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https://issues.apache.org/jira/browse/FLINK-11020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16708452#comment-16708452
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Fabian Hueske commented on FLINK-11020:
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Hi [~hequn8128], Yes, agreed. It would be good to have an easy option to enable
cross products. However, IMO this is a different issue. Would you mind opening
a Jira and move the discussion there?
> 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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