[
https://issues.apache.org/jira/browse/CASSANDRA-2062?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13047901#comment-13047901
]
Jonathan Ellis commented on CASSANDRA-2062:
-------------------------------------------
bq. hasNext() will not be cheap unless the current item has been consumed
I don't think I understand the problem we're trying to solve here. If hasNext
is expensive on a given iterator, the straightforward fix is to cache the
answer (and invalidate it on next()). The most obvious way to do this is to
use the guava AbstractIterator which implements both hasNext and next in terms
of computeNext.
> 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.
--
This message is automatically generated by JIRA.
For more information on JIRA, see: http://www.atlassian.com/software/jira
