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From "Christopher Tubbs (JIRA)" <>
Subject [jira] [Created] (ACCUMULO-775) Optimize iterator seek() method when seeking forward
Date Tue, 25 Sep 2012 10:56:07 GMT
Christopher Tubbs created ACCUMULO-775:

             Summary: Optimize iterator seek() method when seeking forward
                 Key: ACCUMULO-775
             Project: Accumulo
          Issue Type: Improvement
          Components: tserver
            Reporter: Christopher Tubbs
            Assignee: Keith Turner
             Fix For: 1.5.0

At present, seeking is a very expensive operation. Yet, it is a very common case, especially
when writing filtering/consuming/skipping iterators to seek to the next possible match (perhaps
in the next row, when matching a column family with a regular expression), rather than continuing
to iterate. A common solution is to continue to scan for some threshold (~10-20 entries),
hoping to just "run into" the next possible match, rather than waste resources seeking directly
to it.

This pattern can be rolled in to the lower level iterator, so that iterators on top don't
have to do this. They can seek, and the underlying source iterator can simply consume the
next X entries when it makes sense, rather than waste resources seeking.

I could be wrong (please comment and correct me below if I am), but I imagine that the places
where this would make the most sense is if the data currently being sought (seek'd) is in
the current compressed block from the underlying file, especially if it is forward, relative
to the current pointer. A better seek method should be able to tell where one currently is,
and whether the requested data is within reach without doing all the expensive operations
to re-seek to the same compressed block that is already loaded, reload it, decompress it,
and scan to the requested starting point.

Having such an optimization would eliminate the need for users to try to calibrate their own
such scan vs. seek optimization based on guessing whether their data is in the current block
or another one, while still getting that same performance benefit.

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