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https://issues.apache.org/jira/browse/CALCITE-4634?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17356704#comment-17356704
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Steven Phillips commented on CALCITE-4634:
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[~julianhyde] I agree it's not immediately clear that this rewrite is a win. It
would depend on somewhat on the physical implementation. If we are using
HashAgg as the physical implementation, we would be replacing a hash function
with a count function, so I believe this would be a small win. But since it may
be implementation specific, maybe it makes sense to add a flag and have two
versions of this rule.
> Improve AggregateProjectPullUpConstantsRule to remove all constant keys
> -----------------------------------------------------------------------
>
> Key: CALCITE-4634
> URL: https://issues.apache.org/jira/browse/CALCITE-4634
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Affects Versions: 1.16.0
> Reporter: Yingyu
> Priority: Major
>
> Optimizer rule *AggregateProjectPullUpConstantsRule* removes constant keys
> from an Aggregate. However, this rules never removes the last column, because
> Aggregate([]) returns 1 row even if its input is empty.
> An enhancement could be done to AggregateProjectPullUpConstantsRule so that
> it removes all constant keys:
> - Remove all constant keys from an Aggregate
> - (Only)When no key is left from previous step (Aggregate becomes
> Aggregate([])), add a filter to the Aggregate([]) so that it returns empty
> dataset if its input is empty.
> E.g.
> {noformat}
> SELECT 'abc' FROM table_name GROUP BY ('abc')
> {noformat}
> will be translated to:
> {noformat}
> SELECT 'abc' FROM table_name GROUP BY () having count(*)>0
> {noformat}
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