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https://issues.apache.org/jira/browse/BEAM-2478?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16062450#comment-16062450
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Julian Hyde commented on BEAM-2478:
-----------------------------------
Your rewrite for hierarchical calculation is slightly wrong.
{code}
select a, count(distinct b) from t group by a
becomes
select a, count(distinct_b) from (
select a, b as distinct_b
from t
group by a, b)
group by a)
{code}
This correctly ignores rows where b is null.
Calcite's
[AggregateExpandDistinctAggregatesRule|https://insight.io/github.com/apache/calcite/blob/master/core/src/main/java/org/apache/calcite/rel/rules/AggregateExpandDistinctAggregatesRule.java]
does this rewrite; it can also do a more complex rewrite using GROUPING SETS
if there are multiple distinct-counts in the same query. See also CALCITE-1588
for approximate distinct-count.
> Distinct Aggregates
> -------------------
>
> Key: BEAM-2478
> URL: https://issues.apache.org/jira/browse/BEAM-2478
> Project: Beam
> Issue Type: New Feature
> Components: dsl-sql
> Reporter: Jingsong Lee
> Assignee: Tarush Grover
>
> eg: COUNT(DISTINCT empno)
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