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Refactor size estimation of Hashset into a function #8779

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Refactor size estimation of Hashset into a function #8779

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estimation into function
yyy1000 Jan 7, 2024
0ec8a57
Update datafusion/physical-expr/src/aggregate/count_distinct.rs
yyy1000 Jan 8, 2024
22f885a
Update datafusion/physical-expr/src/aggregate/count_distinct.rs
yyy1000 Jan 8, 2024
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13 changes: 7 additions & 6 deletions datafusion/physical-expr/src/aggregate/count_distinct.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -83,6 +83,11 @@ macro_rules! float_distinct_count_accumulator {
}};
}

/// Returns the estimated number of hashbrown hashtables.
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@alamb alamb Jan 8, 2024

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I think we could potentially re-use the comments here:

https://github.com/apache/arrow-datafusion/blob/819d3577872a082f2aea7a68ae83d68534049662/datafusion/physical-plan/src/joins/hash_join.rs#L734-L749

I think the value of this PR / issue is to consolidate the logic in datafusion/physical-expr/src/aggregate/count_distinct.rs with the logic in arrow-datafusion/datafusion/physical-plan/src/joins /hash_join.rs with comments explaining the rationale (aka answering @crepererum 's in comments)

To that end what would you think about:

  1. Adding the code to arrow-datafusion/datafusion/physical-plan/src/common.rs
  2. Use the comments from https://github.com/apache/arrow-datafusion/blob/819d3577872a082f2aea7a68ae83d68534049662/datafusion/physical-plan/src/joins/hash_join.rs#L734-L749 to explain the calculation
  3. Change the code in hash_join.rs to use it too

I think this may require changing the signature to something like

/// Estimates the memory allocated by a [`hashbrown::HashTable`].
///
/// (add explanation about size calculation here)
///
/// Note a [`hashbrown::HashSet`] is implemented as a HashTable with a zero sized key
pub fn estimated_hashtable_size<T>(table: &HashTable<T, RandomState>) -> usize {
...
}

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@yyy1000 yyy1000 Jan 8, 2024

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I'm 100% cool with the changes!
Thank you all for the review. :)

fn estimated_buckets<T>(hashset: &HashSet<T, RandomState>) -> usize {
(hashset.len().checked_mul(8).unwrap_or(usize::MAX) / 7).next_power_of_two()
}

impl AggregateExpr for DistinctCount {
/// Return a reference to Any that can be used for downcasting
fn as_any(&self) -> &dyn Any {
Expand Down Expand Up @@ -336,9 +341,7 @@ where
}

fn size(&self) -> usize {
let estimated_buckets = (self.values.len().checked_mul(8).unwrap_or(usize::MAX)
/ 7)
.next_power_of_two();
let estimated_buckets = estimated_buckets(&self.values);

// Size of accumulator
// + size of entry * number of buckets
Expand Down Expand Up @@ -423,9 +426,7 @@ where
}

fn size(&self) -> usize {
let estimated_buckets = (self.values.len().checked_mul(8).unwrap_or(usize::MAX)
/ 7)
.next_power_of_two();
let estimated_buckets = estimated_buckets(&self.values);

// Size of accumulator
// + size of entry * number of buckets
Expand Down