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https://issues.apache.org/jira/browse/FLINK-33485?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17789280#comment-17789280
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Sergey Nuyanzin commented on FLINK-33485:
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Merged to master as
[a1d4dc0a4b05164aee41c781e75c9e9aeae5d758|https://github.com/apache/flink/commit/a1d4dc0a4b05164aee41c781e75c9e9aeae5d758]
> Optimize the EXISTS sub-query by Metadata RowCount
> --------------------------------------------------
>
> Key: FLINK-33485
> URL: https://issues.apache.org/jira/browse/FLINK-33485
> Project: Flink
> Issue Type: Improvement
> Components: Table SQL / API
> Affects Versions: 1.18.0
> Reporter: Sergey Nuyanzin
> Assignee: Sergey Nuyanzin
> Priority: Major
> Labels: pull-request-available
>
> If the sub-query is guaranteed to produce at least one row, just return TRUE.
> If the sub-query is guaranteed to produce no row, just return FALSE.
> inspired by CALCITE-5117 however since there is {{FlinkSubQueryRemoveRule}}
> then it shold be adopted accordingly
> examples
> {code:sql}
> SELECT * FROM T2 WHERE EXISTS (SELECT SUM(a1), COUNT(*) FROM T1 WHERE 1=2)
> {code}
> aggregation functions always return 1 row even if there is an empty table
> then we could just replace this query with
> {code:sql}
> SELECT * FROM T2
> {code}
> another example
> {code:sql}
> SELECT * FROM MyTable WHERE NOT EXISTS (SELECT a FROM MyTable LIMIT 0)
> {code}
> {{LIMIT 0}} means no rows so it cold be optimized to
> {code:sql}
> SELECT * FROM MyTable
> {code}
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