[jira] [Commented] (HBASE-9778) Avoid seeking to next column in ExplicitColumnTracker when possible

["Lars Hofhansl (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-9778?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13923502#comment-13923502 ] 

Lars Hofhansl commented on HBASE-9778:
--------------------------------------

[~jesse_yates],

bq. Why not keep a count of the versions seen and only skip past if approaching (at?) the number of max versions? Shouldn't need any synchronization on that member variable, so access should be fast

I found a cheap way of doing this after all. We can keep a counter and reset it every time this.index is changed (i.e. we move on to a new column). That way we can limit the number of skips per selected column. We can even set the number of opportunistic skip per scan operation. Patch coming soon.


> 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-v2.txt, 9778-0.94-v3.txt, 9778-0.94-v4.txt, 9778-0.94.txt, 9778-trunk-v2.txt, 9778-trunk-v3.txt, 9778-trunk.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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