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https://issues.apache.org/jira/browse/FLINK-11020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16701876#comment-16701876
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Fabian Hueske commented on FLINK-11020:
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I would not do that. Join reordering without statistics is like rolling a dice,
i.e., we would end up with random plans.
Currently, joins are done in the order in which they are specified in the
query, i.e., the behavior is deterministic and can be controlled by the user.
I think we would lose that if we add rules for join reordering.
> 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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