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https://issues.apache.org/jira/browse/CALCITE-7242?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18032763#comment-18032763
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Julian Hyde commented on CALCITE-7242:
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You should add tests in RelOptRulesTest and elsewhere. Make sure that the rule
does the right thing when the input relation is empty, and when, after removing
the literals, the resulting Aggregate has zero columns.
> Implement a rule to eliminate LITERAL_AGG so that other databases can handle
> it
> -------------------------------------------------------------------------------
>
> Key: CALCITE-7242
> URL: https://issues.apache.org/jira/browse/CALCITE-7242
> Project: Calcite
> Issue Type: Improvement
> Components: core
> Reporter: weihua zhang
> Priority: Major
> Labels: pull-request-available
>
> Calcite can create an Aggregate RelNode with LITERAL_AGG AggregateCall to
> check for existence in a subquery. In this case, the Aggregate will do a
> group by, and just spit out a literal value for each row.
> The LITERAL_AGG function does not exist in other databases. In order to
> create a RelNode structure that other databases supports, this Aggregate is
> turned into an Aggregate that still does the "group by" on the relevant
> groups, but places a Project RelNode on top that returns the literal value.
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