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https://issues.apache.org/jira/browse/FLINK-11020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16701884#comment-16701884
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Timo Walther commented on FLINK-11020:
--------------------------------------
I'm not suggesting to introduce join reordering in general but only in cases
where we would fail otherwise. We could execute the query above if we would
allow for join reordering in cases where a cross join would be the result
otherwise.
> Reorder joins only to eliminate cross joins
> --------------------------------------------
>
> Key: FLINK-11020
> URL: https://issues.apache.org/jira/browse/FLINK-11020
> Project: Flink
> Issue Type: Improvement
> Components: Table API & SQL
> Reporter: Timo Walther
> Priority: Major
>
> Currently, we don't reorder join and rely on the order provided by the user.
> This is fine for most of the cases, however, it limits the set of supported
> SQL queries.
> Example:
> {code}
> val streamUtil: StreamTableTestUtil = streamTestUtil()
> streamUtil.addTable[(Int, String, Long)]("MyTable", 'a, 'b, 'c.rowtime,
> 'proctime.proctime)
> streamUtil.addTable[(Int, String, Long)]("MyTable2", 'a, 'b, 'c.rowtime,
> 'proctime.proctime)
> streamUtil.addTable[(Int, String, Long)]("MyTable3", 'a, 'b, 'c.rowtime,
> 'proctime.proctime)
> val sqlQuery =
> """
> |SELECT t1.a, t3.b
> |FROM MyTable3 t3, MyTable2 t2, MyTable t1
> |WHERE t1.a = t3.a AND t1.a = t2.a
> |""".stripMargin
> streamUtil.printSql(sqlQuery)
> {code}
> Given the current rule sets, this query produces a cross join which is not
> supported and thus leads to:
> {code}
> org.apache.flink.table.api.TableException: Cannot generate a valid execution
> plan for the given query:
> LogicalProject(a=[$8], b=[$1])
> LogicalFilter(condition=[AND(=($8, $0), =($8, $4))])
> LogicalJoin(condition=[true], joinType=[inner])
> LogicalJoin(condition=[true], joinType=[inner])
> LogicalTableScan(table=[[_DataStreamTable_2]])
> LogicalTableScan(table=[[_DataStreamTable_1]])
> LogicalTableScan(table=[[_DataStreamTable_0]])
> This exception indicates that the query uses an unsupported SQL feature.
> Please check the documentation for the set of currently supported SQL
> features.
> {code}
> Introducing {{JoinPushThroughJoinRule}} would help but should only be applied
> if a cross join is the only alternative.
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