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https://issues.apache.org/jira/browse/HBASE-9778?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13798131#comment-13798131
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Lars Hofhansl commented on HBASE-9778:
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I'd like to do some brainstorming about how we can make reseek faster if the
"reseek distance" is expected to be small. Currently the absolute worst case is
when reseek just seeks to the next KV, where next is orders of magnitudes
faster.
Unfortunately we hit this worst case all the time if there is only one version
of each KV.
> Avoid seeking to next column in ExplicitColumnTracker when possible
> -------------------------------------------------------------------
>
> Key: HBASE-9778
> URL: https://issues.apache.org/jira/browse/HBASE-9778
> Project: HBase
> Issue Type: Bug
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Attachments: 9778-0.94.txt, 9778-0.94-v2.txt, 9778-0.94-v3.txt,
> 9778-trunk.txt, 9778-trunk-v2.txt, 9778-trunk-v3.txt
>
>
> The issue of slow seeking in ExplicitColumnTracker was brought up by
> [~vrodionov] on the dev list.
> My idea here is to avoid the seeking if we know that there aren't many
> versions to skip.
> How do we know? We'll use the column family's VERSIONS setting as a hint. If
> VERSIONS is set to 1 (or maybe some value < 10) we'll avoid the seek and call
> SKIP repeatedly.
> HBASE-9769 has some initial number for this approach:
> Interestingly it depends on which column(s) is (are) selected.
> Some numbers: 4m rows, 5 cols each, 1 cf, 10 bytes values, VERSIONS=1,
> everything filtered at the server with a ValueFilter. Everything measured in
> seconds.
> Without patch:
> ||Wildcard||Col 1||Col 2||Col 4||Col 5||Col 2+4||
> |6.4|8.5|14.3|14.6|11.1|20.3|
> With patch:
> ||Wildcard||Col 1||Col 2||Col 4||Col 5||Col 2+4||
> |6.4|8.4|8.9|9.9|6.4|10.0|
> Variation here was +- 0.2s.
> So with this patch scanning is 2x faster than without in some cases, and
> never slower. No special hint needed, beyond declaring VERSIONS correctly.
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