Zhong Yanghong created KYLIN-3487:

             Summary: Create a new measure for count distinct
                 Key: KYLIN-3487
                 URL: https://issues.apache.org/jira/browse/KYLIN-3487
             Project: Kylin
          Issue Type: Improvement
            Reporter: Zhong Yanghong
            Assignee: Zhong Yanghong

In eBay, there's a requirement to calculate the count distinct of sessions. 
Each day there'll be 20M sessions. For deep dive, users want to get the session 
cardinality in a month, or even several months. If just for one month, the 
total cardinality will be around 20M*30

To calculate the count distinct of session, if a session never crosses days, 
it's meaningless to merge the related counter, bitmap or hll, across days.

For count distinct session, it's meaningless to merge across days, for session 
is never across days. Therefore, we may need a new measure containing a map, 
using the date info as the key, and using bitmap or hll as the value. When 
calculating count distinct, it's only need to get the state for each key-value 
entry and then to summarize the states. And we don't need merge bitmap or hll 
across different key-value entries.

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