Why is queryWeight included for some result scores, but not others, in the same query? - elasticsearch

Question

I'm executing a query_string query with one term on multiple fields, _all and tags.name, and trying to understand the scoring. Query: {"query":{"query_string":{"query":"animal","fields":["_all","tags.name"]}}}. Here are the documents returned by the query:
Document 1 has an exact match on tags.name, but not on _all.
Document 8 has an exact match on both tags.name and on _all.
Document 8 should win, and it does, but I'm confused by how the scoring works out. It seems like Document 1 is getting penalized by having its tags.name score multiplied by the IDF twice, whereas Document 8's tags.name score is only multiplied by the IDF once. In short:
They both have a component weight(tags.name:animal in 0) [PerFieldSimilarity].
In Document 1, we have weight = score = queryWeight x fieldWeight.
In Document 8, we have weight = fieldWeight!
Since queryWeight contains idf, this results in Document 1 getting penalized by its idf twice.
Can anyone make sense of this?
Additional information
If I remove _all from the fields of the query, queryWeight is completely gone from the explain.
Adding "use_dis_max":true as an option has no effect.
However, additionally adding "tie_breaker":0.7 (or any value) does affect Document 8 by giving it the more-complicated formula we see in Document 1.
Thoughts: It's plausible that a boolean query (which this is) might do this on purpose to give more weight to queries that match more than one sub-query. However, this doesn't make any sense for a dis_max query, which is supposed to just return the maximum of the sub-queries.
Here are the relevant explain requests. Look for embedded comments.
Document 1 (match only on tags.name):
curl -XGET 'http://localhost:9200/questions/question/1/_explain?pretty' -d '{"query":{"query_string":{"query":"animal","fields":["_all","tags.name"]}}}':
{
"ok" : true,
"_index" : "questions_1390104463",
"_type" : "question",
"_id" : "1",
"matched" : true,
"explanation" : {
"value" : 0.058849156,
"description" : "max of:",
"details" : [ {
"value" : 0.058849156,
"description" : "weight(tags.name:animal in 0) [PerFieldSimilarity], result of:",
// weight = score = queryWeight x fieldWeight
"details" : [ {
// score and queryWeight are NOT a part of the other explain!
"value" : 0.058849156,
"description" : "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
"details" : [ {
"value" : 0.30685282,
"description" : "queryWeight, product of:",
"details" : [ {
// This idf is NOT a part of the other explain!
"value" : 0.30685282,
"description" : "idf(docFreq=1, maxDocs=1)"
}, {
"value" : 1.0,
"description" : "queryNorm"
} ]
}, {
"value" : 0.19178301,
"description" : "fieldWeight in 0, product of:",
"details" : [ {
"value" : 1.0,
"description" : "tf(freq=1.0), with freq of:",
"details" : [ {
"value" : 1.0,
"description" : "termFreq=1.0"
} ]
}, {
"value" : 0.30685282,
"description" : "idf(docFreq=1, maxDocs=1)"
}, {
"value" : 0.625,
"description" : "fieldNorm(doc=0)"
} ]
} ]
} ]
} ]
}
Document 8 (match on both _all and tags.name):
curl -XGET 'http://localhost:9200/questions/question/8/_explain?pretty' -d '{"query":{"query_string":{"query":"animal","fields":["_all","tags.name"]}}}':
{
"ok" : true,
"_index" : "questions_1390104463",
"_type" : "question",
"_id" : "8",
"matched" : true,
"explanation" : {
"value" : 0.15342641,
"description" : "max of:",
"details" : [ {
"value" : 0.033902764,
"description" : "btq, product of:",
"details" : [ {
"value" : 0.033902764,
"description" : "weight(_all:anim in 0) [PerFieldSimilarity], result of:",
"details" : [ {
"value" : 0.033902764,
"description" : "fieldWeight in 0, product of:",
"details" : [ {
"value" : 0.70710677,
"description" : "tf(freq=0.5), with freq of:",
"details" : [ {
"value" : 0.5,
"description" : "phraseFreq=0.5"
} ]
}, {
"value" : 0.30685282,
"description" : "idf(docFreq=1, maxDocs=1)"
}, {
"value" : 0.15625,
"description" : "fieldNorm(doc=0)"
} ]
} ]
}, {
"value" : 1.0,
"description" : "allPayload(...)"
} ]
}, {
"value" : 0.15342641,
"description" : "weight(tags.name:animal in 0) [PerFieldSimilarity], result of:",
// weight = fieldWeight
// No score or queryWeight in sight!
"details" : [ {
"value" : 0.15342641,
"description" : "fieldWeight in 0, product of:",
"details" : [ {
"value" : 1.0,
"description" : "tf(freq=1.0), with freq of:",
"details" : [ {
"value" : 1.0,
"description" : "termFreq=1.0"
} ]
}, {
"value" : 0.30685282,
"description" : "idf(docFreq=1, maxDocs=1)"
}, {
"value" : 0.5,
"description" : "fieldNorm(doc=0)"
} ]
} ]
} ]
}
}

Answer 1

I've no answer. Just want to mention I posted question to the Elasticsearch forum: https://groups.google.com/forum/#!topic/elasticsearch/xBKlFkq0SP0
I'll notify here when I'll get the answer.