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Date: Fri, 3 Apr 2015 09:39:53 +0000 (UTC)
From: "Benedict (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
Message-ID: <JIRA.12779758.1425550560000.116414.1428053993467@Atlassian.JIRA>
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Subject: [jira] [Commented] (CASSANDRA-8915) Improve MergeIterator
 performance
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    [ https://issues.apache.org/jira/browse/CASSANDRA-8915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14394250#comment-14394250 ] 

Benedict commented on CASSANDRA-8915:
-------------------------------------

I perhaps should have commented when I first saw the link. It should be quite viable to merge the behaviours; the Candidate just needs to have a flag indicating if the value is "real" or not, and to just discard the not-real values it encounters.

> Improve MergeIterator performance
> ---------------------------------
>
>                 Key: CASSANDRA-8915
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8915
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Branimir Lambov
>            Assignee: Branimir Lambov
>            Priority: Minor
>
> The implementation of {{MergeIterator}} uses a priority queue and applies a pair of {{poll}}+{{add}} operations for every item in the resulting sequence. This is quite inefficient as {{poll}} necessarily applies at least {{log N}} comparisons (up to {{2log N}}), and {{add}} often requires another {{log N}}, for example in the case where the inputs largely don't overlap (where {{N}} is the number of iterators being merged).
> This can easily be replaced with a simple custom structure that can perform replacement of the top of the queue in a single step, which will very often complete after a couple of comparisons and in the worst case scenarios will match the complexity of the current implementation.
> This should significantly improve merge performance for iterators with limited overlap (e.g. levelled compaction).


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