refactor(rust): Add elementwise select and with_columns to new streaming engine

crates/polars-core/src/frame/from.rs

@@ -28,24 +28,3 @@ impl TryFrom<StructArray> for DataFrame {
         DataFrame::new(columns)
     }
 }
-
-impl From<&Schema> for DataFrame {
-    fn from(schema: &Schema) -> Self {
-        let cols = schema
-            .iter()
-            .map(|(name, dtype)| Series::new_empty(name, dtype))
-            .collect();
-        unsafe { DataFrame::new_no_checks(cols) }
-    }
-}
-
-impl From<&ArrowSchema> for DataFrame {
-    fn from(schema: &ArrowSchema) -> Self {
-        let cols = schema
-            .fields
-            .iter()
-            .map(|fld| Series::new_empty(fld.name.as_str(), &(fld.data_type().into())))
-            .collect();
-        unsafe { DataFrame::new_no_checks(cols) }
-    }
-}

crates/polars-core/src/frame/mod.rs

@@ -318,6 +318,25 @@ impl DataFrame {
         unsafe { DataFrame::new_no_checks(Vec::new()) }
     }
 
+    /// Create an empty `DataFrame` with empty columns as per the `schema`.
+    pub fn empty_with_schema(schema: &Schema) -> Self {
+        let cols = schema
+            .iter()
+            .map(|(name, dtype)| Series::new_empty(name, dtype))
+            .collect();
+        unsafe { DataFrame::new_no_checks(cols) }
+    }
+
+    /// Create an empty `DataFrame` with empty columns as per the `schema`.
+    pub fn empty_with_arrow_schema(schema: &ArrowSchema) -> Self {
+        let cols = schema
+            .fields
+            .iter()
+            .map(|fld| Series::new_empty(fld.name.as_str(), &(fld.data_type().into())))
+            .collect();
+        unsafe { DataFrame::new_no_checks(cols) }
+    }
+
     /// Removes the last `Series` from the `DataFrame` and returns it, or [`None`] if it is empty.
     ///
     /// # Example

crates/polars-io/src/csv/read/read_impl.rs

@@ -475,9 +475,9 @@ impl<'a> CoreReader<'a> {
         // An empty file with a schema should return an empty DataFrame with that schema
         if bytes.is_empty() {
             let mut df = if projection.len() == self.schema.len() {
-                DataFrame::from(self.schema.as_ref())
+                DataFrame::empty_with_schema(self.schema.as_ref())
             } else {
-                DataFrame::from(
+                DataFrame::empty_with_schema(
                     &projection
                         .iter()
                         .map(|&i| self.schema.get_at_index(i).unwrap())

crates/polars-io/src/parquet/read/utils.rs

@@ -17,7 +17,7 @@ pub fn materialize_empty_df(
     } else {
         Cow::Borrowed(reader_schema)
     };
-    let mut df = DataFrame::from(schema.as_ref());
+    let mut df = DataFrame::empty_with_arrow_schema(&schema);
 
     if let Some(row_index) = row_index {
         df.insert_column(0, Series::new_empty(&row_index.name, &IDX_DTYPE))

crates/polars-pipe/src/executors/sinks/group_by/generic/hash_table.rs

@@ -215,7 +215,7 @@ impl<const FIXED: bool> AggHashTable<FIXED> {
         let (skip_len, take_len) = if let Some((offset, slice_len)) = slice {
             if *offset as usize >= local_len {
                 *offset -= local_len as i64;
-                return DataFrame::from(self.output_schema.as_ref());
+                return DataFrame::empty_with_schema(&self.output_schema);
             } else {
                 let out = (*offset as usize, *slice_len);
                 *offset = 0;

crates/polars-pipe/src/executors/sinks/group_by/utils.rs

@@ -57,7 +57,7 @@ pub(super) fn finalize_group_by(
     ooc_payload: Option<(IOThread, Box<dyn Sink>)>,
 ) -> PolarsResult<FinalizedSink> {
     let df = if dfs.is_empty() {
-        DataFrame::from(output_schema)
+        DataFrame::empty_with_schema(output_schema)
     } else {
         let mut df = accumulate_dataframes_vertical_unchecked(dfs);
         // re init to check duplicates

crates/polars-pipe/src/executors/sinks/ordered.rs

@@ -48,8 +48,8 @@ impl Sink for OrderedSink {
     }
     fn finalize(&mut self, _context: &PExecutionContext) -> PolarsResult<FinalizedSink> {
         if self.chunks.is_empty() {
-            return Ok(FinalizedSink::Finished(DataFrame::from(
-                self.schema.as_ref(),
+            return Ok(FinalizedSink::Finished(DataFrame::empty_with_schema(
+                &self.schema,
             )));
         }
         self.sort();

crates/polars-pipe/src/executors/sinks/slice.rs

@@ -94,8 +94,8 @@ impl Sink for SliceSink {
         let mut chunks = chunks.lock().unwrap();
         let chunks: Vec<DataChunk> = std::mem::take(chunks.as_mut());
         if chunks.is_empty() {
-            return Ok(FinalizedSink::Finished(DataFrame::from(
-                self.schema.as_ref(),
+            return Ok(FinalizedSink::Finished(DataFrame::empty_with_schema(
+                &self.schema,
             )));
         }
 

crates/polars-plan/src/plans/ir/schema.rs

@@ -1,3 +1,5 @@
+use recursive::recursive;
+
 use super::*;
 
 impl IR {
@@ -61,6 +63,7 @@ impl IR {
     }
 
     /// Get the schema of the logical plan node.
+    #[recursive]
     pub fn schema<'a>(&'a self, arena: &'a Arena<IR>) -> Cow<'a, SchemaRef> {
         use IR::*;
         let schema = match self {

crates/polars-plan/src/plans/optimizer/predicate_pushdown/mod.rs

@@ -380,7 +380,7 @@ impl<'a> PredicatePushDown<'a> {
                             }
                             if new_paths.is_empty() {
                                 let schema = output_schema.as_ref().unwrap_or(&file_info.schema);
-                                let df = DataFrame::from(schema.as_ref());
+                                let df = DataFrame::empty_with_schema(schema);
 
                                 return Ok(DataFrameScan {
                                     df: Arc::new(df),

crates/polars-sql/src/context.rs

@@ -478,7 +478,7 @@ impl SQLContext {
                 None => {
                     let tbl = table_name.to_string();
                     if let Some(lf) = self.table_map.get_mut(&tbl) {
-                        *lf = DataFrame::from(
+                        *lf = DataFrame::empty_with_schema(
                             lf.schema_with_arenas(&mut self.lp_arena, &mut self.expr_arena)
                                 .unwrap()
                                 .as_ref(),

crates/polars-stream/src/graph.rs

@@ -71,6 +71,7 @@ pub struct GraphNode {
 }
 
 /// A pipe sends data between nodes.
+#[allow(unused)] // TODO: remove.
 pub struct LogicalPipe {
     // Node that we send data to.
     sender: GraphNodeKey,

crates/polars-stream/src/lib.rs

@@ -1,6 +1,5 @@
 #![allow(unused)] // TODO: remove.
 
-#[allow(unused)] // TODO: remove.
 mod async_executor;
 #[allow(unused)] // TODO: remove.
 mod async_primitives;

crates/polars-stream/src/nodes/in_memory_sink.rs

@@ -1,25 +1,31 @@
-use std::cmp::Reverse;
-use std::collections::{BinaryHeap, VecDeque};
+use std::sync::Arc;
 
 use parking_lot::Mutex;
 use polars_core::frame::DataFrame;
+use polars_core::schema::Schema;
+use polars_core::series::Series;
 use polars_core::utils::accumulate_dataframes_vertical_unchecked;
-use polars_core::utils::rayon::iter::{IntoParallelIterator, ParallelIterator};
-use polars_core::POOL;
 use polars_error::PolarsResult;
 use polars_expr::state::ExecutionState;
-use polars_utils::priority::Priority;
-use polars_utils::sync::SyncPtr;
 
 use super::ComputeNode;
 use crate::async_executor::{JoinHandle, TaskScope};
 use crate::async_primitives::pipe::{Receiver, Sender};
 use crate::morsel::Morsel;
 use crate::utils::in_memory_linearize::linearize;
 
-#[derive(Default)]
 pub struct InMemorySink {
     morsels_per_pipe: Mutex<Vec<Vec<Morsel>>>,
+    schema: Arc<Schema>,
+}
+
+impl InMemorySink {
+    pub fn new(schema: Arc<Schema>) -> Self {
+        Self {
+            morsels_per_pipe: Mutex::default(),
+            schema,
+        }
+    }
 }
 
 impl ComputeNode for InMemorySink {
@@ -51,8 +57,12 @@ impl ComputeNode for InMemorySink {
     }
 
     fn finalize(&mut self) -> PolarsResult<Option<DataFrame>> {
-        let mut morsels_per_pipe = core::mem::take(&mut *self.morsels_per_pipe.get_mut());
+        let morsels_per_pipe = core::mem::take(&mut *self.morsels_per_pipe.get_mut());
         let dataframes = linearize(morsels_per_pipe);
-        Ok(Some(accumulate_dataframes_vertical_unchecked(dataframes)))
+        if dataframes.is_empty() {
+            Ok(Some(DataFrame::empty_with_schema(&self.schema)))
+        } else {
+            Ok(Some(accumulate_dataframes_vertical_unchecked(dataframes)))
+        }
     }
 }

crates/polars-stream/src/nodes/mod.rs

@@ -9,6 +9,7 @@ use crate::morsel::Morsel;
 pub mod filter;
 pub mod in_memory_sink;
 pub mod in_memory_source;
+pub mod select;
 pub mod simple_projection;
 
 pub trait ComputeNode {

crates/polars-stream/src/nodes/select.rs

@@ -0,0 +1,90 @@
+use std::sync::Arc;
+
+use polars_core::frame::DataFrame;
+use polars_core::schema::Schema;
+use polars_core::series::Series;
+use polars_error::PolarsResult;
+use polars_expr::prelude::PhysicalExpr;
+use polars_expr::state::ExecutionState;
+
+use super::ComputeNode;
+use crate::async_executor::{JoinHandle, TaskScope};
+use crate::async_primitives::pipe::{Receiver, Sender};
+use crate::morsel::Morsel;
+
+pub struct SelectNode {
+    selectors: Vec<Arc<dyn PhysicalExpr>>,
+    schema: Arc<Schema>,
+    extend_original: bool,
+}
+
+impl SelectNode {
+    pub fn new(
+        selectors: Vec<Arc<dyn PhysicalExpr>>,
+        schema: Arc<Schema>,
+        extend_original: bool,
+    ) -> Self {
+        Self {
+            selectors,
+            schema,
+            extend_original,
+        }
+    }
+}
+
+impl ComputeNode for SelectNode {
+    fn spawn<'env, 's>(
+        &'env self,
+        scope: &'s TaskScope<'s, 'env>,
+        _pipeline: usize,
+        recv: Vec<Receiver<Morsel>>,
+        send: Vec<Sender<Morsel>>,
+        state: &'s ExecutionState,
+    ) -> JoinHandle<PolarsResult<()>> {
+        let [mut recv] = <[_; 1]>::try_from(recv).ok().unwrap();
+        let [mut send] = <[_; 1]>::try_from(send).ok().unwrap();
+
+        scope.spawn_task(true, async move {
+            while let Ok(morsel) = recv.recv().await {
+                let morsel = morsel.try_map(|df| {
+                    // Select columns.
+                    let mut selected: Vec<Series> = self
+                        .selectors
+                        .iter()
+                        .map(|s| s.evaluate(&df, state))
+                        .collect::<PolarsResult<_>>()?;
+
+                    // Extend or create new dataframe.
+                    let ret = if self.extend_original {
+                        let mut out = df.clone();
+                        out._add_columns(selected, &self.schema)?;
+                        out
+                    } else {
+                        // Broadcast scalars.
+                        let max_non_unit_length = selected
+                            .iter()
+                            .map(|s| s.len())
+                            .filter(|l| *l != 1)
+                            .max()
+                            .unwrap_or(1);
+                        for s in &mut selected {
+                            if s.len() != max_non_unit_length {
+                                assert!(s.len() == 1, "got series of incompatible lengths");
+                                *s = s.new_from_index(0, max_non_unit_length);
+                            }
+                        }
+                        unsafe { DataFrame::new_no_checks(selected) }
+                    };
+
+                    PolarsResult::Ok(ret)
+                })?;
+
+                if send.send(morsel).await.is_err() {
+                    break;
+                }
+            }
+
+            Ok(())
+        })
+    }
+}

crates/polars-stream/src/physical_plan/lower_ir.rs

@@ -17,7 +17,50 @@ pub fn lower_ir(
     expr_arena: &mut Arena<AExpr>,
     phys_sm: &mut SlotMap<PhysNodeKey, PhysNode>,
 ) -> PolarsResult<PhysNodeKey> {
-    match ir_arena.get(node) {
+    let ir_node = ir_arena.get(node);
+    match ir_node {
+        IR::SimpleProjection { input, columns } => {
+            let schema = columns.clone();
+            let input = lower_ir(*input, ir_arena, expr_arena, phys_sm)?;
+            Ok(phys_sm.insert(PhysNode::SimpleProjection { input, schema }))
+        },
+
+        // TODO: split partially streamable selections to avoid fallback as much as possible.
+        IR::Select {
+            input,
+            expr,
+            schema,
+            ..
+        } if expr.iter().all(|e| is_streamable(e.node(), expr_arena)) => {
+            let selectors = expr.clone();
+            let schema = schema.clone();
+            let input = lower_ir(*input, ir_arena, expr_arena, phys_sm)?;
+            Ok(phys_sm.insert(PhysNode::Select {
+                input,
+                selectors,
+                schema,
+                extend_original: false,
+            }))
+        },
+
+        // TODO: split partially streamable selections to avoid fallback as much as possible.
+        IR::HStack {
+            input,
+            exprs,
+            schema,
+            ..
+        } if exprs.iter().all(|e| is_streamable(e.node(), expr_arena)) => {
+            let selectors = exprs.clone();
+            let schema = schema.clone();
+            let input = lower_ir(*input, ir_arena, expr_arena, phys_sm)?;
+            Ok(phys_sm.insert(PhysNode::Select {
+                input,
+                selectors,
+                schema,
+                extend_original: true,
+            }))
+        },
+
         IR::Filter { input, predicate } if is_streamable(predicate.node(), expr_arena) => {
             let predicate = predicate.clone();
             let input = lower_ir(*input, ir_arena, expr_arena, phys_sm)?;
@@ -57,8 +100,9 @@ pub fn lower_ir(
 
         IR::Sink { input, payload } => {
             if *payload == SinkType::Memory {
+                let schema = ir_node.schema(ir_arena).into_owned();
                 let input = lower_ir(*input, ir_arena, expr_arena, phys_sm)?;
-                return Ok(phys_sm.insert(PhysNode::InMemorySink { input }));
+                return Ok(phys_sm.insert(PhysNode::InMemorySink { input, schema }));
             }
 
             todo!()