perf(expr): new interface for expression directly returning scalar

src/expr/benches/expr.rs

@@ -205,6 +205,15 @@ fn bench_expr(c: &mut Criterion) {
             .to_async(FuturesExecutor)
             .iter(|| constant.eval(&input))
     });
+    c.bench_function("extract(constant)", |bencher| {
+        let extract = build_from_pretty(format!(
+            "(extract:decimal HOUR:varchar ${}:timestamp)",
+            input_index_for_type(DataType::Timestamp)
+        ));
+        bencher
+            .to_async(FuturesExecutor)
+            .iter(|| extract.eval(&input))
+    });
 
     let sigs = func_sigs();
     let sigs = sigs.sorted_by_cached_key(|sig| format!("{sig:?}"));

src/expr/src/expr/expr_literal.rs

@@ -13,16 +13,15 @@
 // limitations under the License.
 
 use std::convert::TryFrom;
-use std::sync::Arc;
 
-use risingwave_common::array::{ArrayBuilder, ArrayBuilderImpl, ArrayRef, DataChunk};
-use risingwave_common::for_all_variants;
+use risingwave_common::array::DataChunk;
 use risingwave_common::row::OwnedRow;
-use risingwave_common::types::{literal_type_match, DataType, Datum, Scalar, ScalarImpl};
+use risingwave_common::types::{literal_type_match, DataType, Datum};
 use risingwave_common::util::value_encoding::deserialize_datum;
 use risingwave_pb::expr::expr_node::{RexNode, Type};
 use risingwave_pb::expr::ExprNode;
 
+use super::ValueImpl;
 use crate::expr::Expression;
 use crate::{bail, ensure, ExprError, Result};
 
@@ -39,33 +38,11 @@ impl Expression for LiteralExpression {
         self.return_type.clone()
     }
 
-    async fn eval(&self, input: &DataChunk) -> Result<ArrayRef> {
-        let mut array_builder = self.return_type.create_array_builder(input.capacity());
-        let capacity = input.capacity();
-        let builder = &mut array_builder;
-        let literal = &self.literal;
-
-        macro_rules! array_impl_literal_append {
-            ($( { $variant_name:ident, $suffix_name:ident, $array:ty, $builder:ty } ),*) => {
-                match (builder, literal) {
-                    $(
-                        (ArrayBuilderImpl::$variant_name(inner), Some(ScalarImpl::$variant_name(v))) => {
-                            inner.append_n(capacity, Some(v.as_scalar_ref()));
-                        }
-                        (ArrayBuilderImpl::$variant_name(inner), None) => {
-                            inner.append_n(capacity, None);
-                        }
-                    )*
-                    (_, _) => $crate::bail!(
-                        "Do not support values in insert values executor".to_string()
-                    ),
-                }
-            };
-        }
-
-        for_all_variants! { array_impl_literal_append }
-
-        Ok(Arc::new(array_builder.finish()))
+    async fn eval_new(&self, input: &DataChunk) -> Result<ValueImpl> {
+        Ok(ValueImpl::Scalar {
+            value: self.literal.clone(),
+            capacity: input.capacity(),
+        })
     }
 
     async fn eval_row(&self, _input: &OwnedRow) -> Result<Datum> {
@@ -126,7 +103,7 @@ mod tests {
     use risingwave_common::array::{I32Array, StructValue};
     use risingwave_common::array_nonnull;
     use risingwave_common::types::test_utils::IntervalTestExt;
-    use risingwave_common::types::{Decimal, Interval, IntoOrdered};
+    use risingwave_common::types::{Decimal, Interval, IntoOrdered, Scalar, ScalarImpl};
     use risingwave_common::util::value_encoding::serialize_datum;
     use risingwave_pb::data::data_type::{IntervalType, TypeName};
     use risingwave_pb::data::{PbDataType, PbDatum};

src/expr/src/expr/mod.rs

@@ -64,9 +64,11 @@ pub(crate) mod data_types;
 pub(crate) mod template;
 pub(crate) mod template_fast;
 pub mod test_utils;
+mod value;
 
 use std::sync::Arc;
 
+use futures_util::TryFutureExt;
 use risingwave_common::array::{ArrayRef, DataChunk};
 use risingwave_common::row::{OwnedRow, Row};
 use risingwave_common::types::{DataType, Datum};
@@ -76,9 +78,14 @@ pub use self::agg::AggKind;
 pub use self::build::*;
 pub use self::expr_input_ref::InputRefExpression;
 pub use self::expr_literal::LiteralExpression;
+pub use self::value::{ValueImpl, ValueRef};
 use super::{ExprError, Result};
 
-/// Instance of an expression
+/// Interface of an expression.
+///
+/// There're two functions to evaluate an expression: `eval` and `eval_new`, exactly one of them
+/// should be implemented. Prefer calling and implementing `eval_new` instead of `eval` if possible,
+/// to gain the performance benefit of scalar expression.
 #[async_trait::async_trait]
 pub trait Expression: std::fmt::Debug + Sync + Send {
     /// Get the return data type.
@@ -94,14 +101,30 @@ pub trait Expression: std::fmt::Debug + Sync + Send {
         Ok(res)
     }
 
-    /// Evaluate the expression
+    /// Evaluate the expression in vectorized execution. Returns an array.
     ///
-    /// # Arguments
+    /// The default implementation calls `eval_new` and always converts the result to an array.
+    async fn eval(&self, input: &DataChunk) -> Result<ArrayRef> {
+        let value = self.eval_new(input).await?;
+        Ok(match value {
+            ValueImpl::Array(array) => array,
+            ValueImpl::Scalar { value, capacity } => {
+                let mut builder = self.return_type().create_array_builder(capacity);
+                builder.append_datum_n(capacity, value);
+                builder.finish().into()
+            }
+        })
+    }
+
+    /// Evaluate the expression in vectorized execution. Returns a value that can be either an
+    /// array, or a scalar if all values in the array are the same.
     ///
-    /// * `input` - input data of the Project Executor
-    async fn eval(&self, input: &DataChunk) -> Result<ArrayRef>;
+    /// The default implementation calls `eval` and puts the result into the `Array` variant.
+    async fn eval_new(&self, input: &DataChunk) -> Result<ValueImpl> {
+        self.eval(input).map_ok(ValueImpl::Array).await
+    }
 
-    /// Evaluate the expression in row-based execution.
+    /// Evaluate the expression in row-based execution. Returns a nullable scalar.
     async fn eval_row(&self, input: &OwnedRow) -> Result<Datum>;
 
     /// Evaluate if the expression is constant.

src/expr/src/expr/template.rs

@@ -19,49 +19,73 @@ use std::future::Future;
 use std::pin::Pin;
 use std::sync::Arc;
 
-use itertools::multizip;
+use itertools::{multizip, Itertools};
 use paste::paste;
-use risingwave_common::array::{Array, ArrayBuilder, ArrayImpl, ArrayRef, DataChunk, Utf8Array};
+use risingwave_common::array::{Array, ArrayBuilder, ArrayImpl, DataChunk, Utf8Array};
 use risingwave_common::row::OwnedRow;
 use risingwave_common::types::{option_as_scalar_ref, DataType, Datum, Scalar};
 use risingwave_common::util::iter_util::ZipEqDebug;
 
-use crate::expr::{BoxedExpression, Expression};
+use crate::expr::{BoxedExpression, Expression, ValueImpl, ValueRef};
 
 macro_rules! gen_eval {
     { ($macro:ident, $macro_row:ident), $ty_name:ident, $OA:ty, $($arg:ident,)* } => {
-        fn eval<'a, 'b, 'async_trait>(&'a self, data_chunk: &'b DataChunk)
-            -> Pin<Box<dyn Future<Output = $crate::Result<ArrayRef>> + Send + 'async_trait>>
+        fn eval_new<'a, 'b, 'async_trait>(&'a self, data_chunk: &'b DataChunk)
+            -> Pin<Box<dyn Future<Output = $crate::Result<ValueImpl>> + Send + 'async_trait>>
         where
             'a: 'async_trait,
             'b: 'async_trait,
         {
             Box::pin(async move { paste! {
                 $(
-                    let [<ret_ $arg:lower>] = self.[<expr_ $arg:lower>].eval_checked(data_chunk).await?;
-                    let [<arr_ $arg:lower>]: &$arg = [<ret_ $arg:lower>].as_ref().into();
+                    let [<ret_ $arg:lower>] = self.[<expr_ $arg:lower>].eval_new(data_chunk).await?;
+                    let [<val_ $arg:lower>]: ValueRef<'_, $arg> = (&[<ret_ $arg:lower>]).into();
                 )*
 
-                let bitmap = data_chunk.visibility();
-                let mut output_array = <$OA as Array>::Builder::with_meta(data_chunk.capacity(), (&self.return_type).into());
-                Ok(Arc::new(match bitmap {
-                    Some(bitmap) => {
-                        for (($([<v_ $arg:lower>], )*), visible) in multizip(($([<arr_ $arg:lower>].iter(), )*)).zip_eq_debug(bitmap.iter()) {
-                            if !visible {
-                                output_array.append_null();
-                                continue;
-                            }
-                            $macro!(self, output_array, $([<v_ $arg:lower>],)*)
+                Ok(match ($([<val_ $arg:lower>], )*) {
+                    // If all arguments are scalar, we can directly compute the result.
+                    ($(ValueRef::Scalar { value: [<scalar_ref_ $arg:lower>], capacity: [<cap_ $arg:lower>] }, )*) => {
+                        let output_scalar = $macro_row!(self, $([<scalar_ref_ $arg:lower>],)*);
+                        let output_datum = output_scalar.map(|s| s.to_scalar_value());
+                        let capacity = data_chunk.capacity();
+
+                        if cfg!(debug_assertions) {
+                            let all_capacities = [capacity, $([<cap_ $arg:lower>], )*];
+                            assert!(all_capacities.into_iter().all_equal(), "capacities mismatched: {:?}", all_capacities);
                         }
-                        output_array.finish().into()
+
+                        ValueImpl::Scalar { value: output_datum, capacity }
                     }
-                    None => {
-                        for ($([<v_ $arg:lower>], )*) in multizip(($([<arr_ $arg:lower>].iter(), )*)) {
-                            $macro!(self, output_array, $([<v_ $arg:lower>],)*)
-                        }
-                        output_array.finish().into()
+
+                    // Otherwise, fallback to array computation.
+                    // TODO: match all possible combinations to further get rid of the overhead of `Either` iterators.
+                    ($([<val_ $arg:lower>], )*) => {
+                        let bitmap = data_chunk.visibility();
+                        let mut output_array = <$OA as Array>::Builder::with_meta(data_chunk.capacity(), (&self.return_type).into());
+                        let array = match bitmap {
+                            Some(bitmap) => {
+                                // TODO: use `izip` here.
+                                for (($([<v_ $arg:lower>], )*), visible) in multizip(($([<val_ $arg:lower>].iter(), )*)).zip_eq_debug(bitmap.iter()) {
+                                    if !visible {
+                                        output_array.append_null();
+                                        continue;
+                                    }
+                                    $macro!(self, output_array, $([<v_ $arg:lower>],)*)
+                                }
+                                output_array.finish().into()
+                            }
+                            None => {
+                                // TODO: use `izip` here.
+                                for ($([<v_ $arg:lower>], )*) in multizip(($([<val_ $arg:lower>].iter(), )*)) {
+                                    $macro!(self, output_array, $([<v_ $arg:lower>],)*)
+                                }
+                                output_array.finish().into()
+                            }
+                        };
+
+                        ValueImpl::Array(Arc::new(array))
                     }
-                }))
+                })
             }})
         }
 
@@ -141,8 +165,8 @@ macro_rules! gen_expr_normal {
                 F: Fn($($arg::RefItem<'_>, )*) -> $crate::Result<OA::OwnedItem> + Sync + Send,
             > Expression for $ty_name<$($arg, )* OA, F>
             where
-                $(for<'a> &'a $arg: std::convert::From<&'a ArrayImpl>,)*
-                for<'a> &'a OA: std::convert::From<&'a ArrayImpl>,
+                $(for<'a> ValueRef<'a, $arg>: std::convert::From<&'a ValueImpl>,)*
+                for<'a> ValueRef<'a, OA>: std::convert::From<&'a ValueImpl>,
             {
                 fn return_type(&self) -> DataType {
                     self.return_type.clone()
@@ -227,7 +251,10 @@ macro_rules! gen_expr_bytes {
                 F: Fn($($arg::RefItem<'_>, )* &mut dyn std::fmt::Write) -> $crate::Result<()> + Sync + Send,
             > Expression for $ty_name<$($arg, )* F>
             where
-                $(for<'a> &'a $arg: std::convert::From<&'a ArrayImpl>,)*
+                $(
+                    for<'a> &'a $arg: std::convert::From<&'a ArrayImpl>,
+                    for<'a> ValueRef<'a, $arg>: std::convert::From<&'a ValueImpl>,
+                )*
             {
                 fn return_type(&self) -> DataType {
                     self.return_type.clone()
@@ -303,8 +330,8 @@ macro_rules! gen_expr_nullable {
                 F: Fn($(Option<$arg::RefItem<'_>>, )*) -> $crate::Result<Option<OA::OwnedItem>> + Sync + Send,
             > Expression for $ty_name<$($arg, )* OA, F>
             where
-                $(for<'a> &'a $arg: std::convert::From<&'a ArrayImpl>,)*
-                for<'a> &'a OA: std::convert::From<&'a ArrayImpl>,
+                $(for<'a> ValueRef<'a, $arg>: std::convert::From<&'a ValueImpl>,)*
+                for<'a> ValueRef<'a, OA>: std::convert::From<&'a ValueImpl>,
             {
                 fn return_type(&self) -> DataType {
                     self.return_type.clone()

src/expr/src/expr/value.rs

@@ -0,0 +1,75 @@
+// Copyright 2023 RisingWave Labs
+//
+// Licensed 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.
+
+use either::Either;
+use risingwave_common::array::*;
+use risingwave_common::for_all_variants;
+use risingwave_common::types::{Datum, Scalar};
+
+/// The type-erased return value of an expression.
+///
+/// It can be either an array, or a scalar if all values in the array are the same.
+#[derive(Debug, Clone)]
+pub enum ValueImpl {
+    Array(ArrayRef),
+    Scalar { value: Datum, capacity: usize },
+}
+
+/// The generic reference type of [`ValueImpl`]. Used as the arguments of expressions.
+#[derive(Debug, Clone, Copy)]
+pub enum ValueRef<'a, A: Array> {
+    Array(&'a A),
+    Scalar {
+        value: Option<<A as Array>::RefItem<'a>>,
+        capacity: usize,
+    },
+}
+
+impl<'a, A: Array> ValueRef<'a, A> {
+    /// Iterates over all scalars in this value.
+    pub fn iter(self) -> impl Iterator<Item = Option<A::RefItem<'a>>> + 'a {
+        match self {
+            Self::Array(array) => Either::Left(array.iter()),
+            Self::Scalar { value, capacity } => {
+                Either::Right(std::iter::repeat(value).take(capacity))
+            }
+        }
+    }
+}
+
+macro_rules! impl_convert {
+    ($( { $variant_name:ident, $suffix_name:ident, $array:ty, $builder:ty } ),*) => {
+        $(
+            paste::paste! {
+                /// Converts a type-erased value to a reference of a specific array type.
+                impl<'a> From<&'a ValueImpl> for ValueRef<'a, $array> {
+                    fn from(value: &'a ValueImpl) -> Self {
+                        match value {
+                            ValueImpl::Array(array) => {
+                                let array = array.[<as_ $suffix_name>]();
+                                ValueRef::Array(array)
+                            },
+                            ValueImpl::Scalar { value, capacity } => {
+                                let value = value.as_ref().map(|v| v.[<as_ $suffix_name>]().as_scalar_ref());
+                                ValueRef::Scalar { value, capacity: *capacity }
+                            },
+                        }
+                    }
+                }
+            }
+        )*
+    };
+}
+
+for_all_variants! { impl_convert }