refactor: update_coalesce_ctx_children to more tightly define the context, selectively indicating when coalesce should be removed

Author: wiedld

datafusion/physical-optimizer/src/enforce_sorting/mod.rs

@@ -56,7 +56,7 @@ use datafusion_common::config::ConfigOptions;
 use datafusion_common::plan_err;
 use datafusion_common::tree_node::{Transformed, TransformedResult, TreeNode};
 use datafusion_common::Result;
-use datafusion_physical_expr::{Distribution, Partitioning};
+use datafusion_physical_expr::Distribution;
 use datafusion_physical_expr_common::sort_expr::{LexOrdering, LexRequirement};
 use datafusion_physical_plan::coalesce_partitions::CoalescePartitionsExec;
 use datafusion_physical_plan::limit::{GlobalLimitExec, LocalLimitExec};
@@ -126,29 +126,67 @@ fn update_sort_ctx_children(
 /// [`CoalescePartitionsExec`] descendant(s) for every child of a plan. The data
 /// attribute stores whether the plan is a `CoalescePartitionsExec` or is
 /// connected to a `CoalescePartitionsExec` via its children.
+///
+/// The tracker halts at each [`SortExec`] (where the SPM will act to replace the coalesce).
+///
+/// This requires a bottom-up traversal was previously performed, updating the
+/// children previously.
 pub type PlanWithCorrespondingCoalescePartitions = PlanContext<bool>;
 
+/// Determines if the coalesce may be safely removed.
+fn is_coalesce_to_remove(
+    node: &Arc<dyn ExecutionPlan>,
+    parent: &Arc<dyn ExecutionPlan>,
+) -> bool {
+    node.as_any()
+        .downcast_ref::<CoalescePartitionsExec>()
+        .map(|_coalesce| {
+            // TODO(wiedld): find a more generalized approach that does not rely on
+            // pattern matching the structure of the DAG
+            // Note that the `Partitioning::satisfy()` (parent vs. coalesce.child) cannot be used for cases of:
+            //      * Repartition -> Coalesce -> Repartition
+
+            let parent_req_single_partition = matches!(
+                parent.required_input_distribution()[0],
+                Distribution::SinglePartition
+            );
+
+            // node above does not require single distribution
+            !parent_req_single_partition
+            // it doesn't immediately repartition
+            || is_repartition(parent)
+            // any adjacent Coalesce->Sort can be replaced
+            || is_sort(parent)
+        })
+        .unwrap_or(false)
+}
+
 fn update_coalesce_ctx_children(
     coalesce_context: &mut PlanWithCorrespondingCoalescePartitions,
 ) {
-    let children = &coalesce_context.children;
-    coalesce_context.data = if children.is_empty() {
-        // Plan has no children, it cannot be a `CoalescePartitionsExec`.
-        false
-    } else if is_coalesce_partitions(&coalesce_context.plan) {
-        // Initiate a connection:
-        true
-    } else {
-        children.iter().enumerate().any(|(idx, node)| {
-            // Only consider operators that don't require a single partition,
-            // and connected to some `CoalescePartitionsExec`:
-            node.data
-                && !matches!(
-                    coalesce_context.plan.required_input_distribution()[idx],
-                    Distribution::SinglePartition
-                )
-        })
-    };
+    // perform lookahead(1) during bottom up traversal
+    // since we are checking distribution requirements after the coalesce occurs
+    let parent = &coalesce_context.plan;
+
+    for child_context in coalesce_context.children.iter_mut() {
+        // determine if child, or it's descendents, are a coalesce to be removed
+        child_context.data = if child_context.children.is_empty() {
+            // Plan has no children, it cannot be a `CoalescePartitionsExec`.
+            false
+        } else if is_coalesce_to_remove(&child_context.plan, parent) {
+            // Initiate a connection:
+            true
+        } else if is_sort(&child_context.plan) {
+            // halt coalesce removals at the sort
+            false
+        } else {
+            // propogate
+            child_context
+                .children
+                .iter()
+                .any(|grandchild| grandchild.data)
+        };
+    }
 }
 
 /// The boolean flag `repartition_sorts` defined in the config indicates
@@ -246,32 +284,50 @@ fn replace_with_partial_sort(
 /// This function turns plans of the form
 /// ```text
 ///      "SortExec: expr=\[a@0 ASC\]",
-///      "  CoalescePartitionsExec",
-///      "    RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1",
+///      "  ...nodes..."
+///      "    CoalescePartitionsExec",
+///      "      RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1",
 /// ```
 /// to
 /// ```text
 ///      "SortPreservingMergeExec: \[a@0 ASC\]",
 ///      "  SortExec: expr=\[a@0 ASC\]",
-///      "    RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1",
+///      "    ...nodes..."
+///      "      RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1",
 /// ```
 /// by following connections from [`CoalescePartitionsExec`]s to [`SortExec`]s.
 /// By performing sorting in parallel, we can increase performance in some scenarios.
+///
+/// This requires that there are no nodes between the [`SortExec`] and [`CoalescePartitionsExec`]
+/// which require single partitioning. Do not parallelize when the following scenario occurs:
+/// ```text
+///      "SortExec: expr=\[a@0 ASC\]",
+///      "  ...nodes requiring single partitioning..."
+///      "    CoalescePartitionsExec",
+///      "      RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1",
+/// ```
 pub fn parallelize_sorts(
     mut requirements: PlanWithCorrespondingCoalescePartitions,
 ) -> Result<Transformed<PlanWithCorrespondingCoalescePartitions>> {
+    requirements = requirements.update_plan_from_children()?;
     update_coalesce_ctx_children(&mut requirements);
+    let coalesce_can_be_removed = requirements.children.iter().any(|child| child.data);
+
+    let should_parallelize_sort = (is_sort(&requirements.plan)
+        || is_sort_preserving_merge(&requirements.plan))
+        && requirements.plan.output_partitioning().partition_count() <= 1
+        && coalesce_can_be_removed;
+
+    // Repartition -> Coalesce -> Repartition
+    let unneeded_coalesce = is_repartition(&requirements.plan) && coalesce_can_be_removed;
 
     if requirements.children.is_empty() || !requirements.children[0].data {
         // We only take an action when the plan is either a `SortExec`, a
         // `SortPreservingMergeExec` or a `CoalescePartitionsExec`, and they
         // all have a single child. Therefore, if the first child has no
         // connection, we can return immediately.
         Ok(Transformed::no(requirements))
-    } else if (is_sort(&requirements.plan)
-        || is_sort_preserving_merge(&requirements.plan))
-        && requirements.plan.output_partitioning().partition_count() <= 1
-    {
+    } else if should_parallelize_sort {
         // Take the initial sort expressions and requirements
         let (sort_exprs, fetch) = get_sort_exprs(&requirements.plan)?;
         let sort_reqs = LexRequirement::from(sort_exprs.clone());
@@ -286,8 +342,11 @@ pub fn parallelize_sorts(
         // We also need to remove the self node since `remove_corresponding_coalesce_in_sub_plan`
         // deals with the children and their children and so on.
         requirements = requirements.children.swap_remove(0);
+        // sync the requirements.plan.children with the mutated requirements.children
+        requirements = requirements.update_plan_from_children()?;
 
         requirements = add_sort_above_with_check(requirements, sort_reqs, fetch);
+        requirements = requirements.update_plan_from_children()?;
 
         let spm =
             SortPreservingMergeExec::new(sort_exprs, Arc::clone(&requirements.plan));
@@ -298,20 +357,11 @@ pub fn parallelize_sorts(
                 vec![requirements],
             ),
         ))
-    } else if is_coalesce_partitions(&requirements.plan) {
-        // There is an unnecessary `CoalescePartitionsExec` in the plan.
-        // This will handle the recursive `CoalescePartitionsExec` plans.
+    } else if unneeded_coalesce {
         requirements = remove_bottleneck_in_subplan(requirements)?;
-        // For the removal of self node which is also a `CoalescePartitionsExec`.
-        requirements = requirements.children.swap_remove(0);
+        requirements = requirements.update_plan_from_children()?;
 
-        Ok(Transformed::yes(
-            PlanWithCorrespondingCoalescePartitions::new(
-                Arc::new(CoalescePartitionsExec::new(Arc::clone(&requirements.plan))),
-                false,
-                vec![requirements],
-            ),
-        ))
+        Ok(Transformed::yes(requirements))
     } else {
         Ok(Transformed::yes(requirements))
     }
@@ -546,19 +596,7 @@ fn remove_bottleneck_in_subplan(
             })
             .collect::<Result<_>>()?;
     }
-    let mut new_reqs = requirements.update_plan_from_children()?;
-    if let Some(repartition) = new_reqs.plan.as_any().downcast_ref::<RepartitionExec>() {
-        let input_partitioning = repartition.input().output_partitioning();
-        // We can remove this repartitioning operator if it is now a no-op:
-        let mut can_remove = input_partitioning.eq(repartition.partitioning());
-        // We can also remove it if we ended up with an ineffective RR:
-        if let Partitioning::RoundRobinBatch(n_out) = repartition.partitioning() {
-            can_remove |= *n_out == input_partitioning.partition_count();
-        }
-        if can_remove {
-            new_reqs = new_reqs.children.swap_remove(0)
-        }
-    }
+    let new_reqs = requirements.update_plan_from_children()?;
     Ok(new_reqs)
 }