Hacking Lucene for Custom Search Results

Hacking Lucene for Custom Search Results
Query responsible for specifying search behavior
• Both:
Matching – what documents to include in the results
Scoring – how relevant is a result to the query by assigning a score

How to use it

Use a normal Lucene query for matching

Term t = new Term("tag", "star-trek");

TermQuery tq = new TermQuery(t);

Create & Use a CustomQueryScorer for scoring that

wraps the Lucene query

CountingQuery ct = new CountingQuery(tq);

Extend CustomScoreQuery, provide a CustomScoreProvider
OpenSource Connections
protected CustomScoreProvider getCustomScoreProvider(
AtomicReaderContext context) throws IOException {
return new CountingQueryScoreProvider("tag", context);
}
CustomScoreProvider rescores each doc with IndexReader & docId
OpenSource Connections
// Give all docs a score of 1.0
public float customScore(int doc, float subQueryScore, float
valSrcScores[]) throws IOException {
return (float)(1.0f); // New Score
}

Example: Sort by number of terms in a field
OpenSource Connections
// Rescores by counting the number of terms in the field
public float customScore(int doc, float subQueryScore, float
valSrcScores[]) throws IOException {
IndexReader r = context.reader();
Terms tv = r.getTermVector(doc, _field);
TermsEnum termsEnum = null;
termsEnum = tv.iterator(termsEnum);
int numTerms = 0;
while((termsEnum.next()) != null) {
 numTerms++;
}
return (float)(numTerms); // New Score
}

CustomScoreQuery, Takeaway

SIMPLE!

Relatively few gotchas or bells & whistles (we will see lots of gotchas)

• Limited

No tight control on what matches

• If this satisfies your requirements: You should get off the train here

I care about overriding scoring
CustomScoreQuery
• I need to control custom scoring and matching
Custom Lucene Queries!

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