Copilot commented on code in PR #25580:
URL: https://github.com/apache/datafusion/pull/25580#discussion_r4076672824


##########
datafusion/expr/src/logical_plan/materialized_cte.rs:
##########
@@ -0,0 +1,186 @@
+// 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.
+
+//! Logical nodes for `WITH x AS MATERIALIZED (...)`.
+//!
+//! A materialized CTE is planned as one [`MaterializedCte`] node, which owns 
the
+//! CTE body and the rest of the query (the "continuation"), and one
+//! [`MaterializedCteScan`] leaf for each reference to the CTE inside the
+//! continuation. The body is therefore optimized and executed once, and the
+//! scans read its buffered output.
+//!
+//! The scans are leaves, so filters and projections of one reference are not
+//! pushed into the shared body. This is the same optimization fence that
+//! PostgreSQL applies to `MATERIALIZED` CTEs.
+
+use std::cmp::Ordering;
+use std::fmt;
+use std::hash::Hash;
+use std::sync::atomic::{AtomicU64, Ordering as AtomicOrdering};
+
+use datafusion_common::{DFSchemaRef, Result, assert_eq_or_internal_err};
+
+use crate::{Expr, LogicalPlan, UserDefinedLogicalNodeCore};
+
+/// Identifies one materialized CTE and binds its scans to it.
+///
+/// CTE names are not unique (two sibling subqueries can each declare `WITH 
t`),
+/// so the planner allocates a process-unique id.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
+pub struct MaterializedCteId(u64);
+
+impl MaterializedCteId {
+    /// Allocate a new unique id.
+    pub fn next() -> Self {
+        static NEXT: AtomicU64 = AtomicU64::new(0);
+        Self(NEXT.fetch_add(1, AtomicOrdering::Relaxed))
+    }
+
+    /// The numeric value of this id.
+    pub fn as_u64(self) -> u64 {
+        self.0
+    }
+}
+
+impl fmt::Display for MaterializedCteId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.0)
+    }
+}
+
+/// Computes `cte` once and evaluates `continuation`, in which every
+/// [`MaterializedCteScan`] with the same `id` reads the buffered output of
+/// `cte`. The output of this node is the output of `continuation`.
+#[derive(Debug, Clone, PartialEq, Eq, Hash)]
+pub struct MaterializedCte {
+    pub id: MaterializedCteId,
+    pub name: String,
+    pub cte: LogicalPlan,
+    pub continuation: LogicalPlan,
+}
+
+impl PartialOrd for MaterializedCte {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        match self.id.cmp(&other.id) {
+            Ordering::Equal => self.name.partial_cmp(&other.name),
+            ord => Some(ord),
+        }

Review Comment:
   `UserDefinedLogicalNode::dyn_ord` requires `Some(Equal)` iff `dyn_eq` is 
true, but this comparison ignores `cte` and `continuation`. Two nodes retaining 
the same id/name after an optimizer rewrites a child therefore compare equal 
even though derived `PartialEq` says they differ, which can make logical-plan 
ordering/equality-based rewrites misidentify plans. Preserve the cheap id/name 
ordering but return `None` when those keys match and the full nodes differ.
   
   This issue also appears on line 145 of the same file.



##########
datafusion/physical-plan/src/materialized_cte.rs:
##########
@@ -0,0 +1,606 @@
+// 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.
+
+//! Execution plans for `WITH x AS MATERIALIZED (...)`.
+//!
+//! [`MaterializedCteExec`] owns the CTE body and the continuation (the rest of
+//! the query). Every [`MaterializedCteScanExec`] in the continuation shares 
one
+//! [`MaterializedCteBuffer`] with it. The first consumer to execute runs every
+//! partition of the body to completion and buffers the output; every scan then
+//! replays that buffer.
+//!
+//! The body is fully materialized before any scan yields a row. This is a
+//! pipeline break, but it cannot deadlock when two consumers of the same CTE
+//! are read at different rates (for example the build and probe side of one
+//! hash join), which a bounded fan-out channel can.
+//!
+//! Buffered batches are accounted in the memory pool. When a reservation
+//! fails, the rest of that partition is written to a spill file, and the scans
+//! read the in-memory prefix and then the spill file, so row order within a
+//! partition is kept.
+
+use std::fmt;
+use std::sync::Arc;
+
+use arrow::datatypes::SchemaRef;
+use arrow::record_batch::RecordBatch;
+use datafusion_common::config::ConfigOptions;
+use datafusion_common::tree_node::TreeNodeRecursion;
+use datafusion_common::{Result, Statistics, internal_err};
+use datafusion_common_runtime::JoinSet;
+use datafusion_execution::memory_pool::{MemoryConsumer, MemoryReservation};
+use datafusion_execution::{SendableRecordBatchStream, SpillFile, TaskContext};
+use datafusion_physical_expr::{EquivalenceProperties, Partitioning, 
PhysicalExpr};
+use futures::{StreamExt, TryStreamExt};
+use parking_lot::Mutex;
+
+use crate::coop::cooperative;
+use crate::execution_plan::{
+    Boundedness, CardinalityEffect, EmissionType, ExecutionPlan, 
ExecutionPlanProperties,
+    PlanProperties, SchedulingType,
+};
+use crate::filter_pushdown::{
+    ChildFilterDescription, ChildPushdownResult, FilterDescription, 
FilterPushdownPhase,
+    FilterPushdownPropagation,
+};
+use crate::joins::utils::{OnceAsync, OnceFut};
+use crate::metrics::{
+    BaselineMetrics, ExecutionPlanMetricsSet, MetricBuilder, MetricsSet, 
SpillMetrics,
+};
+use crate::spill::spill_manager::SpillManager;
+use crate::statistics::{ChildStats, StatisticsArgs};
+use crate::stream::RecordBatchStreamAdapter;
+use crate::{
+    ChildrenPropertiesMode, DisplayAs, DisplayFormatType, 
ReplaceChildrenOptions,
+};
+
+/// The buffered output of one body partition: an in-memory prefix followed by
+/// an optional spill file with the remaining batches.
+struct BufferedPartition {
+    batches: Vec<RecordBatch>,
+    spill_file: Option<Arc<dyn SpillFile>>,
+    /// Released when the buffer is dropped.
+    _reservation: MemoryReservation,
+}
+
+/// The materialized output of a CTE body.
+struct MaterializedOutput {
+    partitions: Vec<BufferedPartition>,
+    spill_manager: SpillManager,
+}
+
+/// State shared by one [`MaterializedCteExec`] and all its
+/// [`MaterializedCteScanExec`]s.
+pub struct MaterializedCteBuffer {
+    id: u64,
+    name: String,
+    /// The body to run. [`MaterializedCteExec`] updates it every time it is
+    /// rebuilt (for example by a physical optimizer rule), so it is the body 
of
+    /// the final plan. A scan can run the body before the owning
+    /// `MaterializedCteExec` executes, for example from a scalar subquery.
+    body: Mutex<Option<Arc<dyn ExecutionPlan>>>,
+    output: Mutex<Arc<OnceAsync<MaterializedOutput>>>,
+    metrics: ExecutionPlanMetricsSet,
+}
+
+impl fmt::Debug for MaterializedCteBuffer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MaterializedCteBuffer")
+            .field("name", &self.name)
+            .finish_non_exhaustive()
+    }
+}
+
+impl MaterializedCteBuffer {
+    pub fn new(id: u64, name: impl Into<String>) -> Self {
+        Self {
+            id,
+            name: name.into(),
+            body: Mutex::new(None),
+            output: Mutex::new(Arc::default()),
+            metrics: ExecutionPlanMetricsSet::new(),
+        }
+    }
+
+    /// The id that binds scans to this buffer.
+    pub fn id(&self) -> u64 {
+        self.id
+    }
+
+    fn set_body(&self, body: Arc<dyn ExecutionPlan>) {
+        *self.body.lock() = Some(body);
+    }
+
+    fn reset(&self) {
+        *self.output.lock() = Arc::default();
+    }
+
+    /// Return a future that resolves once the body has been fully buffered.
+    /// The body runs at most once, whichever consumer asks first.
+    fn materialize(
+        self: &Arc<Self>,
+        context: Arc<TaskContext>,
+    ) -> Result<OnceFut<MaterializedOutput>> {
+        let Some(body) = self.body.lock().clone() else {
+            return internal_err!("MaterializedCte {} has no body", self.name);
+        };
+        let once = Arc::clone(&self.output.lock());
+        let name = self.name.clone();
+        let metrics = self.metrics.clone();
+        once.try_once(move || Ok(buffer_body(name, body, context, metrics)))
+    }
+}
+
+async fn buffer_body(
+    name: String,
+    body: Arc<dyn ExecutionPlan>,
+    context: Arc<TaskContext>,
+    metrics: ExecutionPlanMetricsSet,
+) -> Result<MaterializedOutput> {
+    let spill_manager = SpillManager::new(
+        context.runtime_env(),
+        SpillMetrics::new(&metrics, 0),
+        body.schema(),
+    )
+    .with_compression_type(context.session_config().spill_compression());
+    let buffered_rows = 
MetricBuilder::new(&metrics).global_counter("buffered_rows");
+
+    let mut join_set = JoinSet::new();
+    for partition in 0..body.output_partitioning().partition_count() {
+        let stream = body.execute(partition, Arc::clone(&context))?;
+        let reservation =
+            
MemoryConsumer::new(format!("MaterializedCte[{name}][{partition}]"))
+                .with_can_spill(true)
+                .register(context.memory_pool());
+        let spill_manager = spill_manager.clone();
+        let buffered_rows = buffered_rows.clone();
+        join_set.spawn(async move {
+            let result =
+                buffer_partition(stream, reservation, &spill_manager, 
&buffered_rows)
+                    .await;
+            (partition, result)
+        });
+    }
+
+    let mut partitions = Vec::with_capacity(join_set.len());
+    while let Some(joined) = join_set.join_next().await {
+        match joined {
+            Ok((partition, result)) => partitions.push((partition, result?)),
+            Err(e) if e.is_panic() => 
std::panic::resume_unwind(e.into_panic()),
+            Err(e) => return internal_err!("MaterializedCte task failed: {e}"),
+        }
+    }
+    partitions.sort_by_key(|(partition, _)| *partition);
+    Ok(MaterializedOutput {
+        partitions: partitions.into_iter().map(|(_, p)| p).collect(),
+        spill_manager,
+    })
+}
+
+async fn buffer_partition(
+    mut stream: SendableRecordBatchStream,
+    reservation: MemoryReservation,
+    spill_manager: &SpillManager,
+    buffered_rows: &crate::metrics::Count,
+) -> Result<BufferedPartition> {
+    let mut batches = vec![];
+    let mut spill = None;
+    while let Some(batch) = stream.next().await.transpose()? {
+        buffered_rows.add(batch.num_rows());
+        // Once a partition spills, every later batch goes to the same file,
+        // so replay keeps the order of the partition.
+        if spill.is_none() && 
reservation.try_grow(batch.get_array_memory_size()).is_ok()
+        {
+            batches.push(batch);
+            continue;
+        }
+        if spill.is_none() {
+            spill = 
Some(spill_manager.create_in_progress_file("MaterializedCte")?);
+        }
+        if let Some(file) = spill.as_mut() {
+            file.append_batch(&batch)?;
+        }
+    }
+    let spill_file = match spill {
+        Some(mut file) => file.finish()?,
+        None => None,
+    };
+    Ok(BufferedPartition {
+        batches,
+        spill_file,
+        _reservation: reservation,
+    })
+}
+
+/// Stream the buffered partitions `partition, partition + n, ...` of `output`,
+/// where `n` is the number of output partitions of the scan.
+fn replay(
+    output: &MaterializedOutput,
+    partition: usize,
+    output_partitions: usize,
+) -> Result<Vec<SendableRecordBatchStream>> {
+    let mut streams = vec![];
+    for buffered in output
+        .partitions
+        .iter()
+        .skip(partition)
+        .step_by(output_partitions)
+    {
+        let schema = Arc::clone(output.spill_manager.schema());
+        let batches = buffered.batches.clone();
+        streams.push(Box::pin(RecordBatchStreamAdapter::new(
+            schema,
+            futures::stream::iter(batches.into_iter().map(Ok)),
+        )) as SendableRecordBatchStream);
+        if let Some(file) = &buffered.spill_file {
+            streams.push(
+                output
+                    .spill_manager
+                    .read_spill_as_stream(Arc::clone(file), None)?,
+            );

Review Comment:
   Each replay calls the buffered spill reader, which decodes up to two batches 
ahead, but this memory is not covered by any `MemoryReservation`. The retained 
in-memory prefix can already consume nearly the entire pool, and every 
concurrent CTE reference adds another untracked read-ahead buffer, so a query 
can substantially exceed `memory_limit` after spilling. Replay needs accounted 
headroom/reservations (or an unbuffered, bounded-memory strategy), especially 
for multiple readers.



##########
datafusion/physical-plan/src/materialized_cte.rs:
##########
@@ -0,0 +1,606 @@
+// 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.
+
+//! Execution plans for `WITH x AS MATERIALIZED (...)`.
+//!
+//! [`MaterializedCteExec`] owns the CTE body and the continuation (the rest of
+//! the query). Every [`MaterializedCteScanExec`] in the continuation shares 
one
+//! [`MaterializedCteBuffer`] with it. The first consumer to execute runs every
+//! partition of the body to completion and buffers the output; every scan then
+//! replays that buffer.
+//!
+//! The body is fully materialized before any scan yields a row. This is a
+//! pipeline break, but it cannot deadlock when two consumers of the same CTE
+//! are read at different rates (for example the build and probe side of one
+//! hash join), which a bounded fan-out channel can.
+//!
+//! Buffered batches are accounted in the memory pool. When a reservation
+//! fails, the rest of that partition is written to a spill file, and the scans
+//! read the in-memory prefix and then the spill file, so row order within a
+//! partition is kept.
+
+use std::fmt;
+use std::sync::Arc;
+
+use arrow::datatypes::SchemaRef;
+use arrow::record_batch::RecordBatch;
+use datafusion_common::config::ConfigOptions;
+use datafusion_common::tree_node::TreeNodeRecursion;
+use datafusion_common::{Result, Statistics, internal_err};
+use datafusion_common_runtime::JoinSet;
+use datafusion_execution::memory_pool::{MemoryConsumer, MemoryReservation};
+use datafusion_execution::{SendableRecordBatchStream, SpillFile, TaskContext};
+use datafusion_physical_expr::{EquivalenceProperties, Partitioning, 
PhysicalExpr};
+use futures::{StreamExt, TryStreamExt};
+use parking_lot::Mutex;
+
+use crate::coop::cooperative;
+use crate::execution_plan::{
+    Boundedness, CardinalityEffect, EmissionType, ExecutionPlan, 
ExecutionPlanProperties,
+    PlanProperties, SchedulingType,
+};
+use crate::filter_pushdown::{
+    ChildFilterDescription, ChildPushdownResult, FilterDescription, 
FilterPushdownPhase,
+    FilterPushdownPropagation,
+};
+use crate::joins::utils::{OnceAsync, OnceFut};
+use crate::metrics::{
+    BaselineMetrics, ExecutionPlanMetricsSet, MetricBuilder, MetricsSet, 
SpillMetrics,
+};
+use crate::spill::spill_manager::SpillManager;
+use crate::statistics::{ChildStats, StatisticsArgs};
+use crate::stream::RecordBatchStreamAdapter;
+use crate::{
+    ChildrenPropertiesMode, DisplayAs, DisplayFormatType, 
ReplaceChildrenOptions,
+};
+
+/// The buffered output of one body partition: an in-memory prefix followed by
+/// an optional spill file with the remaining batches.
+struct BufferedPartition {
+    batches: Vec<RecordBatch>,
+    spill_file: Option<Arc<dyn SpillFile>>,
+    /// Released when the buffer is dropped.
+    _reservation: MemoryReservation,
+}
+
+/// The materialized output of a CTE body.
+struct MaterializedOutput {
+    partitions: Vec<BufferedPartition>,
+    spill_manager: SpillManager,
+}
+
+/// State shared by one [`MaterializedCteExec`] and all its
+/// [`MaterializedCteScanExec`]s.
+pub struct MaterializedCteBuffer {
+    id: u64,
+    name: String,
+    /// The body to run. [`MaterializedCteExec`] updates it every time it is
+    /// rebuilt (for example by a physical optimizer rule), so it is the body 
of
+    /// the final plan. A scan can run the body before the owning
+    /// `MaterializedCteExec` executes, for example from a scalar subquery.
+    body: Mutex<Option<Arc<dyn ExecutionPlan>>>,
+    output: Mutex<Arc<OnceAsync<MaterializedOutput>>>,
+    metrics: ExecutionPlanMetricsSet,
+}
+
+impl fmt::Debug for MaterializedCteBuffer {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("MaterializedCteBuffer")
+            .field("name", &self.name)
+            .finish_non_exhaustive()
+    }
+}
+
+impl MaterializedCteBuffer {
+    pub fn new(id: u64, name: impl Into<String>) -> Self {
+        Self {
+            id,
+            name: name.into(),
+            body: Mutex::new(None),
+            output: Mutex::new(Arc::default()),
+            metrics: ExecutionPlanMetricsSet::new(),
+        }
+    }
+
+    /// The id that binds scans to this buffer.
+    pub fn id(&self) -> u64 {
+        self.id
+    }
+
+    fn set_body(&self, body: Arc<dyn ExecutionPlan>) {
+        *self.body.lock() = Some(body);
+    }
+
+    fn reset(&self) {
+        *self.output.lock() = Arc::default();
+    }
+
+    /// Return a future that resolves once the body has been fully buffered.
+    /// The body runs at most once, whichever consumer asks first.
+    fn materialize(
+        self: &Arc<Self>,
+        context: Arc<TaskContext>,
+    ) -> Result<OnceFut<MaterializedOutput>> {
+        let Some(body) = self.body.lock().clone() else {
+            return internal_err!("MaterializedCte {} has no body", self.name);
+        };
+        let once = Arc::clone(&self.output.lock());
+        let name = self.name.clone();
+        let metrics = self.metrics.clone();
+        once.try_once(move || Ok(buffer_body(name, body, context, metrics)))
+    }
+}
+
+async fn buffer_body(
+    name: String,
+    body: Arc<dyn ExecutionPlan>,
+    context: Arc<TaskContext>,
+    metrics: ExecutionPlanMetricsSet,
+) -> Result<MaterializedOutput> {
+    let spill_manager = SpillManager::new(
+        context.runtime_env(),
+        SpillMetrics::new(&metrics, 0),
+        body.schema(),
+    )
+    .with_compression_type(context.session_config().spill_compression());
+    let buffered_rows = 
MetricBuilder::new(&metrics).global_counter("buffered_rows");
+
+    let mut join_set = JoinSet::new();
+    for partition in 0..body.output_partitioning().partition_count() {
+        let stream = body.execute(partition, Arc::clone(&context))?;
+        let reservation =
+            
MemoryConsumer::new(format!("MaterializedCte[{name}][{partition}]"))
+                .with_can_spill(true)
+                .register(context.memory_pool());
+        let spill_manager = spill_manager.clone();
+        let buffered_rows = buffered_rows.clone();
+        join_set.spawn(async move {
+            let result =
+                buffer_partition(stream, reservation, &spill_manager, 
&buffered_rows)
+                    .await;
+            (partition, result)
+        });
+    }
+
+    let mut partitions = Vec::with_capacity(join_set.len());
+    while let Some(joined) = join_set.join_next().await {
+        match joined {
+            Ok((partition, result)) => partitions.push((partition, result?)),
+            Err(e) if e.is_panic() => 
std::panic::resume_unwind(e.into_panic()),
+            Err(e) => return internal_err!("MaterializedCte task failed: {e}"),
+        }
+    }
+    partitions.sort_by_key(|(partition, _)| *partition);
+    Ok(MaterializedOutput {
+        partitions: partitions.into_iter().map(|(_, p)| p).collect(),
+        spill_manager,
+    })
+}
+
+async fn buffer_partition(
+    mut stream: SendableRecordBatchStream,
+    reservation: MemoryReservation,
+    spill_manager: &SpillManager,
+    buffered_rows: &crate::metrics::Count,
+) -> Result<BufferedPartition> {
+    let mut batches = vec![];
+    let mut spill = None;
+    while let Some(batch) = stream.next().await.transpose()? {
+        buffered_rows.add(batch.num_rows());
+        // Once a partition spills, every later batch goes to the same file,
+        // so replay keeps the order of the partition.
+        if spill.is_none() && 
reservation.try_grow(batch.get_array_memory_size()).is_ok()
+        {
+            batches.push(batch);
+            continue;
+        }
+        if spill.is_none() {
+            spill = 
Some(spill_manager.create_in_progress_file("MaterializedCte")?);
+        }
+        if let Some(file) = spill.as_mut() {
+            file.append_batch(&batch)?;
+        }
+    }
+    let spill_file = match spill {
+        Some(mut file) => file.finish()?,
+        None => None,
+    };
+    Ok(BufferedPartition {
+        batches,
+        spill_file,
+        _reservation: reservation,
+    })
+}
+
+/// Stream the buffered partitions `partition, partition + n, ...` of `output`,
+/// where `n` is the number of output partitions of the scan.
+fn replay(
+    output: &MaterializedOutput,
+    partition: usize,
+    output_partitions: usize,
+) -> Result<Vec<SendableRecordBatchStream>> {
+    let mut streams = vec![];
+    for buffered in output
+        .partitions
+        .iter()
+        .skip(partition)
+        .step_by(output_partitions)
+    {
+        let schema = Arc::clone(output.spill_manager.schema());
+        let batches = buffered.batches.clone();
+        streams.push(Box::pin(RecordBatchStreamAdapter::new(
+            schema,
+            futures::stream::iter(batches.into_iter().map(Ok)),
+        )) as SendableRecordBatchStream);
+        if let Some(file) = &buffered.spill_file {
+            streams.push(
+                output
+                    .spill_manager
+                    .read_spill_as_stream(Arc::clone(file), None)?,
+            );
+        }
+    }
+    Ok(streams)
+}
+
+/// Computes a CTE body once and runs the continuation, whose
+/// [`MaterializedCteScanExec`]s read the buffered body output.
+///
+/// Children: `[body, continuation]`. The output is the output of the
+/// continuation.
+#[derive(Debug)]
+pub struct MaterializedCteExec {
+    body: Arc<dyn ExecutionPlan>,
+    continuation: Arc<dyn ExecutionPlan>,
+    buffer: Arc<MaterializedCteBuffer>,
+    cache: Arc<PlanProperties>,
+}
+
+impl MaterializedCteExec {
+    pub fn new(
+        body: Arc<dyn ExecutionPlan>,
+        continuation: Arc<dyn ExecutionPlan>,
+        buffer: Arc<MaterializedCteBuffer>,
+    ) -> Self {
+        buffer.set_body(Arc::clone(&body));
+        let cache = Arc::clone(continuation.properties());
+        Self {
+            body,
+            continuation,
+            buffer,
+            cache,
+        }
+    }
+
+    pub fn buffer(&self) -> &Arc<MaterializedCteBuffer> {
+        &self.buffer
+    }
+}
+
+impl DisplayAs for MaterializedCteExec {
+    fn fmt_as(&self, t: DisplayFormatType, f: &mut fmt::Formatter) -> 
fmt::Result {
+        match t {
+            DisplayFormatType::Default | DisplayFormatType::Verbose => {
+                write!(f, "MaterializedCteExec: name={}", self.buffer.name)
+            }
+            DisplayFormatType::TreeRender => write!(f, "name={}", 
self.buffer.name),
+        }
+    }
+}
+
+impl ExecutionPlan for MaterializedCteExec {
+    fn name(&self) -> &'static str {
+        "MaterializedCteExec"
+    }
+
+    fn properties(&self) -> &Arc<PlanProperties> {
+        &self.cache
+    }
+
+    fn children(&self) -> Vec<&Arc<dyn ExecutionPlan>> {
+        vec![&self.body, &self.continuation]
+    }
+
+    fn apply_expressions(
+        &self,
+        _f: &mut dyn FnMut(&Arc<dyn PhysicalExpr>) -> 
Result<TreeNodeRecursion>,
+    ) -> Result<TreeNodeRecursion> {
+        Ok(TreeNodeRecursion::Continue)
+    }
+
+    fn replace_children(
+        self: Arc<Self>,
+        mut children: Vec<Arc<dyn ExecutionPlan>>,
+        _: ReplaceChildrenOptions,
+    ) -> Result<Arc<dyn ExecutionPlan>> {
+        if children.len() != 2 {
+            return internal_err!("MaterializedCteExec takes 2 children");
+        }
+        let continuation = children.pop().unwrap();
+        let body = children.pop().unwrap();
+        Ok(Arc::new(Self::new(
+            body,
+            continuation,
+            Arc::clone(&self.buffer),
+        )))
+    }
+
+    fn with_new_children(
+        self: Arc<Self>,
+        children: Vec<Arc<dyn ExecutionPlan>>,
+    ) -> Result<Arc<dyn ExecutionPlan>> {
+        self.replace_children(
+            children,
+            ReplaceChildrenOptions::new(ChildrenPropertiesMode::Recompute),
+        )
+    }
+
+    fn reset_state(self: Arc<Self>) -> Result<Arc<dyn ExecutionPlan>> {
+        self.buffer.reset();
+        Ok(self)
+    }
+
+    fn maintains_input_order(&self) -> Vec<bool> {
+        vec![false, true]
+    }
+
+    fn benefits_from_input_partitioning(&self) -> Vec<bool> {
+        vec![true, false]
+    }
+
+    fn execute(
+        &self,
+        partition: usize,
+        context: Arc<TaskContext>,
+    ) -> Result<SendableRecordBatchStream> {
+        // The body runs lazily, when the first scan polls it.
+        self.buffer.set_body(Arc::clone(&self.body));
+        self.continuation.execute(partition, context)
+    }
+
+    fn metrics(&self) -> Option<MetricsSet> {
+        Some(self.buffer.metrics.clone_inner())
+    }
+
+    fn child_stats_requests(&self, partition: Option<usize>) -> 
Vec<ChildStats> {
+        vec![ChildStats::Skip, ChildStats::At(partition)]
+    }
+
+    fn statistics_from_inputs(
+        &self,
+        input_stats: &[Arc<Statistics>],
+        _args: &StatisticsArgs,
+    ) -> Result<Arc<Statistics>> {
+        Ok(Arc::clone(&input_stats[1]))
+    }
+
+    fn cardinality_effect(&self) -> CardinalityEffect {
+        CardinalityEffect::Equal
+    }
+
+    /// The output is the output of the continuation, so parent filters go to
+    /// the continuation only. The body is shared by every scan and must not
+    /// be filtered for one consumer.
+    fn gather_filters_for_pushdown(
+        &self,
+        _phase: FilterPushdownPhase,
+        parent_filters: Vec<Arc<dyn PhysicalExpr>>,
+        _config: &ConfigOptions,
+    ) -> Result<FilterDescription> {
+        Ok(FilterDescription::new()
+            
.with_child(ChildFilterDescription::all_unsupported(&parent_filters))
+            .with_child(ChildFilterDescription::from_child(
+                &parent_filters,
+                &self.continuation,
+            )?))
+    }
+
+    fn handle_child_pushdown_result(
+        &self,
+        _phase: FilterPushdownPhase,
+        child_pushdown_result: ChildPushdownResult,
+        _config: &ConfigOptions,
+    ) -> Result<FilterPushdownPropagation<Arc<dyn ExecutionPlan>>> {
+        // The body always reports `No`, so a filter is handled when the
+        // continuation handles it.
+        Ok(FilterPushdownPropagation::if_any(child_pushdown_result))
+    }
+}
+
+/// Reads the buffered output of a [`MaterializedCteExec`] body.
+#[derive(Debug)]
+pub struct MaterializedCteScanExec {
+    id: u64,
+    name: String,
+    schema: SchemaRef,
+    buffer: Option<Arc<MaterializedCteBuffer>>,
+    metrics: ExecutionPlanMetricsSet,
+    cache: Arc<PlanProperties>,
+}
+
+impl MaterializedCteScanExec {
+    /// Create a scan that is not yet bound to a buffer. See [`Self::bind`].
+    pub fn new(
+        id: u64,
+        name: impl Into<String>,
+        schema: SchemaRef,
+        partitions: usize,
+    ) -> Self {
+        let cache = PlanProperties::new(
+            EquivalenceProperties::new(Arc::clone(&schema)),
+            Partitioning::UnknownPartitioning(partitions.max(1)),
+            EmissionType::Final,
+            Boundedness::Bounded,

Review Comment:
   The scan is declared bounded even when the CTE body is an unbounded source. 
Because the full-buffering pipeline break happens inside `materialize()` rather 
than in the scan's child tree, `SanityCheckPlan` sees a bounded final scan and 
accepts the query; execution then buffers/spills forever and never emits a row. 
The bound body must be propagated during binding, or materialized CTE planning 
must reject unbounded bodies.



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