Christoph Goller created LUCENE-8943:
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Summary: Incorrect IDF in MultiPhraseQuery and SpanOrQuery
Key: LUCENE-8943
URL: https://issues.apache.org/jira/browse/LUCENE-8943
Project: Lucene - Core
Issue Type: Bug
Components: core/query/scoring
Affects Versions: 8.0
Reporter: Christoph Goller
I recently stumbled across a very old bug in the IDF computation for
MultiPhraseQuery and SpanOrQuery.
BM25Similarity and TFIDFSimilarity / ClassicSimilarity have a method for
combining IDF values from more than on term / TermStatistics.
I mean the method:
Explanation idfExplain(CollectionStatistics collectionStats, TermStatistics
termStats[])
It simply adds up the IDFs from all termStats[].
This method is used e.g. in PhraseQuery where it makes sense. If we assume that
for the phrase "New York" the occurrences of both words are independent, we can
multiply their probabilitis and since IDFs are logarithmic we add them up.
Seems to be a reasonable approximation. However, this method is also used to
add up the IDFs of all terms in a MultiPhraseQuery as can be seen in:
Similarity.SimScorer getStats(IndexSearcher searcher)
A MultiPhraseQuery is actually a PhraseQuery with alternatives at individual
positions. IDFs of alternative terms for one position should not be added up.
Instead we should use the minimum value as an approcimation because this
corresponds to the docFreq of the most frequent term and we know that this is a
lower bound for the docFreq for this position.
In SpanOrQuerry we have the same problem It uses buildSimWeight(...) from
SpanWeight and adds up all IDFs of all OR-clauses.
If my arguments are not convincing, look at SynonymQuery / SynonymWeight in the
constructor:
SynonymWeight(Query query, IndexSearcher searcher, ScoreMode scoreMode, float
boost)
A SynonymQuery is also a kind of OR-query and it uses the maximum of the
docFreq of all its alternative terms. I think this is how it should be.
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