FrankChen021 commented on code in PR #19678:
URL: https://github.com/apache/druid/pull/19678#discussion_r3570769775
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processing/src/main/java/org/apache/druid/hll/HyperLogLogCollector.java:
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@@ -190,12 +191,22 @@ private static double estimateSparse(
}
e += 1.0d / Math.pow(2, upperNibble) + 1.0d / Math.pow(2, lowerNibble);
zeroCount += (((upperNibble & 0xf0) == 0) ? 1 : 0) + (((lowerNibble &
0x0f) == 0) ? 1 : 0);
+ overflowRegisterApplied = true;
Review Comment:
[P2] Normalize sparse positions before marking overflow applied
`position` is the serialized byte offset, which `toByteBuffer()` writes as
the payload index plus `getNumHeaderBytes()` (7 for a V1 sketch), while
`overflowPosition` is only `overflowRegister >>> 1`. Consequently this flag can
remain false after visiting the overflow bucket's actual sparse byte, or become
true for an unrelated entry. For example, if a bucket first receives an
in-range value and then an overflow value, a sparse round-trip stores its tuple
at `overflowPosition + 7`; the new fallback then treats the overflow as a
second previously empty register and decrements `zeroCount` again, making one
populated HLL register estimate approximately 2 instead of 1. Compare positions
in the same coordinate system before deciding whether standalone compensation
is needed.
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