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https://issues.apache.org/jira/browse/ACCUMULO-625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13552913#comment-13552913
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William Slacum commented on ACCUMULO-625:
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You are correct that you can only safely assume that your iterator function
will only apply to a single row. It is actually on a tablet, but that's a
transient thing because it can contain one to many rows.
In regards to unique columns, preserving session state would not allow you to
wrap all of that functionality in an iterator. You'll have an iterator stack
running on multiple tservers that are unable to communicate with each other.
There will always have to be some client work going on to merge the results.
> consider augmenting session state with "breadcrumbs"
> ----------------------------------------------------
>
> Key: ACCUMULO-625
> URL: https://issues.apache.org/jira/browse/ACCUMULO-625
> Project: Accumulo
> Issue Type: Improvement
> Components: tserver
> Reporter: Eric Newton
> Assignee: Keith Turner
>
> Presently, the iterator stack can be created and destroyed at the whim of the
> tserver and its buffering needs. In complex iterations, lower-level
> iterators can make significant progress which is not inherently obvious in
> any returned key. When the iterator stack is re-created to continue a query,
> the last key returned is used to {{seek()}} the iterators. Lower-level
> iterators must re-scan their data to move back to the old position.
> Consider a mechanism to save progress beyond the last key returned.
>
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