feat: Add eliminate group by constant optimizer rule

datafusion/optimizer/src/eliminate_group_by_constant.rs

@@ -0,0 +1,318 @@
+// 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.
+
+//! [`EliminateGroupByConstant`] removes constant expressions from `GROUP BY` clause
+use crate::optimizer::ApplyOrder;
+use crate::{OptimizerConfig, OptimizerRule};
+
+use datafusion_common::tree_node::Transformed;
+use datafusion_common::{internal_err, Result};
+use datafusion_expr::{Aggregate, Expr, LogicalPlan, LogicalPlanBuilder, Volatility};
+
+/// Optimizer rule that removes constant expressions from `GROUP BY` clause
+/// and places additional projection on top of aggregation, to preserve
+/// original schema
+#[derive(Default)]
+pub struct EliminateGroupByConstant {}
+
+impl EliminateGroupByConstant {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+impl OptimizerRule for EliminateGroupByConstant {
+    fn supports_rewrite(&self) -> bool {
+        true
+    }
+
+    fn rewrite(
+        &self,
+        plan: LogicalPlan,
+        _config: &dyn OptimizerConfig,
+    ) -> Result<Transformed<LogicalPlan>> {
+        match plan {
+            LogicalPlan::Aggregate(aggregate) => {
+                let (const_group_expr, nonconst_group_expr): (Vec<_>, Vec<_>) = aggregate
+                    .group_expr
+                    .iter()
+                    .partition(|expr| is_constant_expression(expr));
+
+                // If no constant expressions found (nothing to optimize) or
+                // constant expression is the only expression in aggregate,
+                // optimization is skipped
+                if const_group_expr.is_empty()
+                    || (!const_group_expr.is_empty()
+                        && nonconst_group_expr.is_empty()
+                        && aggregate.aggr_expr.is_empty())
+                {
+                    return Ok(Transformed::no(LogicalPlan::Aggregate(aggregate)));
+                }
+
+                let simplified_aggregate = LogicalPlan::Aggregate(Aggregate::try_new(
+                    aggregate.input,
+                    nonconst_group_expr.into_iter().cloned().collect(),
+                    aggregate.aggr_expr.clone(),
+                )?);
+
+                let projection_expr =
+                    aggregate.group_expr.into_iter().chain(aggregate.aggr_expr);
+
+                let projection = LogicalPlanBuilder::from(simplified_aggregate)
+                    .project(projection_expr)?
+                    .build()?;
+
+                Ok(Transformed::yes(projection))
+            }
+            _ => Ok(Transformed::no(plan)),
+        }
+    }
+
+    fn try_optimize(
+        &self,
+        _plan: &LogicalPlan,
+        _config: &dyn OptimizerConfig,
+    ) -> Result<Option<LogicalPlan>> {
+        internal_err!("Should have called EliminateGroupByConstant::rewrite")
+    }
+
+    fn name(&self) -> &str {
+        "eliminate_group_by_constant"
+    }
+
+    fn apply_order(&self) -> Option<ApplyOrder> {
+        Some(ApplyOrder::BottomUp)
+    }
+}
+
+/// Checks if expression is constant, and can be eliminated from group by.
+///
+/// Intended to be used only within this rule, helper function, which heavily
+/// reiles on `SimplifyExpressions` result.
+fn is_constant_expression(expr: &Expr) -> bool {
+    match expr {
+        Expr::Alias(e) => is_constant_expression(&e.expr),
+        Expr::BinaryExpr(e) => {
+            is_constant_expression(&e.left) && is_constant_expression(&e.right)
+        }
+        Expr::Literal(_) => true,
+        Expr::ScalarFunction(e) => {
+            matches!(
+                e.func.signature().volatility,
+                Volatility::Immutable | Volatility::Stable
+            ) && e.args.iter().all(is_constant_expression)
+        }
+        _ => false,
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::test::*;
+
+    use arrow::datatypes::DataType;
+    use datafusion_common::Result;
+    use datafusion_expr::expr::ScalarFunction;
+    use datafusion_expr::{
+        col, count, lit, ColumnarValue, LogicalPlanBuilder, ScalarUDF, ScalarUDFImpl,
+        Signature, TypeSignature,
+    };
+
+    use std::sync::Arc;
+
+    #[derive(Debug)]
+    struct ScalarUDFMock {
+        signature: Signature,
+    }
+
+    impl ScalarUDFMock {
+        fn new_with_volatility(volatility: Volatility) -> Self {
+            Self {
+                signature: Signature::new(TypeSignature::Any(1), volatility),
+            }
+        }
+    }
+
+    impl ScalarUDFImpl for ScalarUDFMock {
+        fn as_any(&self) -> &dyn std::any::Any {
+            self
+        }
+        fn name(&self) -> &str {
+            "scalar_fn_mock"
+        }
+        fn signature(&self) -> &Signature {
+            &self.signature
+        }
+        fn return_type(&self, _args: &[DataType]) -> Result<DataType> {
+            Ok(DataType::Int32)
+        }
+        fn invoke(&self, _args: &[ColumnarValue]) -> Result<ColumnarValue> {
+            unimplemented!()
+        }
+    }
+
+    #[test]
+    fn test_eliminate_gby_literal() -> Result<()> {
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(vec![col("a"), lit(1u32)], vec![count(col("c"))])?
+            .build()?;
+
+        let expected = "\
+            Projection: test.a, UInt32(1), COUNT(test.c)\
+            \n  Aggregate: groupBy=[[test.a]], aggr=[[COUNT(test.c)]]\
+            \n    TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+
+    #[test]
+    fn test_eliminate_constant() -> Result<()> {
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(vec![lit("test"), lit(123u32)], vec![count(col("c"))])?
+            .build()?;
+
+        let expected = "\
+            Projection: Utf8(\"test\"), UInt32(123), COUNT(test.c)\
+            \n  Aggregate: groupBy=[[]], aggr=[[COUNT(test.c)]]\
+            \n    TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+
+    #[test]
+    fn test_no_op_no_constants() -> Result<()> {
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(vec![col("a"), col("b")], vec![count(col("c"))])?
+            .build()?;
+
+        let expected = "\
+            Aggregate: groupBy=[[test.a, test.b]], aggr=[[COUNT(test.c)]]\
+            \n  TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+
+    #[test]
+    fn test_no_op_only_constant() -> Result<()> {
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(vec![lit(123u32)], Vec::<Expr>::new())?
+            .build()?;
+
+        let expected = "\
+            Aggregate: groupBy=[[UInt32(123)]], aggr=[[]]\
+            \n  TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+
+    #[test]
+    fn test_eliminate_constant_with_alias() -> Result<()> {
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(
+                vec![lit(123u32).alias("const"), col("a")],
+                vec![count(col("c"))],
+            )?
+            .build()?;
+
+        let expected = "\
+            Projection: UInt32(123) AS const, test.a, COUNT(test.c)\
+            \n  Aggregate: groupBy=[[test.a]], aggr=[[COUNT(test.c)]]\
+            \n    TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+
+    #[test]
+    fn test_eliminate_scalar_fn_with_constant_arg() -> Result<()> {
+        let udf = ScalarUDF::new_from_impl(ScalarUDFMock::new_with_volatility(
+            Volatility::Immutable,
+        ));
+        let udf_expr =
+            Expr::ScalarFunction(ScalarFunction::new_udf(udf.into(), vec![lit(123u32)]));
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(vec![udf_expr, col("a")], vec![count(col("c"))])?
+            .build()?;
+
+        let expected = "\
+            Projection: scalar_fn_mock(UInt32(123)), test.a, COUNT(test.c)\
+            \n  Aggregate: groupBy=[[test.a]], aggr=[[COUNT(test.c)]]\
+            \n    TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+
+    #[test]
+    fn test_no_op_volatile_scalar_fn_with_constant_arg() -> Result<()> {
+        let udf = ScalarUDF::new_from_impl(ScalarUDFMock::new_with_volatility(
+            Volatility::Volatile,
+        ));
+        let udf_expr =
+            Expr::ScalarFunction(ScalarFunction::new_udf(udf.into(), vec![lit(123u32)]));
+        let scan = test_table_scan()?;
+        let plan = LogicalPlanBuilder::from(scan)
+            .aggregate(vec![udf_expr, col("a")], vec![count(col("c"))])?
+            .build()?;
+
+        let expected = "\
+            Aggregate: groupBy=[[scalar_fn_mock(UInt32(123)), test.a]], aggr=[[COUNT(test.c)]]\
+            \n  TableScan: test\
+        ";
+
+        assert_optimized_plan_eq(
+            Arc::new(EliminateGroupByConstant::new()),
+            plan,
+            expected,
+        )
+    }
+}

datafusion/optimizer/src/lib.rs

@@ -35,6 +35,7 @@ pub mod decorrelate_predicate_subquery;
 pub mod eliminate_cross_join;
 pub mod eliminate_duplicated_expr;
 pub mod eliminate_filter;
+pub mod eliminate_group_by_constant;
 pub mod eliminate_join;
 pub mod eliminate_limit;
 pub mod eliminate_nested_union;

datafusion/optimizer/src/optimizer.rs

@@ -35,6 +35,7 @@ use crate::decorrelate_predicate_subquery::DecorrelatePredicateSubquery;
 use crate::eliminate_cross_join::EliminateCrossJoin;
 use crate::eliminate_duplicated_expr::EliminateDuplicatedExpr;
 use crate::eliminate_filter::EliminateFilter;
+use crate::eliminate_group_by_constant::EliminateGroupByConstant;
 use crate::eliminate_join::EliminateJoin;
 use crate::eliminate_limit::EliminateLimit;
 use crate::eliminate_nested_union::EliminateNestedUnion;
@@ -262,6 +263,7 @@ impl Optimizer {
             Arc::new(SimplifyExpressions::new()),
             Arc::new(UnwrapCastInComparison::new()),
             Arc::new(CommonSubexprEliminate::new()),
+            Arc::new(EliminateGroupByConstant::new()),
             Arc::new(OptimizeProjections::new()),
         ];
 

datafusion/sqllogictest/test_files/explain.slt

@@ -209,6 +209,7 @@ logical_plan after single_distinct_aggregation_to_group_by SAME TEXT AS ABOVE
 logical_plan after simplify_expressions SAME TEXT AS ABOVE
 logical_plan after unwrap_cast_in_comparison SAME TEXT AS ABOVE
 logical_plan after common_sub_expression_eliminate SAME TEXT AS ABOVE
+logical_plan after eliminate_group_by_constant SAME TEXT AS ABOVE
 logical_plan after optimize_projections TableScan: simple_explain_test projection=[a, b, c]
 logical_plan after eliminate_nested_union SAME TEXT AS ABOVE
 logical_plan after simplify_expressions SAME TEXT AS ABOVE
@@ -235,6 +236,7 @@ logical_plan after single_distinct_aggregation_to_group_by SAME TEXT AS ABOVE
 logical_plan after simplify_expressions SAME TEXT AS ABOVE
 logical_plan after unwrap_cast_in_comparison SAME TEXT AS ABOVE
 logical_plan after common_sub_expression_eliminate SAME TEXT AS ABOVE
+logical_plan after eliminate_group_by_constant SAME TEXT AS ABOVE
 logical_plan after optimize_projections SAME TEXT AS ABOVE
 logical_plan TableScan: simple_explain_test projection=[a, b, c]
 initial_physical_plan CsvExec: file_groups={1 group: [[WORKSPACE_ROOT/datafusion/core/tests/data/example.csv]]}, projection=[a, b, c], has_header=true