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From "Kannan Muthukkaruppan (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-4433) avoid extra next (potentially a seek) if done with column/row
Date Mon, 19 Sep 2011 00:20:09 GMT
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: Bug
            Reporter: Kannan Muthukkaruppan
            Assignee: Kannan Muthukkaruppan


[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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