datafusion_physical_plan/
visitor.rs

1// Licensed to the Apache Software Foundation (ASF) under one
2// or more contributor license agreements.  See the NOTICE file
3// distributed with this work for additional information
4// regarding copyright ownership.  The ASF licenses this file
5// to you under the Apache License, Version 2.0 (the
6// "License"); you may not use this file except in compliance
7// with the License.  You may obtain a copy of the License at
8//
9//   http://www.apache.org/licenses/LICENSE-2.0
10//
11// Unless required by applicable law or agreed to in writing,
12// software distributed under the License is distributed on an
13// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
14// KIND, either express or implied.  See the License for the
15// specific language governing permissions and limitations
16// under the License.
17
18use super::ExecutionPlan;
19
20/// Visit all children of this plan, according to the order defined on `ExecutionPlanVisitor`.
21// Note that this would be really nice if it were a method on
22// ExecutionPlan, but it can not be because it takes a generic
23// parameter and `ExecutionPlan` is a trait
24pub fn accept<V: ExecutionPlanVisitor>(
25    plan: &dyn ExecutionPlan,
26    visitor: &mut V,
27) -> Result<(), V::Error> {
28    visitor.pre_visit(plan)?;
29    for child in plan.children() {
30        visit_execution_plan(child.as_ref(), visitor)?;
31    }
32    visitor.post_visit(plan)?;
33    Ok(())
34}
35
36/// Trait that implements the [Visitor
37/// pattern](https://en.wikipedia.org/wiki/Visitor_pattern) for a
38/// depth first walk of `ExecutionPlan` nodes. `pre_visit` is called
39/// before any children are visited, and then `post_visit` is called
40/// after all children have been visited.
41///
42/// To use, define a struct that implements this trait and then invoke
43/// ['accept'].
44///
45/// For example, for an execution plan that looks like:
46///
47/// ```text
48/// ProjectionExec: id
49///    FilterExec: state = CO
50///       DataSourceExec:
51/// ```
52///
53/// The sequence of visit operations would be:
54/// ```text
55/// visitor.pre_visit(ProjectionExec)
56/// visitor.pre_visit(FilterExec)
57/// visitor.pre_visit(DataSourceExec)
58/// visitor.post_visit(DataSourceExec)
59/// visitor.post_visit(FilterExec)
60/// visitor.post_visit(ProjectionExec)
61/// ```
62pub trait ExecutionPlanVisitor {
63    /// The type of error returned by this visitor
64    type Error;
65
66    /// Invoked on an `ExecutionPlan` plan before any of its child
67    /// inputs have been visited. If Ok(true) is returned, the
68    /// recursion continues. If Err(..) or Ok(false) are returned, the
69    /// recursion stops immediately and the error, if any, is returned
70    /// to `accept`
71    fn pre_visit(&mut self, plan: &dyn ExecutionPlan) -> Result<bool, Self::Error>;
72
73    /// Invoked on an `ExecutionPlan` plan *after* all of its child
74    /// inputs have been visited. The return value is handled the same
75    /// as the return value of `pre_visit`. The provided default
76    /// implementation returns `Ok(true)`.
77    fn post_visit(&mut self, _plan: &dyn ExecutionPlan) -> Result<bool, Self::Error> {
78        Ok(true)
79    }
80}
81
82/// Recursively calls `pre_visit` and `post_visit` for this node and
83/// all of its children, as described on [`ExecutionPlanVisitor`]
84pub fn visit_execution_plan<V: ExecutionPlanVisitor>(
85    plan: &dyn ExecutionPlan,
86    visitor: &mut V,
87) -> Result<(), V::Error> {
88    visitor.pre_visit(plan)?;
89    for child in plan.children() {
90        visit_execution_plan(child.as_ref(), visitor)?;
91    }
92    visitor.post_visit(plan)?;
93    Ok(())
94}
datafusion_physical_plan/visitor.rs

datafusion_physical_plan/
visitor.rs
