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https://issues.apache.org/jira/browse/LUCENE-4396?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14018417#comment-14018417
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Da Huang commented on LUCENE-4396:
----------------------------------
About scores diff. on BS/BS2 (the same as BNS/BS2)
Now, there's scores diff. on BS/BS2, when excuting query like "+a b c d ...".
I have been told that the reason is indicate by
the TODO on ReqOptSumScorer.score() which says that
{code}
// TODO: sum into a double and cast to float if we ever send required clauses
to BS1
{code}
However, I don't think so, as the score bias is due to
different score calculating orders.
Supposed that a doc hits the query "+a b c d", the score calculated by BS is
{code}
BS.score(doc) = ((a.score() + b.score()) + c.score()) + d.score()
{code}
while the score calculated by BS2 is
{code}
BS2.score(doc) = a.score() + (float)(b.score() + c.score() + d.score())
{code}
Notice that, in BS2, we can only get the float value of (b.score() + c.score()
+ d.score())
by reqScorer.score().
Furthermore, I have noticed that actually we can control the BS's
score calulating order, so that
{code}
BS.score(doc) = a.score() + ((b.score() + c.score()) + d.score())
{code}
However, for BS2, we do not know the calculating order of
(b.score() + c.score() + d.score()), as the order is determined by
scorer's position in a heap. I still think this matters little.
I will rearrange the calculating order of BS.score() at next patch,
to see whether it works.
> BooleanScorer should sometimes be used for MUST clauses
> -------------------------------------------------------
>
> Key: LUCENE-4396
> URL: https://issues.apache.org/jira/browse/LUCENE-4396
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Attachments: And.tasks, AndOr.tasks, AndOr.tasks, LUCENE-4396.patch,
> LUCENE-4396.patch, LUCENE-4396.patch, LUCENE-4396.patch, LUCENE-4396.patch,
> LUCENE-4396.patch, luceneutil-score-equal.patch, luceneutil-score-equal.patch
>
>
> Today we only use BooleanScorer if the query consists of SHOULD and MUST_NOT.
> If there is one or more MUST clauses we always use BooleanScorer2.
> But I suspect that unless the MUST clauses have very low hit count compared
> to the other clauses, that BooleanScorer would perform better than
> BooleanScorer2. BooleanScorer still has some vestiges from when it used to
> handle MUST so it shouldn't be hard to bring back this capability ... I think
> the challenging part might be the heuristics on when to use which (likely we
> would have to use firstDocID as proxy for total hit count).
> Likely we should also have BooleanScorer sometimes use .advance() on the subs
> in this case, eg if suddenly the MUST clause skips 1000000 docs then you want
> to .advance() all the SHOULD clauses.
> I won't have near term time to work on this so feel free to take it if you
> are inspired!
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