[GitHub] [arrow-datafusion] alamb commented on a change in pull request #1729: Add approx-median operator

[GitBox <gi...@apache.org>]

alamb commented on a change in pull request #1729:
URL: https://github.com/apache/arrow-datafusion/pull/1729#discussion_r801549186



##########
File path: datafusion/src/physical_plan/aggregates.rs
##########
@@ -924,6 +939,62 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn test_median_expr() -> Result<()> {
+        let funcs = vec![AggregateFunction::ApproxMedian];
+        let data_types = vec![
+            DataType::UInt32,
+            DataType::UInt64,
+            DataType::Int32,
+            DataType::Int64,
+            DataType::Float32,
+            DataType::Float64,
+        ];
+        for fun in funcs {
+            for data_type in &data_types {
+                let input_schema =
+                    Schema::new(vec![Field::new("c1", data_type.clone(), true)]);
+                let input_phy_exprs: Vec<Arc<dyn PhysicalExpr>> = vec![Arc::new(
+                    expressions::Column::new_with_schema("c1", &input_schema).unwrap(),
+                )];
+                let result_agg_phy_exprs = create_aggregate_expr(
+                    &fun,
+                    false,
+                    &input_phy_exprs[0..1],
+                    &input_schema,
+                    "c1",
+                )?;
+
+                if fun == AggregateFunction::ApproxMedian {
+                    assert!(result_agg_phy_exprs.as_any().is::<ApproxMedian>());
+                    assert_eq!("c1", result_agg_phy_exprs.name());
+                    assert_eq!(
+                        Field::new("c1", data_type.clone(), true),
+                        result_agg_phy_exprs.field().unwrap()
+                    );
+                }
+            }
+        }
+        Ok(())
+    }
+
+    #[test]
+    fn test_median() -> Result<()> {
+        let observed = return_type(&AggregateFunction::ApproxMedian, &[DataType::Utf8]);
+        assert!(observed.is_err());

Review comment:
       👍 

##########
File path: datafusion/src/physical_plan/expressions/approx_median.rs
##########
@@ -0,0 +1,75 @@
+// Licensed to the Apache Software Foundation (ASF) under one

Review comment:
       Is this file needed anymore? I think it can be removed

##########
File path: datafusion/src/optimizer/to_approx_perc.rs
##########
@@ -0,0 +1,161 @@
+// 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.
+
+//! espression/function to approx_percentile optimizer rule
+
+use crate::error::Result;
+use crate::execution::context::ExecutionProps;
+use crate::logical_plan::plan::Aggregate;
+use crate::logical_plan::{Expr, LogicalPlan};
+use crate::optimizer::optimizer::OptimizerRule;
+use crate::optimizer::utils;
+use crate::physical_plan::aggregates;
+use crate::scalar::ScalarValue;
+
+/// espression/function to approx_percentile optimizer rule
+///  ```text
+///    SELECT F1(s)
+///    ...
+///
+///    Into
+///
+///    SELECT APPROX_PERCENTILE_CONT(s, lit(n)) as "F1(s)"
+///    ...
+///  ```
+pub struct ToApproxPerc {}
+
+impl ToApproxPerc {
+    #[allow(missing_docs)]
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+
+impl Default for ToApproxPerc {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+fn optimize(plan: &LogicalPlan) -> Result<LogicalPlan> {
+    match plan {
+        LogicalPlan::Aggregate(Aggregate {
+            input,
+            aggr_expr,
+            schema,
+            group_expr,
+        }) => {
+            let new_aggr_expr = aggr_expr
+                .iter()
+                .map(|agg_expr| replace_with_percentile(agg_expr).unwrap())
+                .collect::<Vec<_>>();
+
+            Ok(LogicalPlan::Aggregate(Aggregate {
+                input: input.clone(),
+                aggr_expr: new_aggr_expr,
+                schema: schema.clone(),
+                group_expr: group_expr.clone(),
+            }))
+        }
+        _ => optimize_children(plan),
+    }
+}
+
+fn optimize_children(plan: &LogicalPlan) -> Result<LogicalPlan> {
+    let expr = plan.expressions();
+    let inputs = plan.inputs();
+    let new_inputs = inputs
+        .iter()
+        .map(|plan| optimize(plan))
+        .collect::<Result<Vec<_>>>()?;
+    utils::from_plan(plan, &expr, &new_inputs)
+}
+
+fn replace_with_percentile(expr: &Expr) -> Result<Expr> {
+    match expr {
+        Expr::AggregateFunction {
+            fun,
+            args,
+            distinct,
+        } => {
+            let mut new_args = args.clone();
+            let mut new_func = fun.clone();
+            if fun == &aggregates::AggregateFunction::ApproxMedian {
+                new_args.push(Expr::Literal(ScalarValue::Float64(Some(0.5_f64))));
+                new_func = aggregates::AggregateFunction::ApproxPercentileCont;
+            }
+
+            Ok(Expr::AggregateFunction {
+                fun: new_func,
+                args: new_args,
+                distinct: *distinct,
+            })
+        }
+        _ => Ok(expr.clone()),
+    }
+}
+
+impl OptimizerRule for ToApproxPerc {
+    fn optimize(
+        &self,
+        plan: &LogicalPlan,
+        _execution_props: &ExecutionProps,
+    ) -> Result<LogicalPlan> {
+        optimize(plan)
+    }
+    fn name(&self) -> &str {
+        "ToApproxPerc"
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::logical_plan::{col, LogicalPlanBuilder};
+    use crate::physical_plan::aggregates;
+    use crate::test::*;
+
+    fn assert_optimized_plan_eq(plan: &LogicalPlan, expected: &str) {
+        let rule = ToApproxPerc::new();
+        let optimized_plan = rule
+            .optimize(plan, &ExecutionProps::new())
+            .expect("failed to optimize plan");
+        let formatted_plan = format!("{}", optimized_plan.display_indent_schema());
+        assert_eq!(formatted_plan, expected);
+    }
+
+    #[test]
+    fn median_1() -> Result<()> {
+        let table_scan = test_table_scan()?;
+        let expr = Expr::AggregateFunction {
+            fun: aggregates::AggregateFunction::ApproxMedian,
+            distinct: false,
+            args: vec![col("b")],
+        };
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .aggregate(Vec::<Expr>::new(), vec![expr])?
+            .build()?;
+
+        // Do nothing

Review comment:
       ```suggestion
           // Rewrite to use approx_percentile
   ```

##########
File path: datafusion/src/physical_plan/expressions/approx_percentile_cont.rs
##########
@@ -203,6 +203,10 @@ impl ApproxPercentileAccumulator {
             return_type,
         }
     }
+

Review comment:
       left over?




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