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https://issues.apache.org/jira/browse/CASSANDRA-13911?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17776257#comment-17776257
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Szymon Miezal commented on CASSANDRA-13911:
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Does anyone remember what was the motivation for this particular condition
change
[https://github.com/apache/cassandra/commit/1efdf330e291a41cd8051e0c1195f75b5d352370#diff-212cd446e1b8c2aeb27818def0ba8c67370a1e00c5f657c85e42f0d43adfe05bR559?]
I have found that
[https://github.com/apache/cassandra-dtest/commit/51ad68ec45c7a40de1c51b31651632f2e87ceaa4]
passes without it as well which is suspicious.
The motivation for the questions is that I have found that in case of DISTINCT
queries we are making an additional SRP call for every call of
https://github.com/apache/cassandra/blob/cassandra-3.11/src/java/org/apache/cassandra/service/DataResolver.java#L729
due to exactly that condition.
> IllegalStateException thrown by UPI.Serializer.hasNext() for some SELECT
> queries
> --------------------------------------------------------------------------------
>
> Key: CASSANDRA-13911
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13911
> Project: Cassandra
> Issue Type: Bug
> Components: Legacy/Coordination
> Reporter: Aleksey Yeschenko
> Assignee: Aleksey Yeschenko
> Priority: Normal
> Fix For: 3.0.15, 3.11.1
>
>
> Certain combinations of rows, in presence of per partition limit (set
> explicitly in 3.6+ or implicitly to 1 via DISTINCT) cause
> {{UnfilteredPartitionIterators.Serializer.hasNext()}} to throw
> {{IllegalStateException}} .
> Relevant code snippet:
> {code}
> // We can't answer this until the previously returned iterator has been fully
> consumed,
> // so complain if that's not the case.
> if (next != null && next.hasNext())
> throw new IllegalStateException("Cannot call hasNext() until the previous
> iterator has been fully consumed");
> {code}
> Since {{UnfilteredPartitionIterators.Serializer}} and
> {{UnfilteredRowIteratorSerializer.serializer}} deserialize partitions/rows
> lazily, it is required for correct operation of the partition iterator to
> have the previous partition fully consumed, so that deserializing the next
> one can start from the correct position in the byte buffer. However, that
> condition won’t always be satisfied, as there are legitimate combinations of
> rows that do not consume every row in every partition.
> For example, look at [this
> dtest|https://github.com/iamaleksey/cassandra-dtest/commits/13911].
> In case we end up with a following pattern of rows:
> {code}
> node1, partition 0 | 0
> node2, partition 0 | x x
> {code}
> , where {{x}} and {{x}} a row tombstones for rows 1 and 2, it’s sufficient
> for {{MergeIterator}} to only look at row 0 in partition from node1 and at
> row tombstone 1 from node2 to satisfy the per partition limit of 1. The
> stopping merge result counter will stop iteration right there, leaving row
> tombstone 2 from node2 unvisited and not deseiralized. Switching to the next
> partition will in turn trigger the {{IllegalStateException}} because we
> aren’t done yet.
> The stopping counter is behaving correctly, so is the {{MergeIterator}}. I’ll
> note that simply removing that condition is not enough to fix the problem
> properly - it’d just cause us to deseiralize garbage, trying to deserialize a
> new partition from a position in the bytebuffer that precedes remaining rows
> in the previous partition.
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