlan) -> Result<f64> {
+    match plan {
+        LogicalPlan::Filter(filter) => {
+            let input_cardinality = estimate_cardinality(&filter.input)?;
+            Ok(0.1 * input_cardinality)
+        }
+        LogicalPlan::Aggregate(agg) => {
+            let input_cardinality = estimate_cardinality(&agg.input)?;
+            Ok(0.1 * input_cardinality)
+        }
+        LogicalPlan::TableScan(_) => {
+            // The logical-plan-level `TableSource` trait doesn't expose row
+            // statistics. To use cardinalities for base relations, override
+            // `JoinCostEstimator::cardinality`.
+            plan_err!(
+                "Default JoinCostEstimator cannot size TableScan; \
+                 override `cardinality` to provide statistics"
+            )
+        }
+        x => {
+            let inputs = x.inputs();
+            if inputs.len() == 1 {
+                estimate_cardinality(inputs[0])
+            } else {
+                plan_err!("Cannot estimate cardinality for plan with multiple inputs")
+            }
+        }
+    }
+}