add optimzer rule

Author: JanKaul

datafusion/optimizer/src/reorder_join/mod.rs

@@ -20,3 +20,6 @@
 pub mod cost;
 pub mod join_graph;
 pub mod left_deep_join_plan;
+pub mod rule;
+
+pub use rule::ReorderJoinRule;

datafusion/optimizer/src/reorder_join/rule.rs

@@ -0,0 +1,84 @@
+// 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.
+
+//! `OptimizerRule` wrapper for join reordering.
+//!
+//! Append this to an `Optimizer`'s rule list (or to a
+//! `SessionStateBuilder` via `with_optimizer_rule`) so the IK84 reorder
+//! runs *after* `ExtractEquijoinPredicate` has lifted equi-conditions
+//! into the joins' `on` clauses. Running it before that point leaves the
+//! reorder with empty-`on` cross-products and a disconnected join graph.
+
+use std::sync::Arc;
+
+use datafusion_common::{Result, tree_node::Transformed};
+use datafusion_expr::LogicalPlan;
+
+use crate::{OptimizerConfig, OptimizerRule};
+
+use super::{
+    cost::{DefaultCostEstimator, JoinCostEstimator},
+    left_deep_join_plan::optimal_left_deep_join_plan,
+};
+
+/// Optimizer-rule wrapper around [`optimal_left_deep_join_plan`].
+#[derive(Debug)]
+pub struct ReorderJoinRule {
+    estimator: Arc<dyn JoinCostEstimator + Send + Sync>,
+}
+
+impl ReorderJoinRule {
+    pub fn new(estimator: Arc<dyn JoinCostEstimator + Send + Sync>) -> Self {
+        Self { estimator }
+    }
+}
+
+impl Default for ReorderJoinRule {
+    fn default() -> Self {
+        Self::new(Arc::new(DefaultCostEstimator))
+    }
+}
+
+impl OptimizerRule for ReorderJoinRule {
+    fn name(&self) -> &str {
+        "reorder_join"
+    }
+
+    // `optimal_left_deep_join_plan` does its own top-level traversal and
+    // short-circuits when the plan has no joins, so we don't want the
+    // framework to walk the tree on our behalf.
+    fn apply_order(&self) -> Option<crate::optimizer::ApplyOrder> {
+        None
+    }
+
+    fn rewrite(
+        &self,
+        plan: LogicalPlan,
+        _config: &dyn OptimizerConfig,
+    ) -> Result<Transformed<LogicalPlan>> {
+        let before = plan.clone();
+        let after = optimal_left_deep_join_plan(plan, self.estimator.as_ref())?;
+        // IK84 is deterministic on a stable graph, so a second pass over
+        // an already-optimal plan reproduces the same chain and we
+        // converge by reporting no change.
+        if after == before {
+            Ok(Transformed::no(after))
+        } else {
+            Ok(Transformed::yes(after))
+        }
+    }
+}