[jira] [Commented] (CALCITE-1495) Add a rule to convert INNER JOIN preceded by GROUP BY to appropriate SEMI-JOIN

["Julian Hyde (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CALCITE-1495?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15710404#comment-15710404 ] 

Julian Hyde commented on CALCITE-1495:
--------------------------------------

bq. This rule should also check for join type and should allow conversion only for inner join. Currently, it can convert an outer join which results in wrong result set. 

I agree. But suppose the rule matches a LEFT join whose right-hand columns are not used, and whose right input is unique (i.e. returns at most one row per key). That is a join you can remove, and replace by the left input. The rule might as well do that.

> 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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