Elastic eng I, should not (should + must_not)

Elastic Community and Ecosystem Elastic Training
1

hello

reviewing the elasting eng I material
I came across the topic "nest bool query (Slide 170)" ...
Why does this example influence max_score (is higher) but not the hits total?

Is there a simple explantation?

3

Adding a should clause to a bool query that contains a must clause does not change the number of hits, only the scoring.

For example, the following match query searches for blogs that mention "logstash" in the content field. Suppose you get 140 hits:

GET blogs/_search
{
  "query": {
    "match": {
      "content": "logstash"
    }
  }
}

Now let's add a should clause, so that blogs with "logstash" in the title field of the blog score higher (we are still requiring the "logstash" is in the content field):

GET blogs/_search
{
  "query": {
    "bool": {
      "must": {
        "match": {
          "content": "logstash"
        }
      },
      "should": {
        "match": {
          "title": "logstash"
        }
      }
    }
  }
}

The should clause does not add any hits to the results. Think about the English word "should" - it is not a requirement that "logstash" be in the title, rather it is something that we would prefer to have in our hits.

So in the bool query above, you would still get 140 hits. But if a blog has "logstash" in the title also, then the score from that should clause is added to the score from the must clause. In this example above, the max_score would be considerably higher for blogs that have "logstash" in the title. If a blog does not have "logstash" in the title, then its score from the bool query will be the same as if you ran the match query without a should clause.

4

Hi nsam,
it is because the match_phrase is inside a should clause. Any clause inside the must must happen. Any clause inside the should should happen. In other words, any clause inside a should will not discard results, but will impact the score of the documents that match the should. Not sure that is clear, but hopefully you could understand.

Cheers

5

@raposa & @pmusa

both explanation help me to undesrstand ...
so to sum up, the should clause boost the _score and don't influence total count.

thank you

6

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