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https://issues.apache.org/jira/browse/HBASE-8151?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13608677#comment-13608677
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Lars Hofhansl commented on HBASE-8151:
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Unless there are objections I will commit the 1st versions of the 0.94 and 0.96
patches, and then file a separate ticket for #2.
> Small optimization in HFileReaderV2
> -----------------------------------
>
> Key: HBASE-8151
> URL: https://issues.apache.org/jira/browse/HBASE-8151
> Project: HBase
> Issue Type: Bug
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Attachments: 8151-0.94.txt, 8151-0.94-v2.txt, 8151-0.96.txt,
> 8151-0.96-v2.txt, 8151-0.96-v3.txt
>
>
> HFiles V2 store the memstoreTS of each KV.
> In many cases all the KVs in an HFile will have a memstoreTS of 0 (that is
> the case when at the time the HFile was written there are no KVs that were
> created after the oldest still active scanner - which is frequently the case).
> In that case we:
> # do not need to decode the memstoreTS (a vlong), since we know its value is
> 0 and its length is 1 byte.
> # when we compact HFiles and all of the involved files have only KVs with
> memstoreTS = 0 we know ahead of time that all KVs meet this condition and we
> do not need to store the memstoreTS in the new HFile.
> This issue will cover the first part. The performance improvement will be
> modest as it is fairly cheap to decode vlongs of size 1.
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