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https://issues.apache.org/jira/browse/HBASE-4811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13681891#comment-13681891
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chunhui shen commented on HBASE-4811:
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bq.So would: ReversedRegionScanner -> KVHelp -> RerversedStoreScanner ->
ReversedKVHeap -> (MemstoreScanner|StoreFileScanner) work?
Need ReversedRegionScanner -> ReversedKVHeap, because we need a reversed sort
of scanners in ReversedKVHeap and call seekToPreviousRow for
RegionScanner#nextRow, otherwise the scan order would be wrong.
bq.ReversedKeyValueHeap should not need backwardSeek, since all calls to
(re)seek, etc, call backwardSeek anyway.
ReversedKeyValueHeap should support backwardSeek and seekToPreviousRow even if
no calls. Is it better to understand that prohibit calling (re)seek in
ReversedKeyValueHeap and ReversedStoreScanner
bq.do we need NonReversedNonLazyKeyValueScanner? Could add "unsupported"
implementations for these methods to NonLazyKeyValueScanner.
MemStoreScanner extends from NonLazyKeyValueScanner, it seems not appropriate
if add "unsupported" implementations to NonLazyKeyValueScanner
> 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, 0.94.7
> Reporter: John Carrino
> Assignee: Liang Xie
> Attachments: 4811-trunk-v10.txt, 4811-trunk-v5.patch,
> HBase-4811-0.94.3modified.txt, HBase-4811-0.94-v2.txt,
> hbase-4811-trunkv1.patch, hbase-4811-trunkv4.patch, hbase-4811-trunkv6.patch,
> hbase-4811-trunkv7.patch, hbase-4811-trunkv8.patch, hbase-4811-trunkv9.patch
>
>
> 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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