Marko Bonaci created LUCENE-6687:
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Summary: MLT term frequency calculation bug
Key: LUCENE-6687
URL: https://issues.apache.org/jira/browse/LUCENE-6687
Project: Lucene - Core
Issue Type: Bug
Components: core/query/scoring, core/queryparser
Affects Versions: 5.2.1, Trunk
Environment: OS X v10.10.4; Solr 5.2.1
Reporter: Marko Bonaci
In {{org.apache.lucene.queries.mlt.MoreLikeThis}}, there's a method
{{retrieveTerms}} that receives a {{Map}} of fields, i.e. a document basically,
but it doesn't have to be an existing doc.
There are 2 for loops, one inside the other, which both loop through the same
set of fields.
That effectively doubles the term frequency for all the terms from fields that
we provide in MLT QP {{qf}} parameter.
It basically goes two times over the list of fields and accumulates the term
frequencies from all fields into {{termFreqMap}}.
The private method {{retrieveTerms}} is only called from one public method, the
version of overloaded method {{like}} that receives a Map: so that private
class member {{fieldNames}} is always derived from {{retrieveTerms}}'s argument
{{fields}}.
Uh, I don't understand what I wrote myself, but that basically means that, by
the time {{retrieveTerms}} method gets called, its parameter fields and private
member {{fieldNames}} always contain the same list of fields.
Here's the proof:
These are the final results of the calculation:
And this is the actual {{thread_id:TID0009}} document, where those values were
derived from (from fields {{title_mlt}} and {{pagetext_mlt}}):
Now, let's further test this hypothesis by seeing MLT QP in action from the
AdminUI.
Let's try to find docs that are More Like doc {{TID0009}}.
Here's the interesting part, the query:
{{q={!mlt qf=pagetext_mlt,title_mlt mintf=14 mindf=2 minwl=3 maxwl=15}TID0009}}
We just saw, in the last image above, that the term accumulator appears {{7}}
times in {{TID0009}} doc, but the {{accumulator}}'s TF was calculated as {{14}}.
By using {{mintf=14}}, we say that, when calculating similarity, we don't want
to consider terms that appear less than 14 times (when terms from fields
{{title_mlt}} and {{pagetext_mlt}} are merged together) in {{TID0009}}.
I added the term accumulator in only one other document ({{TID0004}}), where it
appears only once, in the field {{title_mlt}}.
Let's see what happens when we use {{mintf=15}}:
Bug, no?
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