dis_max query isn't looking for the best matching clause - elasticsearch

Question

I'm testing the dis_max query in the docs below:
PUT /blog/post/1
{
"title": "Quick brown rabbits",
"body": "Brown rabbits are commonly seen."
}
PUT /blog/post/2
{
"title": "Keeping pets healthy",
"body": "My quick brown fox eats rabbits on a regular basis."
}
This example is extracted from the book "Elasticsearch definitive guide" which explains that the answer from the query below would shows equals _score for both documents.
{
"query": {
"dis_max": {
"queries": [
{ "match": { "title": "Quick pets" }},
{ "match": { "body": "Quick pets" }}
]
}
}}
But, as you could see, the result from the query shows different _score.
{
"took" : 10,
"timed_out" : false,
"_shards" : {
"total" : 5,
"successful" : 5,
"failed" : 0
},
"hits" : {
"total" : 2,
"max_score" : 0.02250402,
"hits" : [ {
"_index" : "blog",
"_type" : "post",
"_id" : "2",
"_score" : 0.02250402,
"_source" : {
"title" : "Keeping pets healthy",
"body" : "My quick brown fox eats rabbits on a regular basis."
}
}, {
"_index" : "blog",
"_type" : "post",
"_id" : "1",
"_score" : 0.016645055,
"_source" : {
"title" : "Quick brown rabbits",
"body" : "Brown rabbits are commonly seen."
}
} ]
}
}
Elasticsearch is not returning the _score from best matching clause but is, somehow, blending the results. How may I fix it?

Answer 1

I've got the answer.
This confusing behavior happens because the index used in the example is using 5 shards (default number of shards). And the _score is not calculated in the index as a whole but in individual shards and then are summarized before the user got the answer.
This problem is not a issue when you have a huge number of documents, what it is not my case.
So, to test my thesis, I deleted my index:
DELETE /blog
And then, created a new index using only 1 shard:
PUT /BLOG
{ "settings" : { "number_of_shards" : 1 }}
So, I performed my query again and got both documents with the same _score: 0.12713557
Sweet =)