physical plan: separate children from children to explain

This patch adds a new method `children_to_explain` to the execution plan trait.
By default it returns the same as `children`. The method is needed in the
situation when plan wants to hide children from the optimizer, e.g. its child
is a plan that should be executed remotely, but still wants to show it in the
explain analysis.

Author: askalt

datafusion/physical-plan/src/display.rs

@@ -413,6 +413,14 @@ struct IndentVisitor<'a, 'b> {
 
 impl ExecutionPlanVisitor for IndentVisitor<'_, '_> {
     type Error = fmt::Error;
+
+    fn f_children<'p>(
+        &mut self,
+        plan: &'p dyn ExecutionPlan,
+    ) -> Vec<&'p std::sync::Arc<dyn ExecutionPlan>> {
+        plan.children_to_explain()
+    }
+
     fn pre_visit(&mut self, plan: &dyn ExecutionPlan) -> Result<bool, Self::Error> {
         write!(self.f, "{:indent$}", "", indent = self.indent * 2)?;
         plan.fmt_as(self.t, self.f)?;
@@ -491,6 +499,13 @@ impl GraphvizVisitor<'_, '_> {
 impl ExecutionPlanVisitor for GraphvizVisitor<'_, '_> {
     type Error = fmt::Error;
 
+    fn f_children<'p>(
+        &mut self,
+        plan: &'p dyn ExecutionPlan,
+    ) -> Vec<&'p std::sync::Arc<dyn ExecutionPlan>> {
+        plan.children_to_explain()
+    }
+
     fn pre_visit(&mut self, plan: &dyn ExecutionPlan) -> Result<bool, Self::Error> {
         let id = self.graphviz_builder.next_id();
 

datafusion/physical-plan/src/execution_plan.rs

@@ -207,6 +207,14 @@ pub trait ExecutionPlan: Debug + DisplayAs + Send + Sync {
     /// joins).
     fn children(&self) -> Vec<&Arc<dyn ExecutionPlan>>;
 
+    /// Get a list of children that must be displayed on the explanation stage.
+    /// Note, that this list can be different from [`Self::children`] if, e.g.,
+    /// some child is executed remotely and it is not required to apply plan
+    /// transformation for it, but it still should be displayed in explain.
+    fn children_to_explain(&self) -> Vec<&Arc<dyn ExecutionPlan>> {
+        self.children()
+    }
+
     /// Apply a closure `f` to each expression (non-recursively) in the current
     /// physical plan node. This does not include expressions in any children.
     ///

datafusion/physical-plan/src/render_tree.rs

@@ -141,7 +141,7 @@ impl RenderTree {
 /// # Returns
 /// * A tuple of (width, height) representing the dimensions needed for the tree
 fn get_tree_width_height(plan: &dyn ExecutionPlan) -> (usize, usize) {
-    let children = plan.children();
+    let children = plan.children_to_explain();
 
     // Leaf nodes take up 1x1 space
     if children.is_empty() {
@@ -210,7 +210,7 @@ fn create_tree_recursive(
 
     let mut node = RenderTreeNode::new(plan.name().to_string(), extra_info);
 
-    let children = plan.children();
+    let children = plan.children_to_explain();
 
     if children.is_empty() {
         result.set_node(x, y, Arc::new(node));

datafusion/physical-plan/src/visitor.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+use std::sync::Arc;
+
 use super::ExecutionPlan;
 
 /// Visit all children of this plan, according to the order defined on `ExecutionPlanVisitor`.
@@ -26,7 +28,7 @@ pub fn accept<V: ExecutionPlanVisitor>(
     visitor: &mut V,
 ) -> Result<(), V::Error> {
     visitor.pre_visit(plan)?;
-    for child in plan.children() {
+    for child in visitor.f_children(plan) {
         visit_execution_plan(child.as_ref(), visitor)?;
     }
     visitor.post_visit(plan)?;
@@ -63,6 +65,14 @@ pub trait ExecutionPlanVisitor {
     /// The type of error returned by this visitor
     type Error;
 
+    /// Invoked on an `ExecutionPlan` to acquire children list to visit.
+    fn f_children<'p>(
+        &mut self,
+        plan: &'p dyn ExecutionPlan,
+    ) -> Vec<&'p Arc<dyn ExecutionPlan>> {
+        plan.children()
+    }
+
     /// Invoked on an `ExecutionPlan` plan before any of its child
     /// inputs have been visited. If Ok(true) is returned, the
     /// recursion continues. If Err(..) or Ok(false) are returned, the
@@ -86,7 +96,7 @@ pub fn visit_execution_plan<V: ExecutionPlanVisitor>(
     visitor: &mut V,
 ) -> Result<(), V::Error> {
     visitor.pre_visit(plan)?;
-    for child in plan.children() {
+    for child in visitor.f_children(plan) {
         visit_execution_plan(child.as_ref(), visitor)?;
     }
     visitor.post_visit(plan)?;