alamb commented on code in PR #11652:
URL: https://github.com/apache/datafusion/pull/11652#discussion_r1693942386
##########
datafusion/sqllogictest/test_files/group_by.slt:
##########
@@ -4334,8 +4335,9 @@ physical_plan
01)GlobalLimitExec: skip=0, fetch=5
02)--SortPreservingMergeExec: [name@0 DESC,time_chunks@1 DESC], fetch=5
03)----ProjectionExec: expr=[name@0 as name, date_bin(IntervalMonthDayNano {
months: 0, days: 0, nanoseconds: 900000000000 }, ts@1) as time_chunks]
-04)------RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1
-05)--------StreamingTableExec: partition_sizes=1, projection=[name, ts],
infinite_source=true, output_ordering=[name@0 DESC, ts@1 DESC]
+04)------LocalLimitExec: fetch=5
+05)--------RepartitionExec: partitioning=RoundRobinBatch(8), input_partitions=1
Review Comment:
If I am able to pull the CoalesceBatch into Repartition I think adding
support for limit in repartition will become quite easy (as the actual limit
code will be reused)
##########
datafusion/core/src/physical_optimizer/limit_pushdown.rs:
##########
@@ -0,0 +1,661 @@
+// 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 rule reduces the amount of data transferred by pushing down limits as
much as possible.
+
+use std::fmt::Debug;
+use std::sync::Arc;
+
+use crate::error::Result;
+use crate::physical_optimizer::PhysicalOptimizerRule;
+use crate::physical_plan::ExecutionPlan;
+
+use datafusion_common::config::ConfigOptions;
+use datafusion_common::plan_datafusion_err;
+use datafusion_common::tree_node::{Transformed, TransformedResult, TreeNode};
+use datafusion_optimizer::push_down_limit::combine_limit;
+use datafusion_physical_plan::coalesce_partitions::CoalescePartitionsExec;
+use datafusion_physical_plan::limit::{GlobalLimitExec, LocalLimitExec};
+use
datafusion_physical_plan::sorts::sort_preserving_merge::SortPreservingMergeExec;
+
+/// This rule inspects [`ExecutionPlan`]'s and pushes down the fetch limit from
+/// the parent to the child if applicable.
+#[derive(Default)]
+pub struct LimitPushdown {}
+
+impl LimitPushdown {
+ #[allow(missing_docs)]
+ pub fn new() -> Self {
+ Self {}
+ }
+}
+
+impl PhysicalOptimizerRule for LimitPushdown {
+ fn optimize(
+ &self,
+ plan: Arc<dyn ExecutionPlan>,
+ _config: &ConfigOptions,
+ ) -> Result<Arc<dyn ExecutionPlan>> {
+ plan.transform_down(push_down_limits).data()
+ }
+
+ fn name(&self) -> &str {
+ "LimitPushdown"
+ }
+
+ fn schema_check(&self) -> bool {
+ true
+ }
+}
+
+/// This enumeration makes `skip` and `fetch` calculations easier by providing
+/// a single API for both local and global limit operators.
+#[derive(Debug)]
+enum LimitExec {
+ Global(GlobalLimitExec),
+ Local(LocalLimitExec),
+}
+
+impl LimitExec {
+ fn input(&self) -> &Arc<dyn ExecutionPlan> {
+ match self {
+ Self::Global(global) => global.input(),
+ Self::Local(local) => local.input(),
+ }
+ }
+
+ fn fetch(&self) -> Option<usize> {
+ match self {
+ Self::Global(global) => global.fetch(),
+ Self::Local(local) => Some(local.fetch()),
+ }
+ }
+
+ fn skip(&self) -> usize {
+ match self {
+ Self::Global(global) => global.skip(),
+ Self::Local(_) => 0,
+ }
+ }
+
+ fn with_child(&self, child: Arc<dyn ExecutionPlan>) -> Self {
+ match self {
+ Self::Global(global) => {
+ Self::Global(GlobalLimitExec::new(child, global.skip(),
global.fetch()))
+ }
+ Self::Local(local) => Self::Local(LocalLimitExec::new(child,
local.fetch())),
+ }
+ }
+}
+
+impl From<LimitExec> for Arc<dyn ExecutionPlan> {
+ fn from(limit_exec: LimitExec) -> Self {
+ match limit_exec {
+ LimitExec::Global(global) => Arc::new(global),
+ LimitExec::Local(local) => Arc::new(local),
+ }
+ }
+}
+
+/// Pushes down the limit through the plan.
+pub fn push_down_limits(
+ plan: Arc<dyn ExecutionPlan>,
+) -> Result<Transformed<Arc<dyn ExecutionPlan>>> {
+ let maybe_modified = if let Some(limit_exec) = extract_limit(&plan) {
+ let child = limit_exec.input();
+ if let Some(child_limit) = extract_limit(child) {
+ let merged = merge_limits(&limit_exec, &child_limit);
+ // Revisit current node in case of consecutive pushdowns
+ Some(push_down_limits(merged)?.data)
+ } else if child.supports_limit_pushdown() {
+ try_push_down_limit(&limit_exec, child.clone())?
+ } else {
+ add_fetch_to_child(&limit_exec, child.clone())
+ }
+ } else {
+ None
+ };
+
+ Ok(maybe_modified.map_or(Transformed::no(plan), Transformed::yes))
+}
+
+/// Transforms the [`ExecutionPlan`] into a [`LimitExec`] if it is a
+/// [`GlobalLimitExec`] or a [`LocalLimitExec`].
+fn extract_limit(plan: &Arc<dyn ExecutionPlan>) -> Option<LimitExec> {
+ if let Some(global_limit) =
plan.as_any().downcast_ref::<GlobalLimitExec>() {
+ Some(LimitExec::Global(GlobalLimitExec::new(
+ global_limit.input().clone(),
+ global_limit.skip(),
+ global_limit.fetch(),
+ )))
+ } else {
+ plan.as_any()
+ .downcast_ref::<LocalLimitExec>()
+ .map(|local_limit| {
+ LimitExec::Local(LocalLimitExec::new(
+ local_limit.input().clone(),
+ local_limit.fetch(),
+ ))
+ })
+ }
+}
+
+/// Merge the limits of the parent and the child. If at least one of them is a
+/// [`GlobalLimitExec`], the result is also a [`GlobalLimitExec`]. Otherwise,
+/// the result is a [`LocalLimitExec`].
+fn merge_limits(
Review Comment:
structurally this could be a method on `LimitExec` too , though this is
totally fine too
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