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Date: Tue, 22 Nov 2011 22:34:44 +0000 (UTC)
From: "John Carrino (Commented) (JIRA)" <jira@apache.org>
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Subject: [jira] [Commented] (HBASE-4811) Support reverse Scan
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    [ https://issues.apache.org/jira/browse/HBASE-4811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13155506#comment-13155506 ] 

John Carrino commented on HBASE-4811:
-------------------------------------

Yeah, I'm not that familiar with the codebase, but I'd assume that in
order to get forward scans you'd have to have the data sorted. And
from what I understand it is internally stored as "sstables" or
HFiles. If you have it sorted to scan in one direction, it seems
pretty easy to go the other direction.  LevelDb uses ssTables and
supports reverse ranges.

The only thing that I could think of from the design (from a high
level) that might make it difficult to do reverse ranges is dealing
with splitting ranges when moving ranges from one region server to
another.

Just from a quick look at MemStore that you mention, it uses a
KeyValueSkipListSet under the covers that is a NavigableSet and
supports descendingSet and descendingIterator.

-jc


On Tue, Nov 22, 2011 at 12:52 PM, stack (Commented) (JIRA)

                
> Support reverse Scan
> --------------------
>
>                 Key: HBASE-4811
>                 URL: https://issues.apache.org/jira/browse/HBASE-4811
>             Project: HBase
>          Issue Type: Improvement
>          Components: client
>    Affects Versions: 0.20.6
>            Reporter: John Carrino
>
> 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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