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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4811) Support reverse Scan
Date Tue, 01 Oct 2013 03:43:24 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13782599#comment-13782599
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Lars Hofhansl commented on HBASE-4811:
--------------------------------------

I will double check this on another machine as well (this was with the 0.94 patch).
Note that if the data is actually returned to the client the difference is not noticeable.

> Support reverse Scan
> --------------------
>
>                 Key: HBASE-4811
>                 URL: https://issues.apache.org/jira/browse/HBASE-4811
>             Project: HBase
>          Issue Type: New Feature
>          Components: Client
>    Affects Versions: 0.20.6, 0.94.7
>            Reporter: John Carrino
>            Assignee: chunhui shen
>             Fix For: 0.98.0
>
>         Attachments: 4811-0.94-v3.txt, 4811-trunk-v10.txt, 4811-trunk-v5.patch, HBase-4811-0.94.3modified.txt,
hbase-4811-0.94 v21.patch, HBase-4811-0.94-v2.txt, hbase-4811-trunkv11.patch, hbase-4811-trunkv12.patch,
hbase-4811-trunkv13.patch, hbase-4811-trunkv14.patch, hbase-4811-trunkv15.patch, hbase-4811-trunkv16.patch,
hbase-4811-trunkv17.patch, hbase-4811-trunkv18.patch, hbase-4811-trunkv19.patch, hbase-4811-trunkv1.patch,
hbase-4811-trunkv20.patch, hbase-4811-trunkv21.patch, hbase-4811-trunkv4.patch, hbase-4811-trunkv6.patch,
hbase-4811-trunkv7.patch, hbase-4811-trunkv8.patch, hbase-4811-trunkv9.patch
>
>
> Reversed scan means scan the rows backward. 
> And StartRow bigger than StopRow in a reversed scan.
> For example, for the following rows:
> aaa/c1:q1/value1
> aaa/c1:q2/value2
> bbb/c1:q1/value1
> bbb/c1:q2/value2
> ccc/c1:q1/value1
> ccc/c1:q2/value2
> ddd/c1:q1/value1
> ddd/c1:q2/value2
> eee/c1:q1/value1
> eee/c1:q2/value2
> you could do a reversed scan from 'ddd' to 'bbb'(exclude) like this:
> Scan scan = new Scan();
> scan.setStartRow('ddd');
> scan.setStopRow('bbb');
> scan.setReversed(true);
> for(Result result:htable.getScanner(scan)){
>  System.out.println(result);
> }
> Aslo you could do the reversed scan with shell like this:
> hbase> scan 'table',{REVERSED => true,STARTROW=>'ddd', STOPROW=>'bbb'}
> And the output is:
> ddd/c1:q1/value1
> ddd/c1:q2/value2
> ccc/c1:q1/value1
> ccc/c1:q2/value2
> NOTE: when setting reversed as true for a client scan, you must set the start row, else
will throw exception. Through {@link Scan#createBiggestByteArray(int)},you could get a big
enough byte array as the start row
> All the documentation I find about HBase says that if you want forward and reverse scans
you should just build 2 tables and one be ascending and one descending.  Is there a fundamental
reason that HBase only supports forward Scan?  It seems like a lot of extra space overhead
and coding overhead (to keep them in sync) to support 2 tables.  
> I am assuming this has been discussed before, but I can't find the discussions anywhere
about it or why it would be infeasible.



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