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https://issues.apache.org/jira/browse/FLINK-23723?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17406509#comment-17406509
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Ada Wong commented on FLINK-23723:
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cc [~jark] We should decouple lookup join from the source table proc time field.
> Look up join support PROCTIME() function
> ----------------------------------------
>
> Key: FLINK-23723
> URL: https://issues.apache.org/jira/browse/FLINK-23723
> Project: Flink
> Issue Type: Improvement
> Components: Table SQL / API
> Reporter: silence
> Priority: Major
>
> Now,when we what to use look up join we must declare procetime attribute on
> source table.
> this is relatively troublesome, and in some complex scenarios(eg. Top-N) ,
> look up join cannot be performed because of the loss of processtime attribute.
> Moreover, in the process of look up join, the processing time is of no
> practical use. can we support specifying `PROCTIME()` directly in the join
> without declaring it in the source table,
> like:
> {code:sql}
> SELECT o.order_id, o.total, c.country, c.zip
> FROM Orders AS o
> JOIN Customers FOR SYSTEM_TIME AS OF PROCTIME() AS c
> ON o.customer_id = c.id;
> {code}
> Now it throws an exception
> {code:java}
> Caused by: java.lang.AssertionError: Temporal table can only be used in
> temporal join and only supports 'FOR SYSTEM_TIME AS OF' left table's time
> attribute field.
> Querying a temporal table using 'FOR SYSTEM TIME AS OF' syntax with an
> expression call 'PROCTIME()' is not supported yet.
> at org.apache.calcite.util.Litmus$1.fail(Litmus.java:31)
> at
> org.apache.flink.table.planner.plan.nodes.logical.FlinkLogicalSnapshot.isValid(FlinkLogicalSnapshot.scala:60)
> {code}
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