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From "Andrew Hudson (JIRA)" <>
Subject [jira] Created: (LUCENE-586) Very inefficient implementation of MultiTermDocs.skipTo
Date Thu, 01 Jun 2006 18:44:30 GMT
Very inefficient implementation of MultiTermDocs.skipTo

         Key: LUCENE-586
     Project: Lucene - Java
        Type: Improvement

  Components: Index, Search  
    Reporter: Andrew Hudson

In our application anytime the index was unoptimized/contained more than one segment there
was a sharp drop in performance, which amounted to over 50ms per search on average.  We would
consistently see this drop anytime an index went from an optimized state to an unoptimized

I tracked down the issue to the implementation of MultiTermDocs.skipTo function (found in  Optimized indexes do not use this class during search but unoptimized
indexes do.  The comment on this function even explicitly states 'As yet unoptimized implementation.'
 It was implemented just by calling 'next' over and over so even if it knew it could skip
ahead hundreds of thousands of hits it would not.

So I re-implemented the function very similar to how the function was implemented
and tested it out on or application for correctness and performance and it passed all our
tests and the performance penalty of having multiple segments vanished.  We have already put
the new jar onto our production machines.

Here is my implementation of skipTo, which closely mirrors the accepted implementation of
'next', please feel free to test it and commit it.

  /** Much more optimized implementation. Could be
   * optimized fairly easily to skip entire segments */
  public boolean skipTo(int target) throws IOException {
    if (current != null && current.skipTo(target-base)) {
      return true;
    } else if (pointer < readers.length) {
      base = starts[pointer];
      current = termDocs(pointer++);
      return skipTo(target);
    } else
      return false;

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