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Specialize Median Accumulator (#7376)
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* Specialize Median Accumulator

* Tweak memory limit test
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@tustvold
tustvold authored Aug 23, 2023
1 parent ae4b52a commit 65821eb
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Showing 2 changed files with 66 additions and 131 deletions.
4 changes: 2 additions & 2 deletions datafusion/core/tests/memory_limit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -68,12 +68,12 @@ async fn oom_sort() {
#[tokio::test]
async fn group_by_none() {
TestCase::new()
.with_query("select median(image) from t")
.with_query("select median(request_bytes) from t")
.with_expected_errors(vec![
"Resources exhausted: Failed to allocate additional",
"AggregateStream",
])
.with_memory_limit(20_000)
.with_memory_limit(2_000)
.run()
.await
}
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193 changes: 64 additions & 129 deletions datafusion/physical-expr/src/aggregate/median.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -20,13 +20,15 @@
use crate::aggregate::utils::down_cast_any_ref;
use crate::expressions::format_state_name;
use crate::{AggregateExpr, PhysicalExpr};
use arrow::array::{Array, ArrayRef, UInt32Array};
use arrow::compute::sort_to_indices;
use arrow::array::{Array, ArrayRef};
use arrow::datatypes::{DataType, Field};
use datafusion_common::internal_err;
use arrow_array::cast::AsArray;
use arrow_array::{downcast_integer, ArrowNativeTypeOp, ArrowNumericType};
use arrow_buffer::ArrowNativeType;
use datafusion_common::{DataFusionError, Result, ScalarValue};
use datafusion_expr::Accumulator;
use std::any::Any;
use std::fmt::Formatter;
use std::sync::Arc;

/// MEDIAN aggregate expression. This uses a lot of memory because all values need to be
Expand Down Expand Up @@ -65,11 +67,29 @@ impl AggregateExpr for Median {
}

fn create_accumulator(&self) -> Result<Box<dyn Accumulator>> {
Ok(Box::new(MedianAccumulator {
data_type: self.data_type.clone(),
arrays: vec![],
all_values: vec![],
}))
use arrow_array::types::*;
macro_rules! helper {
($t:ty, $dt:expr) => {
Ok(Box::new(MedianAccumulator::<$t> {
data_type: $dt.clone(),
all_values: vec![],
}))
};
}
let dt = &self.data_type;
downcast_integer! {
dt => (helper, dt),
DataType::Float16 => helper!(Float16Type, dt),
DataType::Float32 => helper!(Float32Type, dt),
DataType::Float64 => helper!(Float64Type, dt),
DataType::Decimal128(_, _) => helper!(Decimal128Type, dt),
DataType::Decimal256(_, _) => helper!(Decimal256Type, dt),
_ => Err(DataFusionError::NotImplemented(format!(
"MedianAccumulator not supported for {} with {}",
self.name(),
self.data_type
))),
}
}

fn state_fields(&self) -> Result<Vec<Field>> {
Expand Down Expand Up @@ -106,159 +126,75 @@ impl PartialEq<dyn Any> for Median {
}
}

#[derive(Debug)]
/// The median accumulator accumulates the raw input values
/// as `ScalarValue`s
///
/// The intermediate state is represented as a List of scalar values updated by
/// `merge_batch` and a `Vec` of `ArrayRef` that are converted to scalar values
/// in the final evaluation step so that we avoid expensive conversions and
/// allocations during `update_batch`.
struct MedianAccumulator {
struct MedianAccumulator<T: ArrowNumericType> {
data_type: DataType,
arrays: Vec<ArrayRef>,
all_values: Vec<ScalarValue>,
all_values: Vec<T::Native>,
}

impl Accumulator for MedianAccumulator {
impl<T: ArrowNumericType> std::fmt::Debug for MedianAccumulator<T> {
fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
write!(f, "MedianAccumulator({})", self.data_type)
}
}

impl<T: ArrowNumericType> Accumulator for MedianAccumulator<T> {
fn state(&self) -> Result<Vec<ScalarValue>> {
let all_values = to_scalar_values(&self.arrays)?;
let all_values = self
.all_values
.iter()
.map(|x| ScalarValue::new_primitive::<T>(Some(*x), &self.data_type))
.collect();
let state = ScalarValue::new_list(Some(all_values), self.data_type.clone());

Ok(vec![state])
}

fn update_batch(&mut self, values: &[ArrayRef]) -> Result<()> {
assert_eq!(values.len(), 1);
let array = &values[0];

// Defer conversions to scalar values to final evaluation.
assert_eq!(array.data_type(), &self.data_type);
self.arrays.push(array.clone());

let values = values[0].as_primitive::<T>();
self.all_values.reserve(values.len() - values.null_count());
self.all_values.extend(values.iter().flatten());
Ok(())
}

fn merge_batch(&mut self, states: &[ArrayRef]) -> Result<()> {
assert_eq!(states.len(), 1);

let array = &states[0];
assert!(matches!(array.data_type(), DataType::List(_)));
for index in 0..array.len() {
match ScalarValue::try_from_array(array, index)? {
ScalarValue::List(Some(mut values), _) => {
self.all_values.append(&mut values);
}
ScalarValue::List(None, _) => {} // skip empty state
v => {
return internal_err!(
"unexpected state in median. Expected DataType::List, got {v:?}"
)
}
}
let array = states[0].as_list::<i32>();
for v in array.iter().flatten() {
self.update_batch(&[v])?
}
Ok(())
}

fn evaluate(&self) -> Result<ScalarValue> {
let batch_values = to_scalar_values(&self.arrays)?;

if !self
.all_values
.iter()
.chain(batch_values.iter())
.any(|v| !v.is_null())
{
return ScalarValue::try_from(&self.data_type);
}

// Create an array of all the non null values and find the
// sorted indexes
let array = ScalarValue::iter_to_array(
self.all_values
.iter()
.chain(batch_values.iter())
// ignore null values
.filter(|v| !v.is_null())
.cloned(),
)?;

// find the mid point
let len = array.len();
let mid = len / 2;

// only sort up to the top size/2 elements
let limit = Some(mid + 1);
let options = None;
let indices = sort_to_indices(&array, options, limit)?;

// pick the relevant indices in the original arrays
let result = if len >= 2 && len % 2 == 0 {
// even number of values, average the two mid points
let s1 = scalar_at_index(&array, &indices, mid - 1)?;
let s2 = scalar_at_index(&array, &indices, mid)?;
match s1.add(s2)? {
ScalarValue::Int8(Some(v)) => ScalarValue::Int8(Some(v / 2)),
ScalarValue::Int16(Some(v)) => ScalarValue::Int16(Some(v / 2)),
ScalarValue::Int32(Some(v)) => ScalarValue::Int32(Some(v / 2)),
ScalarValue::Int64(Some(v)) => ScalarValue::Int64(Some(v / 2)),
ScalarValue::UInt8(Some(v)) => ScalarValue::UInt8(Some(v / 2)),
ScalarValue::UInt16(Some(v)) => ScalarValue::UInt16(Some(v / 2)),
ScalarValue::UInt32(Some(v)) => ScalarValue::UInt32(Some(v / 2)),
ScalarValue::UInt64(Some(v)) => ScalarValue::UInt64(Some(v / 2)),
ScalarValue::Float32(Some(v)) => ScalarValue::Float32(Some(v / 2.0)),
ScalarValue::Float64(Some(v)) => ScalarValue::Float64(Some(v / 2.0)),
ScalarValue::Decimal128(Some(v), p, s) => {
ScalarValue::Decimal128(Some(v / 2), p, s)
}
v => {
return internal_err!("Unsupported type in MedianAccumulator: {v:?}")
}
}
// TODO: evaluate could pass &mut self
let mut d = self.all_values.clone();
let cmp = |x: &T::Native, y: &T::Native| x.compare(*y);

let len = d.len();
let median = if len == 0 {
None
} else if len % 2 == 0 {
let (low, high, _) = d.select_nth_unstable_by(len / 2, cmp);
let (_, low, _) = low.select_nth_unstable_by(low.len() - 1, cmp);
let median = low.add_wrapping(*high).div_wrapping(T::Native::usize_as(2));
Some(median)
} else {
// odd number of values, pick that one
scalar_at_index(&array, &indices, mid)?
let (_, median, _) = d.select_nth_unstable_by(len / 2, cmp);
Some(*median)
};

Ok(result)
Ok(ScalarValue::new_primitive::<T>(median, &self.data_type))
}

fn size(&self) -> usize {
let arrays_size: usize = self.arrays.iter().map(|a| a.len()).sum();

std::mem::size_of_val(self)
+ ScalarValue::size_of_vec(&self.all_values)
+ arrays_size
- std::mem::size_of_val(&self.all_values)
+ self.data_type.size()
- std::mem::size_of_val(&self.data_type)
}
}

fn to_scalar_values(arrays: &[ArrayRef]) -> Result<Vec<ScalarValue>> {
let num_values: usize = arrays.iter().map(|a| a.len()).sum();
let mut all_values = Vec::with_capacity(num_values);

for array in arrays {
for index in 0..array.len() {
all_values.push(ScalarValue::try_from_array(&array, index)?);
}
+ self.all_values.capacity() * std::mem::size_of::<T::Native>()
}

Ok(all_values)
}

/// Given a returns `array[indicies[indicie_index]]` as a `ScalarValue`
fn scalar_at_index(
array: &dyn Array,
indices: &UInt32Array,
indicies_index: usize,
) -> Result<ScalarValue> {
let array_index = indices
.value(indicies_index)
.try_into()
.expect("Convert uint32 to usize");
ScalarValue::try_from_array(array, array_index)
}

#[cfg(test)]
Expand All @@ -269,7 +205,6 @@ mod tests {
use crate::generic_test_op;
use arrow::record_batch::RecordBatch;
use arrow::{array::*, datatypes::*};
use datafusion_common::Result;

#[test]
fn median_decimal() -> Result<()> {
Expand Down

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