Gopal V created HIVE-12094:
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Summary: nDV of aggregate columns tend to be log scale - not unique
Key: HIVE-12094
URL: https://issues.apache.org/jira/browse/HIVE-12094
Project: Hive
Issue Type: Improvement
Components: Statistics
Affects Versions: 1.3.0, 2.0.0
Reporter: Gopal V
Stats for aggregate columns do not process properly if declared as a simple nDV
{code}
select count(distinct l_suppkey) from lineitem group by l_orderkey having
count(distinct l_suppkey) = 1
{code}
will mis-estimate the cardinality of the output by a significant margin.
The log-scale of the nDV in general skews towards a very low number, which is
not accounted for in the StatsRulesProcFactory.
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