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https://issues.apache.org/jira/browse/CASSANDRA-8915?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14629583#comment-14629583
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Branimir Lambov commented on CASSANDRA-8915:
--------------------------------------------
The rebased and squashed code is
[here|https://github.com/blambov/cassandra/tree/8195-rebase] with
[testall|http://cassci.datastax.com/job/blambov-8195-rebase-testall/] and
[dtest|http://cassci.datastax.com/job/blambov-8195-rebase-dtest/].
> 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
> Labels: compaction, performance
> Fix For: 3.x
>
> Time Spent: 0.2h
> Remaining Estimate: 0h
>
> 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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