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https://issues.apache.org/jira/browse/ACCUMULO-2232?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13880401#comment-13880401
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Christopher Tubbs commented on ACCUMULO-2232:
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This issue would probably go away entirely, if we separated the Combiner from
the scope it's supposed to run at, and made the "full major compaction" scope a
top-level iterator scope, along with "partial major compaction". Obviously,
this is not a simple transition..., but I think it's an (eventual) worthwhile
one.
> Combiners can cause deleted data to come back
> ---------------------------------------------
>
> Key: ACCUMULO-2232
> URL: https://issues.apache.org/jira/browse/ACCUMULO-2232
> Project: Accumulo
> Issue Type: Bug
> Components: client, tserver
> Reporter: John Vines
>
> The case-
> 3 files with-
> * 1 with a key, k, with timestamp 0, value 3
> * 1 with a delete of k with timestamp 1
> * 1 with k with timestamp 2, value 2
> The column of k has a summing combiner set on it. The issue here is that
> depending on how the major compactions play out, differing values with
> result. If all 3 files compact, the correct value of 2 will result. However,
> if 1 & 3 compact first, they will aggregate to 5. And then the delete will
> fall after the combined value, resulting in the result 5 to persist.
> First and foremost, this should be documented. I think to remedy this,
> combiners should only be used on full MajC, not not full ones. This may
> necessitate a special flag or a new combiner that implemented the proper
> semantics.
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