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[jira] [Created] (FLINK-11020) Reorder joins only to eliminate cross joins

Timo Walther created FLINK-11020:

             Summary: Reorder joins only to eliminate cross joins 
                 Key: FLINK-11020
             Project: Flink
          Issue Type: Improvement
          Components: Table API & SQL
            Reporter: Timo Walther

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.

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


Given the current rule sets, this query produces a cross join which is not supported and thus leads to:

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])

This exception indicates that the query uses an unsupported SQL feature.
Please check the documentation for the set of currently supported SQL features.

Introducing {{JoinPushThroughJoinRule}} would help but should only be applied if a cross join is the only alternative.

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