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https://issues.apache.org/jira/browse/CASSANDRA-6933?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13948731#comment-13948731
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Benedict edited comment on CASSANDRA-6933 at 3/27/14 1:34 AM:
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Okay, so with a little wikipedia and wolfram alpha help, we have:
lg N, lg 0.5N, lg 0.25N = sum lg2 (2 ^ m) for m in 1..lg2(N), _which is the
same as_ sum(1..lg2(N)), which 0.5(lg2(N))(1+lg2(N))
Whereas lg 1 + lg 2 + lg 3 ... + lg N _is actually_ sum (lg n) for n in 1..N,
which is (lg (1*2*3*4*5...*N)) / (lg 2)
So, for N = 100 we get an extra 1205 comparisons vs an extra 25.
... and I'll leave it there. if I've still messed it up, it's close enough!
was (Author: benedict):
Okay, so with a little wikipedia and wolfram alpha help, we have:
lg N, lg 0.5N, lg 0.25N = sum lg2 (2 ^ m) for m in 1..sqrt(N), _which is the
same as_ sum(1..sqrt(N)), which is sqrt(N).(sqrt(N)+1)/2. So extra comparisons
are 0.5(N+sqrt(N))
Whereas lg 1 + lg 2 + lg 3 ... + lg N _is actually_ sum (lg n) for n in 1..N,
which is (lg (1*2*3*4*5...*N)) / (lg 2)
So, for N = 100 we get an extra 1205 comparisons vs an extra 55.
> Optimise Read Comparison Costs in collectTimeOrderedData
> --------------------------------------------------------
>
> Key: CASSANDRA-6933
> URL: https://issues.apache.org/jira/browse/CASSANDRA-6933
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Benedict
> Assignee: Benedict
> Priority: Minor
> Labels: performance
> Fix For: 2.1
>
>
> Introduce a new SearchIterator construct, which can be obtained from a
> ColumnFamily, which permits efficiently iterating a subset of the cells in
> ascending order. Essentially, it saves the previously visited position and
> searches from there, but also tries to avoid searching the whole remaining
> space if possible.
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