Timo Walther created FLINK-11020:
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Summary: 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
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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