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Date: Tue, 4 Mar 2014 17:59:23 +0000 (UTC)
From: "Lars Hofhansl (JIRA)" <jira@apache.org>
To: issues@hbase.apache.org
Message-ID: <JIRA.12697596.1393476696612.12144.1393955963727@arcas>
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Subject: [jira] [Commented] (HBASE-10625) Remove unnecessary key compare
 from AbstractScannerV2.reseekTo
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    [ https://issues.apache.org/jira/browse/HBASE-10625?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13919708#comment-13919708 ] 

Lars Hofhansl commented on HBASE-10625:
---------------------------------------

I'll chop away at this as I get time. This change brings the "C1,C3 with FAST_DIFF" case from 43.2s to 36.0s (of course it's of no help if it's not correct :) ).


> Remove unnecessary key compare from AbstractScannerV2.reseekTo
> --------------------------------------------------------------
>
>                 Key: HBASE-10625
>                 URL: https://issues.apache.org/jira/browse/HBASE-10625
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Lars Hofhansl
>         Attachments: 10625-0.94-experimental.txt, 10625-0.94.txt, 10625-trunk-experimental.txt, 10625-trunk.txt
>
>
> In reseekTo we find this
> {code}
> ...
>         compared = compareKey(reader.getComparator(), key, offset, length);
>         if (compared < 1) {
>           // If the required key is less than or equal to current key, then
>           // don't do anything.
>           return compared;
>         } else {
>            ...
>            return loadBlockAndSeekToKey(this.block, this.nextIndexedKey,
>               false, key, offset, length, false);
> ...
> {code}
> loadBlockAndSeekToKey already does the right thing when a we pass a key that sorts before the current key. It's less efficient than this early check, but in the vast (all?) cases we pass forward keys (as required by the reseek contract). We're optimizing the wrong thing.
> Scanning with the ExplicitColumnTracker is 20-30% faster.
> (I tested with rows of 5 short KVs selected the 2nd and or 4th column)
> I propose simply removing that check.


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