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From "Raymond Liu (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4433) avoid extra next (potentially a seek) if done with column/row
Date Tue, 05 Mar 2013 07:29:13 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4433?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13593147#comment-13593147
] 

Raymond Liu commented on HBASE-4433:
------------------------------------

And also , when there are only one version of row exist, no matter how many hfile you have,
a sequence scan operation will always need scan all the hfile row by row. you don't skip any
real seek by lazy seek. And in many case, like hive on top of hbase or a bulkloaded read only
table, I think it's quite normal that a row only got one version.

b.t.w how to modify exist comment? Find no way to do it, while it seems some one could modify
their comment.
                
> avoid extra next (potentially a seek) if done with column/row
> -------------------------------------------------------------
>
>                 Key: HBASE-4433
>                 URL: https://issues.apache.org/jira/browse/HBASE-4433
>             Project: HBase
>          Issue Type: Improvement
>            Reporter: Kannan Muthukkaruppan
>            Assignee: Kannan Muthukkaruppan
>             Fix For: 0.92.0
>
>
> [Noticed this in 89, but quite likely true of trunk as well.]
> When we are done with the requested column(s) the code still does an extra next() call
before it realizes that it is actually done. This extra next() call could potentially result
in an unnecessary extra block load. This is likely to be especially bad for CFs where the
KVs are large blobs where each KV may be occupying a block of its own. So the next() can often
load a new unrelated block unnecessarily.
> --
> For the simple case of reading say the top-most column in a row in a single file, where
each column (KV) was say a block of its own-- it seems that we are reading 3 blocks, instead
of 1 block!
> I am working on a simple patch and with that the number of seeks is down to 2. 
> [There is still an extra seek left.  I think there were two levels of extra/unnecessary
next() we were doing without actually confirming that the next was needed. One at the StoreScanner/ScanQueryMatcher
level which this diff avoids. I think the other is at hfs.next() (at the storefile scanner
level) that's happening whenever a HFile scanner servers out a data-- and perhaps that's the
additional seek that we need to avoid. But I want to tackle this optimization first as the
two issues seem unrelated.]
> -- 
> The basic idea of the patch I am working on/testing is as follows. The ExplicitColumnTracker
currently returns "INCLUDE" to the ScanQueryMatcher if the KV needs to be included and then
if done, only in the the next call it returns the appropriate SEEK_NEXT_COL or SEEK_NEXT_ROW
hint. For the cases when ExplicitColumnTracker knows it is done with a particular column/row,
the patch attempts to combine the INCLUDE code and done hint into a single match code-- INCLUDE_AND_SEEK_NEXT_COL
and INCLUDE_AND_SEEK_NEXT_ROW.

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