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https://issues.apache.org/jira/browse/HBASE-3498?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12989859#comment-12989859
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ryan rawson commented on HBASE-3498:
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After talking to stack about the unstable peek(), this might be ok.
Consider, we have a scanner at the 'current' position that has a peek()ed value
of 'C'. Now later on when we peek() again we get 'B' or some other value < 'C'
(but larger than the previously "gotten" values). This is still the 'current'
heap in the KeyValueHeap, and only during the 'next' would we recheck the
priority queue. At this point a different scanner might become the 'current'
(aka top) scanner, but it could NOT have previously been the current scanner,
because it would have had to have a value < 'C', which it did not.
This only works if only 1 KeyValueScanner has an unstable 'peek' and only if it
is unstable in 1 direction (gets smaller than) only.
> Memstore scanner needs new semantics, which may require new data structure
> --------------------------------------------------------------------------
>
> Key: HBASE-3498
> URL: https://issues.apache.org/jira/browse/HBASE-3498
> Project: HBase
> Issue Type: Sub-task
> Reporter: ryan rawson
> Assignee: ryan rawson
> Fix For: 0.92.0
>
>
> We may need a new memstore datastructure. Much has been written about the
> concurrency and speed and cpu usage, but there are new things that were
> brought to light with HBASE-2856.
> Specifically we need a memstore scanner that serves up to the moment reads,
> with a row-level completeness. Specifically after a memstore scanner goes
> past the end of a row, it should return some kind of 'end of row' token which
> the StoreScanner should trigger on to know it's at the end of the row. The
> next call to memstore scanner.next() should return the _very next available
> row from the start of that row_ at _the time it's requested_.
> It should specifically NOT:
> - return everything but the first column
> - skip a row that was inserted _after_ the previous next() was completed
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