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From "Jonathan Ellis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-6933) Optimise Read Comparison Costs in collectTimeOrderedData
Date Wed, 02 Apr 2014 22:32:15 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-6933?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13958264#comment-13958264
] 

Jonathan Ellis commented on CASSANDRA-6933:
-------------------------------------------

I wrote a quick script and Benedict is right, assuming uniform distribution does help us pick
better bsearch "pivots" than just "half the remaining elements."

{code}
import bisect
import random

error_bisecting = 0
error_range = 0

for _ in xrange(10000):
    universe = range(100)
    sample = sorted(random.sample(universe, 10))

    last_index = 0
    r = None
    for n in sample:
        i = bisect.bisect(universe, n, last_index + 1)
        guess_bisecting = (len(universe) - (last_index + 1)) / 2
        guess_range = last_index + r if r else guess_bisecting
        error_bisecting += abs(i - guess_bisecting)
        error_range = abs(i - guess_range)
        r = i - last_index
        last_index = i

print error_range - error_bisecting
{code}

> 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
>
>         Attachments: 6933-v3.txt
>
>
> 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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