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https://issues.apache.org/jira/browse/FLINK-7919?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16246487#comment-16246487
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Greg Hogan commented on FLINK-7919:
-----------------------------------
[~fhueske] would an alternative be to use inner join semantics (dropping the
unmatched element) and potentially support outer joins in the future?
> Join with Solution Set fails with NPE if Solution Set has no entry
> ------------------------------------------------------------------
>
> Key: FLINK-7919
> URL: https://issues.apache.org/jira/browse/FLINK-7919
> Project: Flink
> Issue Type: Bug
> Components: DataSet API, Local Runtime
> Affects Versions: 1.4.0, 1.3.2
> Reporter: Fabian Hueske
>
> A job with a delta iteration fails hard with a NPE in the solution set join,
> if the solution set has no entry for the join key of the probe side.
> The following program reproduces the problem:
> {code}
> DataSet<Tuple2<Long, Integer>> values = env.fromElements(
> Tuple2.of(1L, 1), Tuple2.of(2L, 1), Tuple2.of(3L, 1));
> DeltaIteration<Tuple2<Long, Integer>, Tuple2<Long, Integer>> di = values
> .iterateDelta(values, 5,0);
> DataSet<Tuple2<Long, Integer>> loop = di.getWorkset()
> .map(new MapFunction<Tuple2<Long, Integer>, Tuple2<Long, Integer>>() {
> @Override
> public Tuple2<Long, Integer> map(Tuple2<Long, Integer> value) throws
> Exception {
> // modifying the key to join on a non existing solution set key
> return Tuple2.of(value.f0 + 1, 1);
> }
> })
> .join(di.getSolutionSet()).where(0).equalTo(0)
> .with(new JoinFunction<Tuple2<Long, Integer>, Tuple2<Long, Integer>,
> Tuple2<Long, Integer>>() {
> @Override
> public Tuple2<Long, Integer> join(
> Tuple2<Long, Integer> first,
> Tuple2<Long, Integer> second) throws Exception {
>
> return Tuple2.of(first.f0, first.f1 + second.f1);
> }
> });
> DataSet<Tuple2<Long, Integer>> result = di.closeWith(loop, loop);
> result.print();
> {code}
> It doesn't matter whether the solution set is managed or not.
> The problem is cause because the solution set hash table prober returns a
> {{null}} value if the solution set does not contain a value for the probe
> side key.
> The join operator does not check if the return value is {{null}} or not but
> immediately tries to create a copy using a {{TypeSerializer}}. This copy
> fails with a NPE.
> I propose to check for {{null}} and call the join function with {{null}} on
> the solution set side. This gives OUTER JOIN semantics for join.
> Since the code was previously failing with a NPE, it is safe to forward the
> {{null}} into the {{JoinFunction}}.
> However, users must be aware that the solution set value may be {{null}} and
> we need to update the documentation (JavaDocs + website) to describe the
> behavior.
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