Rachelint opened a new pull request, #15591:
URL: https://github.com/apache/datafusion/pull/15591
## Which issue does this PR close?
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- Part of #7065
## Rationale for this change
As mentioned in #7065 , we use a single `Vec` to manage `aggregation
intermediate results` both in `GroupAccumulator` and `GroupValues`.
It is simple but not efficient enough in high-cardinality aggregation,
because when `Vec` is not large enough, we need to allocate a new `Vec` and
copy all data from the old one.
- Copying a large amount of data(due to high-cardinality) is obviously
expansive
- And it is also not friendly to cpu (will refresh cache and tlb)
So this pr introduces a `blocked approach` to manage the `aggregation
intermediate results`. We will never resize the `Vec` in the approach, and
instead we split the data to blocks, when the capacity is not enough, we just
allocate a new block. Detail can see #7065
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## What changes are included in this PR?
- Implement the sketch for blocked approach
- Implement blocked groups supporting `PrimitiveGroupsAccumulator` and
`GroupValuesPrimitive` as the example
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## Are these changes tested?
Test by exist tests. And new unit tests, new fuzzy tests.
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## Are there any user-facing changes?
Two functions are added to `GroupValues` and `GroupAccumulator` trait.
But as you can see, there are default implementations for them, and users
can choose to really support the blocked approach when wanting a better
performance for their `udaf`s.
```
/// Returns `true` if this accumulator supports blocked groups.
fn supports_blocked_groups(&self) -> bool {
false
}
/// Alter the block size in the accumulator
///
/// If the target block size is `None`, it will use a single big
/// block(can think it a `Vec`) to manage the state.
///
/// If the target block size` is `Some(blk_size)`, it will try to
/// set the block size to `blk_size`, and the try will only success
/// when the accumulator has supported blocked mode.
///
/// NOTICE: After altering block size, all data in previous will be
cleared.
///
fn alter_block_size(&mut self, block_size: Option<usize>) -> Result<()> {
if block_size.is_some() {
return Err(DataFusionError::NotImplemented(
"this accumulator doesn't support blocked mode
yet".to_string(),
));
}
Ok(())
}
```
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