2010YOUY01 commented on code in PR #23816:
URL: https://github.com/apache/datafusion/pull/23816#discussion_r3725664891
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datafusion/functions-aggregate/src/approx_distinct.rs:
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@@ -654,14 +707,35 @@ impl Default for ApproxDistinct {
#[derive(PartialEq, Eq, Hash)]
pub struct ApproxDistinct {
signature: Signature,
+ /// HLL register precision. Only used for types that take the HLL code path
+ /// (i.e. not Boolean / small-int types, which use exact bitmap counting).
+ hll_precision: usize,
}
impl ApproxDistinct {
pub fn new() -> Self {
Self {
signature: Signature::any(1, Volatility::Immutable),
+ hll_precision: DEFAULT_HLL_P,
}
}
+
+ /// Creates an `ApproxDistinct` that uses HLL sketches with `2^p`
registers.
+ ///
+ /// This only has effect for types that use the HLL accumulator path. Small
+ /// integer and boolean types use exact bitmap counting regardless of this
+ /// value. Valid range: `HLL_P_MIN..=HLL_P_MAX` (4..=18).
+ pub fn with_hll_precision(p: usize) -> Result<Self> {
Review Comment:
Non-blocking suggestion:
It's the only API to use this configuration now, however mostly users are
using higher level SQL/Dataframe API.
So it would be better to be able to set it from global configurations. After
that we can add some end-to-end tests.
I remember there are no existing configurations targeting individual
functions, so perhaps some refactor would be needed.
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