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https://issues.apache.org/jira/browse/HBASE-4811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13782618#comment-13782618
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Lars Hofhansl commented on HBASE-4811:
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On another (slower) machine. 20m rows, 1 col, 10bytes value this time and with
JDK7: with patch: 15.82 without patch: 15.70. So the effect is less pronounced
here, but it repeatable. The shortest run is always without the patch and
longest with the patch as well. Looking for culprits now.
> 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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