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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-1614) Add next() and skipTo() variants to DocIdSetIterator that return the current doc, instead of boolean
Date Thu, 21 May 2009 16:39:45 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-1614?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12711682#action_12711682
] 

Michael McCandless commented on LUCENE-1614:
--------------------------------------------

bq. If they are all on -1 to start with, they are already all sorted.

Right but that defeats the optimization.  I'm talking about this code in ConjunctionScorer:
{code}
Arrays.sort(scorers, new Comparator() {         // sort the array
    public int compare(Object o1, Object o2) {
      return ((Scorer)o1).doc() - ((Scorer)o2).doc();
    }
  });

doNext();

// If first-time skip distance is any predictor of
// scorer sparseness, then we should always try to skip first on
// those scorers.
// Keep last scorer in it's last place (it will be the first
// to be skipped on), but reverse all of the others so that
// they will be skipped on in order of original high skip.
int end=(scorers.length-1);
for (int i=0; i<(end>>1); i++) {
  Scorer tmp = scorers[i];
  scorers[i] = scorers[end-i-1];
  scorers[end-i-1] = tmp;
}
{code}

Ie it sets things up so that "typically" the rarest sub-scorer drives the intersection.  If
they are all on -1 then this heuristic won't work.

{quote}
We could do some smart sorting in the constructor so that we skip in cheap and fast scorers
first (TermScorers first, ordered by df, followed by simple conjunctions of terms, followed
by other more expensive stuff like sloppy phrase queries and complex boolean queries. Perhaps
in the future, even a method on Scorer that estimates it's cost?
{quote}

Right, we'd need to do something along these lines if we switch DISI to start with doc() =
-1.

> Add next() and skipTo() variants to DocIdSetIterator that return the current doc, instead
of boolean
> ----------------------------------------------------------------------------------------------------
>
>                 Key: LUCENE-1614
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1614
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>            Reporter: Shai Erera
>             Fix For: 2.9
>
>         Attachments: LUCENE-1614.patch, LUCENE-1614.patch
>
>
> See http://www.nabble.com/Another-possible-optimization---now-in-DocIdSetIterator-p23223319.html
for the full discussion. The basic idea is to add variants to those two methods that return
the current doc they are at, to save successive calls to doc(). If there are no more docs,
return -1. A summary of what was discussed so far:
> # Deprecate those two methods.
> # Add nextDoc() and skipToDoc(int) that return doc, with default impl in DISI (calls
next() and skipTo() respectively, and will be changed to abstract in 3.0).
> #* I actually would like to propose an alternative to the names: advance() and advance(int)
- the first advances by one, the second advances to target.
> # Wherever these are used, do something like '(doc = advance()) >= 0' instead of comparing
to -1 for improved performance.
> I will post a patch shortly

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