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https://issues.apache.org/jira/browse/ACCUMULO-473?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13233462#comment-13233462
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Keith Turner commented on ACCUMULO-473:
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Aaron, the binary search would always be done after the block was read into
memory. Like the HBase ticket mentioned this is really only useful when the
data is in cache. When the data is not in cache the block needs to be read,
decompressed, and searched. So if the read and decompress is O(N) and the
search is O(log N), then the total operation is O(N + log N). However when the
decompressed block is already in cache the operation is O(log N).
> Support binary search within RFile blocks
> -----------------------------------------
>
> Key: ACCUMULO-473
> URL: https://issues.apache.org/jira/browse/ACCUMULO-473
> Project: Accumulo
> Issue Type: Improvement
> Components: tserver
> Reporter: Keith Turner
> Assignee: Keith Turner
> Fix For: 1.5.0
>
>
> RFiles store blocks of key values using relative encoding. By default these
> blocks are small (100k). To find a key in an RFile the index is used to find
> the block, then the block is scanned for the key. It would be nice for
> iterators that do alot of seeking if a binary search were done instead of a
> scan.
> Relative encoding is a form of compression that serializes a key relative to
> the last key. For example if the row in a key is the same as the previous
> key, then the row is not stored again. This works well with sorted data.
> The current on disk format does not support random access within a block,
> because to read entry N all previous entries must be read. One option would
> be to deserialize the block into a form that supports random access and then
> do binary search. However this will consume memory and CPU. If the relative
> encoding format could be modified to support random access, then binary
> search could be supported in a memory and CPU efficient way.
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