[
https://issues.apache.org/jira/browse/FLINK-11070?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16712454#comment-16712454
]
Fabian Hueske commented on FLINK-11070:
---------------------------------------
Thanks for this issue [~hequn8128]!
I think we definitely have to support cross joins at some point. However, I
think we should not do that (by default) before we added support for join
reordering.
The example that you gave in the description, can be execute with two equi
joins and should not be executed with a cross join. However, the planner will
not find the efficient plan without join reordering. If we enable cross joins
by default, the plan would be simply translated and executed with a
(non-parallel) cross join. A cross join should (typically) only be used, if a
query cannot be executed in any other way.
I would propose that until we have support for proper join reordering, we
1. We give a better error message if a query can only be executed with a cross
join and explain that users should try to reorder the table references or
enable cross joins.
2. Add a switch to support cross joins. I would make this join parallel and
always broadcast the second input table (since we do not reorder the joins,
this will be deterministic).
3. Possibly add a switch to enable join reordering with a BIG warning sign,
that the order can be very suboptimal.
Once we support join reordering (with reasonable cardinality estimates), we can
remove the switches and have a good behavior without any configuration.
What do you think?
> Add stream-stream non-window cross join
> ---------------------------------------
>
> Key: FLINK-11070
> URL: https://issues.apache.org/jira/browse/FLINK-11070
> Project: Flink
> Issue Type: New Feature
> Components: Table API & SQL
> Reporter: Hequn Cheng
> Assignee: Hequn Cheng
> 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:java}
> 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:java}
> 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}
> In order to support more queries, it would be nice to have cross join on
> streaming. We can start from a simple version, for example, call
> forceNonParallel() for connectOperator in `DataStreamJoin` when it is a cross
> join. The performance may be bad. But it works fine if the two tables of
> cross join are small ones.
> We can do some optimizations later, such as broadcasting the smaller side,
> etc.
> Any suggestions are greatly appreciated.
--
This message was sent by Atlassian JIRA
(v7.6.3#76005)
