zabetak commented on PR #6359:
URL: https://github.com/apache/hive/pull/6359#issuecomment-4153794528
> I was unable to find a way to distinguish between "const NULL"
ColStatistics objects and objects that truly have an "unknown NDV" without
changing the ColStatistics class or refactoring the StatEstimator interface,
and making...
@konstantinb Since the previous version of the combiner didn't bother much
about unknown NDV values, I am not too opinionated about handling the case
where countDistinct is zero. Personally, I would be fine even with something
simplistic like the following:
```java
if (stat.getCountDistint() >= 0 && result.getCountDistint() >= 0) {
result.setCountDistint(StatsUtils.safeAdd(result.getCountDistint(),
stat.getCountDistint()));
}
```
From my perspective missing NDVs is an edge case and not something that
should appear too often during optimization.
Having said that, I am not against a more elaborate solution like modifying
`ColStatistics` or the `StatsEstimator` interface as per previous commits so I
will defer the final decision to @konstantinb .
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