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https://issues.apache.org/jira/browse/ACCUMULO-790?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Christopher Tubbs resolved ACCUMULO-790.
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Resolution: Fixed
> RFile should compress using common prefixes of key elements
> -----------------------------------------------------------
>
> Key: ACCUMULO-790
> URL: https://issues.apache.org/jira/browse/ACCUMULO-790
> Project: Accumulo
> Issue Type: Improvement
> Components: tserver
> Reporter: Christopher Tubbs
> Assignee: Christopher Tubbs
> Labels: compression, file, hackathon, optimization, rfile
> Fix For: 1.5.0
>
>
> Relative keys have proven themselves as a great way to compress within
> dimensions of the key. However, we could probably do better, since we know
> that our data is sorted lexicographically, we can make a reasonable
> assumption that we will get better compression if we only store the fact that
> a key (or portion of a key) has a common prefix with the previous key, even
> if it is not an exact match.
> Currently, in RFile, unused bits from the delete flag byte are being used to
> store flags that show whether an element of the key is exactly the same as
> the previous, or if it is different. We can change the semantics of these
> flags to store three states per element of the key: exact match as previous
> key, has a common prefix as previous key, no relative key compression. If we
> don't want to add a byte to store 2 bits for 3 states per element, we can
> just take the ordinal value of the unused 7 bits of the delete flag field and
> map it to an enumeration of relative key flags.
> In the case of a common prefix flag enabled for a given element of the
> current key when reading the RFile, we can interpret the first bytes of that
> element as a VInt expressing the length of the common prefix relative to the
> previous key's same element. Because this will add at least one byte to the
> the length of that element, we will not want to use the common prefix
> compression if the common prefix is less than 2 bytes. For less than 2 bytes
> in common (1 or 0 bytes in common), we'd select the no compression flag for
> that element.
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