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https://issues.apache.org/jira/browse/HBASE-4433?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13589068#comment-13589068
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Raymond Liu commented on HBASE-4433:
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I am wondering, we might add a conf to let user choose the strategy to allow
include_and_seek or just separate include/seek. However, the difference of this
kind of settings might not be easy to be figure out by an end user. And whether
the table have many history versions or not also totally depends on the usage
of the table. Better to have some auto select mechanism to help with it.
If the table is mainly go with one time write/many read mode, only user know
it, I don't know is there any way to find out this by hbase itself?
While if table is configed with MAX history VERSION set to 1 etc, Then for most
chance I guess it is safe for the column tracker to go with separate
include/seek approaching.
> 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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