Preserve constant values across union operations (#13805)

* Add value tracking to ConstExpr for improved union optimization

* Update PartialEq impl

* Minor change

* Add docstring for ConstExpr value

* Improve constant propagation across union partitions

* Add assertion for across_partitions

* fix fmt

* Update properties.rs

* Remove redundant constant removal loop

* Remove unnecessary mut

* Set across_partitions=true when both sides are constant

* Extract and use constant values in filter expressions

* Add initial SLT for constant value tracking across UNION ALL

* Assign values to ConstExpr where possible

* Revert "Set across_partitions=true when both sides are constant"

This reverts commit 3051cd4.

* Temporarily take value from literal

* Lint fixes

* Cargo fmt

* Add get_expr_constant_value

* Make `with_value()` accept optional value

* Add todo

* Move test to union.slt

* Fix changed slt after merge

* Simplify constexpr

* Update properties.rs

---------

Co-authored-by: berkaysynnada <[email protected]>

Author: gokselk

datafusion/physical-expr/src/equivalence/class.rs

@@ -24,7 +24,7 @@ use std::fmt::Display;
 use std::sync::Arc;
 
 use datafusion_common::tree_node::{Transformed, TransformedResult, TreeNode};
-use datafusion_common::JoinType;
+use datafusion_common::{JoinType, ScalarValue};
 use datafusion_physical_expr_common::physical_expr::format_physical_expr_list;
 
 use indexmap::{IndexMap, IndexSet};
@@ -55,13 +55,45 @@ use indexmap::{IndexMap, IndexSet};
 /// // create a constant expression from a physical expression
 /// let const_expr = ConstExpr::from(col);
 /// ```
+// TODO: Consider refactoring the `across_partitions` and `value` fields into an enum:
+//
+// ```
+// enum PartitionValues {
+//     Uniform(Option<ScalarValue>),           // Same value across all partitions
+//     Heterogeneous(Vec<Option<ScalarValue>>) // Different values per partition
+// }
+// ```
+//
+// This would provide more flexible representation of partition values.
+// Note: This is a breaking change for the equivalence API and should be
+// addressed in a separate issue/PR.
 #[derive(Debug, Clone)]
 pub struct ConstExpr {
     /// The  expression that is known to be constant (e.g. a `Column`)
     expr: Arc<dyn PhysicalExpr>,
     /// Does the constant have the same value across all partitions? See
     /// struct docs for more details
-    across_partitions: bool,
+    across_partitions: AcrossPartitions,
+}
+
+#[derive(PartialEq, Clone, Debug)]
+/// Represents whether a constant expression's value is uniform or varies across partitions.
+///
+/// The `AcrossPartitions` enum is used to describe the nature of a constant expression
+/// in a physical execution plan:
+///
+/// - `Heterogeneous`: The constant expression may have different values for different partitions.
+/// - `Uniform(Option<ScalarValue>)`: The constant expression has the same value across all partitions,
+///   or is `None` if the value is not specified.
+pub enum AcrossPartitions {
+    Heterogeneous,
+    Uniform(Option<ScalarValue>),
+}
+
+impl Default for AcrossPartitions {
+    fn default() -> Self {
+        Self::Heterogeneous
+    }
 }
 
 impl PartialEq for ConstExpr {
@@ -79,23 +111,23 @@ impl ConstExpr {
         Self {
             expr,
             // By default, assume constant expressions are not same across partitions.
-            across_partitions: false,
+            across_partitions: Default::default(),
         }
     }
 
     /// Set the `across_partitions` flag
     ///
     /// See struct docs for more details
-    pub fn with_across_partitions(mut self, across_partitions: bool) -> Self {
+    pub fn with_across_partitions(mut self, across_partitions: AcrossPartitions) -> Self {
         self.across_partitions = across_partitions;
         self
     }
 
     /// Is the  expression the same across all partitions?
     ///
     /// See struct docs for more details
-    pub fn across_partitions(&self) -> bool {
-        self.across_partitions
+    pub fn across_partitions(&self) -> AcrossPartitions {
+        self.across_partitions.clone()
     }
 
     pub fn expr(&self) -> &Arc<dyn PhysicalExpr> {
@@ -113,7 +145,7 @@ impl ConstExpr {
         let maybe_expr = f(&self.expr);
         maybe_expr.map(|expr| Self {
             expr,
-            across_partitions: self.across_partitions,
+            across_partitions: self.across_partitions.clone(),
         })
     }
 
@@ -143,14 +175,20 @@ impl ConstExpr {
     }
 }
 
-/// Display implementation for `ConstExpr`
-///
-/// Example `c` or `c(across_partitions)`
 impl Display for ConstExpr {
-    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "{}", self.expr)?;
-        if self.across_partitions {
-            write!(f, "(across_partitions)")?;
+        match &self.across_partitions {
+            AcrossPartitions::Heterogeneous => {
+                write!(f, "(heterogeneous)")?;
+            }
+            AcrossPartitions::Uniform(value) => {
+                if let Some(val) = value {
+                    write!(f, "(uniform: {})", val)?;
+                } else {
+                    write!(f, "(uniform: unknown)")?;
+                }
+            }
         }
         Ok(())
     }

datafusion/physical-expr/src/equivalence/mod.rs

@@ -27,7 +27,7 @@ mod ordering;
 mod projection;
 mod properties;
 
-pub use class::{ConstExpr, EquivalenceClass, EquivalenceGroup};
+pub use class::{AcrossPartitions, ConstExpr, EquivalenceClass, EquivalenceGroup};
 pub use ordering::OrderingEquivalenceClass;
 pub use projection::ProjectionMapping;
 pub use properties::{

datafusion/physical-expr/src/equivalence/ordering.rs

@@ -262,7 +262,7 @@ mod tests {
     };
     use crate::expressions::{col, BinaryExpr, Column};
     use crate::utils::tests::TestScalarUDF;
-    use crate::{ConstExpr, PhysicalExpr, PhysicalSortExpr};
+    use crate::{AcrossPartitions, ConstExpr, PhysicalExpr, PhysicalSortExpr};
 
     use arrow::datatypes::{DataType, Field, Schema};
     use arrow_schema::SortOptions;
@@ -583,9 +583,10 @@ mod tests {
             let eq_group = EquivalenceGroup::new(eq_group);
             eq_properties.add_equivalence_group(eq_group);
 
-            let constants = constants
-                .into_iter()
-                .map(|expr| ConstExpr::from(expr).with_across_partitions(true));
+            let constants = constants.into_iter().map(|expr| {
+                ConstExpr::from(expr)
+                    .with_across_partitions(AcrossPartitions::Uniform(None))
+            });
             eq_properties = eq_properties.with_constants(constants);
 
             let reqs = convert_to_sort_exprs(&reqs);