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From "Branimir Lambov (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8915) Improve MergeIterator performance
Date Thu, 05 Mar 2015 14:36:38 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14348795#comment-14348795
] 

Branimir Lambov commented on CASSANDRA-8915:
--------------------------------------------

Yes, what I'm suggesting is a replacement of the priority queue with a custom version that
supports {{swapHead}}. It actually doesn't have to be a full PQ implementation, {{swapHead}}
is the only operation needed (the initial heap construction can be done by sorting).

> 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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