[CALCITE-4634] Improve AggregateProjectPullUpConstantsRule to remove all constant keys

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.16.0
• Component/s: core
• Labels: pull-request-available
• Assignee: Unassigned
• Reporter: Yingyu Wang
• Created: 03/Jun/21 17:36
• Updated: 09/Jun/21 21:11

Description

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. 

SELECT 'abc' FROM table_name GROUP BY ('abc')

will be translated to:

SELECT 'abc' FROM table_name GROUP BY () having count(*)>0

Issue Links

• is related to: CALCITE-4597 Allow RelNodes to have an empty row type (zero fields)
• links to: GitHub Pull Request #2432