[GitHub] [arrow-datafusion] ozankabak commented on a diff in pull request #5630: Introduce a common trait TreeNode for ExecutionPlan, PhysicalExpr, LogicalExpr, LogicalPlan

["ozankabak (via GitHub)" <gi...@apache.org>]

ozankabak commented on code in PR #5630:
URL: https://github.com/apache/arrow-datafusion/pull/5630#discussion_r1146268669


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datafusion/common/src/tree_node.rs:
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@@ -0,0 +1,261 @@
+// 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.
+
+//! This module provides common traits for visiting or rewriting tree nodes easily.
+
+use crate::{DataFusionError, Result};
+
+/// Trait for tree node. It can be [`ExecutionPlan`], [`PhysicalExpr`], [`LogicalExpr`], etc.
+pub trait TreeNode: Clone {
+    /// Return the children of this tree node
+    fn get_children(&self) -> Vec<Self>;
+
+    /// Use preorder to iterate the node on the tree so that we can stop fast for some cases.
+    ///
+    /// `op` can be used to collect some info from the tree node.
+    fn collect<F>(&self, op: &mut F) -> Result<()>
+    where
+        F: FnMut(&Self) -> Result<Recursion>,
+    {
+        match op(self)? {
+            Recursion::Continue => {}
+            // If the recursion should stop, do not visit children
+            Recursion::Stop => return Ok(()),
+            r => {
+                return Err(DataFusionError::Execution(format!(
+                    "Recursion {r:?} is not supported for collect"
+                )))
+            }
+        };
+
+        for child in self.get_children() {
+            child.collect(op)?;
+        }
+
+        Ok(())
+    }
+
+    /// Visit the tree node using the given [TreeNodeVisitor]
+    /// It performs a depth first walk of an node and its children.
+    ///
+    /// For an node tree such as
+    /// ```text
+    /// ParentNode
+    ///    left: ChildNode1
+    ///    right: ChildNode2
+    /// ```
+    ///
+    /// The nodes are visited using the following order
+    /// ```text
+    /// pre_visit(ParentNode)
+    /// pre_visit(ChildNode1)
+    /// post_visit(ChildNode1)
+    /// pre_visit(ChildNode2)
+    /// post_visit(ChildNode2)
+    /// post_visit(ParentNode)
+    /// ```
+    ///
+    /// If an Err result is returned, recursion is stopped immediately
+    ///
+    /// If [`Recursion::Stop`] is returned on a call to pre_visit, no
+    /// children of that node will be visited, nor is post_visit
+    /// called on that node
+    ///
+    /// If using the default [`post_visit`] with nothing to do, the [`collect`] should be preferred
+    fn visit<V: TreeNodeVisitor<N = Self>>(&self, visitor: &mut V) -> Result<Recursion> {
+        match visitor.pre_visit(self)? {
+            Recursion::Continue => {}
+            // If the recursion should stop, do not visit children
+            Recursion::Stop => return Ok(Recursion::Stop),
+            r => {
+                return Err(DataFusionError::Execution(format!(
+                    "Recursion {r:?} is not supported for collect_using"
+                )))
+            }
+        };
+
+        for child in self.get_children() {
+            match child.visit(visitor)? {
+                Recursion::Continue => {}
+                // If the recursion should stop, do not visit children
+                Recursion::Stop => return Ok(Recursion::Stop),
+                r => {
+                    return Err(DataFusionError::Execution(format!(
+                        "Recursion {r:?} is not supported for collect_using"
+                    )))
+                }
+            }
+        }
+
+        visitor.post_visit(self)
+    }
+
+    /// Convenience utils for writing optimizers rule: recursively apply the given `op` to the node tree.
+    /// When `op` does not apply to a given node, it is left unchanged.
+    /// The default tree traversal direction is transform_up(Postorder Traversal).
+    fn transform<F>(self, op: &F) -> Result<Self>
+    where
+        F: Fn(Self) -> Result<Option<Self>>,
+    {
+        self.transform_up(op)
+    }
+
+    /// Convenience utils for writing optimizers rule: recursively apply the given 'op' to the node and all of its
+    /// children(Preorder Traversal).
+    /// When the `op` does not apply to a given node, it is left unchanged.
+    fn transform_down<F>(self, op: &F) -> Result<Self>
+    where
+        F: Fn(Self) -> Result<Option<Self>>,
+    {
+        let node_cloned = self.clone();

Review Comment:
   I agree with @alamb that this is due to the chosen signature, not an unavoidable clone -- and we should fix it. Same problem in exists in `transform_up` function too. When no change is made, the cloning is done wastefully.



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