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https://issues.apache.org/jira/browse/CASSANDRA-2062?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Stu Hood updated CASSANDRA-2062:
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Attachment: 0004-CASSANDRA-2062-0004-Replace-ReducingIterator-for-lazy-.txt
0003-CASSANDRA-2062-0003-Replace-ReducingIterator-in-QueryF.txt
0002-CASSANDRA-2062-0002-Port-all-collating-consumers-to-Me.txt
0001-CASSANDRA-2062-0001-Improved-iterator-for-merging-sort.txt
bq. [CollatingIterator] calls hasNext() on its child iterator immediately after
pulling off the least value from one.
ReducingIterator also does not know whether there are more items until it has
consumed past the end of the current item, which is why it was necessary to
squash it in.
bq. we still have the RI use in collectCollatedColumns [...] as well as
LazyColumnIterator
Fixed in 0003 and removed in 0004.
bq. It looks to me like the main obstacle to using MI there is making
MI.Reducer support customizable isEqual?
The comparator passed to the MI is used for isEqual, so it should be pluggable
already.
> 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,
> 0003-CASSANDRA-2062-0003-Replace-ReducingIterator-in-QueryF.txt,
> 0004-CASSANDRA-2062-0004-Replace-ReducingIterator-for-lazy-.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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