Function queries producing scores of -inf or NaN (e.g. 1/x) return incorrect
results with TopScoreDocCollector
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Key: LUCENE-2271
URL: https://issues.apache.org/jira/browse/LUCENE-2271
Project: Lucene - Java
Issue Type: Bug
Components: Search
Affects Versions: 2.9
Reporter: Uwe Schindler
Fix For: 2.9.2, 3.0.1, 3.1
This is a foolowup to LUCENE-2270, where a part of this problem was fixed
(boost = 0 leading to NaN scores, which is also un-intuitive), but in general,
function queries in Solr can create these invalid scores easily. In previous
version of Lucene these scores ordered correct (except NaN, which mixes up
results), but never invalid document ids are returned (like Integer.MAX_VALUE).
The problem is: TopScoreDocCollector pre-fills the HitQueue with sentinel
ScoreDocs with a score of -inf and a doc id of Integer.MAX_VALUE. For the HQ to
work, this sentinel must be smaller than all posible values, which is not the
case:
- -inf is equal and the document is not inserted into the HQ, as not
competitive, but the HQ is not yet full, so the sentinel values keep in the HQ
and result is the Integer.MAX_VALUE docs. This problem is solveable (and only
affects the Ordered collector) by chaning the exit condition to:
{code}
if (score <= pqTop.score && pqTop.doc != Integer.MAX_VALUE) {
// Since docs are returned in-order (i.e., increasing doc Id), a document
// with equal score to pqTop.score cannot compete since HitQueue favors
// documents with lower doc Ids. Therefore reject those docs too.
return;
}
{code}
- The NaN case can be fixed in the same way, but then has another problem: all
comparisons with NaN result in false (none of these is true): x < NaN, x > NaN,
NaN == NaN. This leads to the fact that HQ's lessThan always returns false,
leading to unexspected ordering in the PQ and sometimes the sentinel values do
not stay at the top of the queue. A later hit then overrides the top of the
queue but leaves the incorrect sentinels unchanged -> invalid results. This
can be fixed in two ways in HQ:
Force all sentinels to the top:
{code}
protected final boolean lessThan(ScoreDoc hitA, ScoreDoc hitB) {
if (hitA.doc == Integer.MAX_VALUE)
return true;
if (hitB.doc == Integer.MAX_VALUE)
return false;
if (hitA.score == hitB.score)
return hitA.doc > hitB.doc;
else
return hitA.score < hitB.score;
}
{code}
or alternatively have a defined order for NaN (Float.compare sorts them after
+inf):
{code}
protected final boolean lessThan(ScoreDoc hitA, ScoreDoc hitB) {
if (hitA.score == hitB.score)
return hitA.doc > hitB.doc;
else
return Float.compare(hitA.score, hitB.score) < 0;
}
{code}
The problem with both solutions is, that we have now more comparisons per hit
and the use of sentinels is questionable. I would like to remove the sentinels
and use the old pre 2.9 code for comparing and using PQ.add() when a
competitive hit arrives. The order of NaN would be unspecified.
To fix the order of NaN, it would be better to replace all score comparisons by
Float.compare() [also in FieldComparator].
I would like to delay 2.9.2 and 3.0.1 until this problem is discussed and
solved.
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