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https://issues.apache.org/jira/browse/CALCITE-6665?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17906229#comment-17906229
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xiong duan commented on CALCITE-6665:
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If this method returns NULL, it means we don't know. True means it is empty,
and False means it is never empty.
> Add isEmpty metadata to check if a relational expression returns no rows
> ------------------------------------------------------------------------
>
> Key: CALCITE-6665
> URL: https://issues.apache.org/jira/browse/CALCITE-6665
> Project: Calcite
> Issue Type: Improvement
> Reporter: xiong duan
> Priority: Major
>
> In Calcite, there are many times to determine whether a relational expression
> returns 0 rows through maxRowCount.
> But some places use `maxRowCount<=1D` while others use `maxRowCount ==0D`.
> This PR mainly aims to add `@Nullable Boolean
> RelMetadataQuery.isEmpty(RelNode)` to unify this behavior.
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