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From "Nicholas Chammas (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SPARK-25150) Joining DataFrames derived from the same source yields confusing/incorrect results
Date Fri, 28 Sep 2018 20:13:00 GMT

    [ https://issues.apache.org/jira/browse/SPARK-25150?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16632512#comment-16632512
] 

Nicholas Chammas commented on SPARK-25150:
------------------------------------------

Correct, this isn't a cross join. It's just a plain inner join.

In theory, whether cross joins are enabled or not should have no bearing on the result. However,
what we're seeing is that without them enabled we get an incorrect error and with them enabled
we get incorrect results.

If we were actually trying a cross join (i.e. no {{on=(...)}} condition specified) I think
those results (with the 4 output rows) would still be incorrect since you'd expect NH's population
to be combined with RI's stats in one of the output rows, but that's not the case. You'd also
expect MA to show up in the output, too.

> The second join joins on a column in {{states}}, but that is not a DataFrame used in
that join. Is that the problem?

Not sure what you mean here. Both joins join on {{states}}, which is the first DataFrame in
the definition of {{analysis}}.

 

> Joining DataFrames derived from the same source yields confusing/incorrect results
> ----------------------------------------------------------------------------------
>
>                 Key: SPARK-25150
>                 URL: https://issues.apache.org/jira/browse/SPARK-25150
>             Project: Spark
>          Issue Type: Bug
>          Components: SQL
>    Affects Versions: 2.3.1
>            Reporter: Nicholas Chammas
>            Priority: Major
>         Attachments: expected-output.txt, output-with-implicit-cross-join.txt, output-without-implicit-cross-join.txt,
persons.csv, states.csv, zombie-analysis.py
>
>
> I have two DataFrames, A and B. From B, I have derived two additional DataFrames, B1
and B2. When joining A to B1 and B2, I'm getting a very confusing error:
> {code:java}
> Join condition is missing or trivial.
> Either: use the CROSS JOIN syntax to allow cartesian products between these
> relations, or: enable implicit cartesian products by setting the configuration
> variable spark.sql.crossJoin.enabled=true;
> {code}
> Then, when I configure "spark.sql.crossJoin.enabled=true" as instructed, Spark appears
to give me incorrect answers.
> I am not sure if I am missing something obvious, or if there is some kind of bug here.
The "join condition is missing" error is confusing and doesn't make sense to me, and the seemingly
incorrect output is concerning.
> I've attached a reproduction, along with the output I'm seeing with and without the implicit
cross join enabled.
> I realize the join I've written is not "correct" in the sense that it should be left
outer join instead of an inner join (since some of the aggregates are not available for all
states), but that doesn't explain Spark's behavior.



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