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https://issues.apache.org/jira/browse/CASSANDRA-2062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13048286#comment-13048286
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Stu Hood commented on CASSANDRA-2062:
-------------------------------------
This design is intended to preserve the status quo of nested iterators by
adjusting their contract slightly. If we're willing to make more extensive
changes to compaction, I truly believe that the design put forth on
CASSANDRA-847 is an order of magnitude more elegant.
> Better control of iterator consumption
> --------------------------------------
>
> Key: CASSANDRA-2062
> URL: https://issues.apache.org/jira/browse/CASSANDRA-2062
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Stu Hood
> Priority: Minor
> Fix For: 1.0
>
> Attachments:
> 0001-CASSANDRA-2062-0001-Improved-iterator-for-merging-sort.txt,
> 0002-CASSANDRA-2062-0002-Port-all-collating-consumers-to-Me.txt
>
>
> The core reason for this ticket is to gain control over the consumption of
> the lazy nested iterators in the read path.
> {quote}We survive now because we write the size of the row at the front of
> the row (via some serious acrobatics at write time), which gives us hasNext()
> for rows for free. But it became apparent while working on the block-based
> format that hasNext() will not be cheap unless the current item has been
> consumed. "Consumption" of the row is easy, and blocks will be framed so that
> they can be very easily skipped, but you don't want to have to seek to the
> end of the row to answer hasNext, and then seek back to the beginning to
> consume the row, which is what CollatingIterator would have forced us to
> do.{quote}
> While we're at it, we can also improve efficiency: for {{M}} iterators
> containing {{N}} total items, commons.collections.CollatingIterator performs
> a {{O(M*N)}} merge, and calls hasNext multiple times per returned value. We
> can do better.
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