sandeshkr419 opened a new pull request, #23816:
URL: https://github.com/apache/datafusion/pull/23816
## Which issue does this PR close?
Resolves https://github.com/apache/datafusion/issues/23815
## Rationale for this change
Adds runtime-configurable HLL precision to approx_distinct. Previously the
register count was a compile-time constant (p=14, 16 KiB per sketch); now any
precision between 4 to 18 is supported.
## What changes are included in this PR?
- HyperLogLog::with_precision(p): constructs a sketch at the requested
precision; new() still defaults to p=14.
- HyperLogLog::from_registers(vec): replaces new_with_registers([u8;
16384]); infers p from the slice length.
- ApproxDistinct::with_hll_precision(p): forwards p through every HLL
accumulator path (HLLAccumulator, NumericHLLAccumulator, HllGroupsAccumulator).
Has no effect for types that use exact bitmap counting (Boolean, Int8, UInt8,
Int16, UInt16) — those paths are unaffected regardless of the value passed.
## Are these changes tested?
9 new tests cover: construction at non-default precisions, accuracy within
expected error bounds at p=10 and p=12, roundtrip serialization at p=10/12/14,
cross-precision merge panic (programming error guard), and boundary validation.
## Are there any user-facing changes?
None for existing callers. Old ApproxDistinct::new() and HyperLogLog::new()
continue to use p=14.
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