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https://issues.apache.org/jira/browse/CALCITE-1495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15670991#comment-15670991
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Julian Hyde commented on CALCITE-1495:
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Yes, {{SemiJoinRule}} ensures that no columns from the RHS are used.
One improvement: Currently it looks for an {{Aggregate}} as its input. It would
be better if it looked for an input that is unique on the join key.
> Add a rule to convert INNER JOIN preceded by GROUP BY to appropriate SEMI-JOIN
> ------------------------------------------------------------------------------
>
> Key: CALCITE-1495
> URL: https://issues.apache.org/jira/browse/CALCITE-1495
> Project: Calcite
> Issue Type: New Feature
> Components: core
> Reporter: Vineet Garg
> Assignee: Julian Hyde
>
> For IN and EXISTS subqueries Calcite currently generates plan consisting of
> GROUP BY on inner table followed by INNER JOIN with outer table.
> e.g. for following query:
> {noformat} select sal from emp where empno IN (select deptno from dept)
> {noformat}
> Generated plan is:
> {noformat}
> LogicalProject(SAL=[$5])
> LogicalProject(EMPNO=[$0], ENAME=[$1], JOB=[$2], MGR=[$3], HIREDATE=[$4],
> SAL=[$5], COMM=[$6], DEPTNO=[$7], SLACKER=[$8])
> LogicalJoin(condition=[=($0, $9)], joinType=[inner])
> LogicalTableScan(table=[[CATALOG, SALES, EMP]])
> LogicalAggregate(group=[{0}])
> LogicalProject(DEPTNO=[$0])
> LogicalTableScan(table=[[CATALOG, SALES, DEPT]])
> {noformat}
> Such cases could be converted using this rule to use SEMI-JOIN to make it
> more efficient
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