datafusion_functions_aggregate/
approx_median.rs

1// Licensed to the Apache Software Foundation (ASF) under one
2// or more contributor license agreements.  See the NOTICE file
3// distributed with this work for additional information
4// regarding copyright ownership.  The ASF licenses this file
5// to you under the Apache License, Version 2.0 (the
6// "License"); you may not use this file except in compliance
7// with the License.  You may obtain a copy of the License at
8//
9//   http://www.apache.org/licenses/LICENSE-2.0
10//
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12// software distributed under the License is distributed on an
13// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
14// KIND, either express or implied.  See the License for the
15// specific language governing permissions and limitations
16// under the License.
17
18//! Defines physical expressions for APPROX_MEDIAN that can be evaluated MEDIAN at runtime during query execution
19
20use arrow::datatypes::DataType::{Float64, UInt64};
21use arrow::datatypes::{DataType, Field, FieldRef};
22use std::any::Any;
23use std::fmt::Debug;
24use std::sync::Arc;
25
26use datafusion_common::{not_impl_err, plan_err, Result};
27use datafusion_expr::function::{AccumulatorArgs, StateFieldsArgs};
28use datafusion_expr::type_coercion::aggregates::NUMERICS;
29use datafusion_expr::utils::format_state_name;
30use datafusion_expr::{
31    Accumulator, AggregateUDFImpl, Documentation, Signature, Volatility,
32};
33use datafusion_macros::user_doc;
34
35use crate::approx_percentile_cont::ApproxPercentileAccumulator;
36
37make_udaf_expr_and_func!(
38    ApproxMedian,
39    approx_median,
40    expression,
41    "Computes the approximate median of a set of numbers",
42    approx_median_udaf
43);
44
45/// APPROX_MEDIAN aggregate expression
46#[user_doc(
47    doc_section(label = "Approximate Functions"),
48    description = "Returns the approximate median (50th percentile) of input values. It is an alias of `approx_percentile_cont(0.5) WITHIN GROUP (ORDER BY x)`.",
49    syntax_example = "approx_median(expression)",
50    sql_example = r#"```sql
51> SELECT approx_median(column_name) FROM table_name;
52+-----------------------------------+
53| approx_median(column_name)        |
54+-----------------------------------+
55| 23.5                              |
56+-----------------------------------+
57```"#,
58    standard_argument(name = "expression",)
59)]
60#[derive(PartialEq, Eq, Hash)]
61pub struct ApproxMedian {
62    signature: Signature,
63}
64
65impl Debug for ApproxMedian {
66    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
67        f.debug_struct("ApproxMedian")
68            .field("name", &self.name())
69            .field("signature", &self.signature)
70            .finish()
71    }
72}
73
74impl Default for ApproxMedian {
75    fn default() -> Self {
76        Self::new()
77    }
78}
79
80impl ApproxMedian {
81    /// Create a new APPROX_MEDIAN aggregate function
82    pub fn new() -> Self {
83        Self {
84            signature: Signature::uniform(1, NUMERICS.to_vec(), Volatility::Immutable),
85        }
86    }
87}
88
89impl AggregateUDFImpl for ApproxMedian {
90    /// Return a reference to Any that can be used for downcasting
91    fn as_any(&self) -> &dyn Any {
92        self
93    }
94
95    fn state_fields(&self, args: StateFieldsArgs) -> Result<Vec<FieldRef>> {
96        Ok(vec![
97            Field::new(format_state_name(args.name, "max_size"), UInt64, false),
98            Field::new(format_state_name(args.name, "sum"), Float64, false),
99            Field::new(format_state_name(args.name, "count"), UInt64, false),
100            Field::new(format_state_name(args.name, "max"), Float64, false),
101            Field::new(format_state_name(args.name, "min"), Float64, false),
102            Field::new_list(
103                format_state_name(args.name, "centroids"),
104                Field::new_list_field(Float64, true),
105                false,
106            ),
107        ]
108        .into_iter()
109        .map(Arc::new)
110        .collect())
111    }
112
113    fn name(&self) -> &str {
114        "approx_median"
115    }
116
117    fn signature(&self) -> &Signature {
118        &self.signature
119    }
120
121    fn return_type(&self, arg_types: &[DataType]) -> Result<DataType> {
122        if !arg_types[0].is_numeric() {
123            return plan_err!("ApproxMedian requires numeric input types");
124        }
125        Ok(arg_types[0].clone())
126    }
127
128    fn accumulator(&self, acc_args: AccumulatorArgs) -> Result<Box<dyn Accumulator>> {
129        if acc_args.is_distinct {
130            return not_impl_err!(
131                "APPROX_MEDIAN(DISTINCT) aggregations are not available"
132            );
133        }
134
135        Ok(Box::new(ApproxPercentileAccumulator::new(
136            0.5_f64,
137            acc_args.exprs[0].data_type(acc_args.schema)?,
138        )))
139    }
140
141    fn documentation(&self) -> Option<&Documentation> {
142        self.doc()
143    }
144}
datafusion_functions_aggregate/approx_median.rs

datafusion_functions_aggregate/
approx_median.rs
