I'm a little confused about what is the difference between should and boost final score calculation

• when a bool query has a must clause, the should clauses act as a boost factor, meaning none of them have to match but if they do, the relevancy score for that document will be boosted and thus appear higher in the result.
• so,if we have:

one query which contains must and should clauses

vs

second query which contains must clause and boosting clause

1. Is there a difference ?
2. when you recommend to use must and should vs must and boosting clauses in a query ?

CodePudding user response：

You can read the documentation of boolean query here, there is huge difference in the should and boost.

Should and must both contributes to the _score of the document, and as mentioned in the above documentation, follows the

The bool query takes a more-matches-is-better approach, so the score from each matching must or should clause will be added together to provide the final _score for each document.

While boost is a parameter, using which you can increase the weight according to your value, let me explain that using an example.

Index sample docs

POST _doc/1
{
  "brand" : "samsung",
  "name" : "samsung phone"
}

POST _doc/2

{
  "brand" : "apple",
  "name" : "apple phone"
}

Boolean Query using should without boost

{
    "query": {
        "bool": {
            "should": [
                {
                    "match": {
                        "name": {
                            "query": "apple"
                        }
                    }
                },
                {
                    "match": {
                        "brand": {
                            "query": "apple"
                        }
                    }
                }
            ]
        }
    }
}

Search result showing score

"max_score": 1.3862942,

Now in same query use boost of factor 10

{
    "query": {
        "bool": {
            "should": [
                {
                    "match": {
                        "name": {
                            "query": "apple"
                        }
                    }
                },
                {
                    "match": {
                        "brand": {
                            "query": "apple",
                            "boost": 10 --> Note additional boost
                        }
                    }
                }
            ]
        }
    }
}

Query result showing boost

"max_score": 7.624619, (Note considerable high score)

In short, when you want to boost a particular document containing your query term, you can additionally pass the boost param and it will be on top of the normal score calculated by should or must.